Primary & Secondary Loops

8/17/2019 Primary & Secondary Loops

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HYDRONIC BALANCING PART 1: THE STANDARDS AND DRIVING FORCE BEHIND THE

NEW REQUIREMENTS

May 05, 2015 / JMP

By Chad Edmondson

Balancing plays a critical roll in the performance of any hydronic heating and cooling system. For that

reason alone, ASHRAE has made hydronic balancing a non-negotiable stop on the road to compliance

with ASHRAE 90.1-2010 (or 2013), starting with this requirement:

6.4.2.2 Pump Head. Pump differential pressure (head) for the purpose of sizing pumps shall be

determined in accordance with generally accepted engineering standards and handbooks acceptable

to the adopting authority. The pressure drop through each device and pipe segment in the critical

circuit at design conditions shall be calculated.

Furthermore, systems must be balanced :

6.7.2.3 Hydronic System Balancing. Hydronic systems shall be proportionately balanced in a

manner to first minimize throttling losses; then the pump impeller shall be trimmed or pump speed

shall be adjusted to meet design flow conditions.

And finally, there is this:

6.7.2.3.1 General. Construction documents shall require that all HVAC systems be balanced in

accordance with generally accepted engineering standards. Construction documents shall require that

a written balance report be provided to the building owner or the designated representative of the

building owner for HVAC systems serving zones with a total conditions area exceeding 5000 ft2.

All of these standards are interrelated. Accurately calculating pressure drops ensures that pumps arenot oversized. Oversized pumps can lead to inefficient pump operation over the life of the

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system. Flow balancing (during and after commissioning) is how we make sure that we only put as

much energy into the system as we are take out. Flow balancing is also how we begin to benchmark

the performance of our buildings, a practice that ASHRAE aspires to make commonplace. It is all part

of a long-term plan to drive more and more buildings to net zero energy performance.

In all likelihood the standards as written above are now part of your own building code since the U.S.Department of Energy (DOE) has required that states update their building codes to meet or exceed

Standard 90.1-2010 (or ask for an extension) by October 18, 2013.

What does this mean to mechanical engineers designing commercial buildings larger than 5000 sq.

ft.? Among other things, it means that you must now incorporate a balancing procedure into your

mechanical specifications and include in the design plan all the necessary instrumentation to perform

that procedure.

Over the next several blogs we’ll dig deeper into what you, as a designer or as a commissioner, need

to know and understand in order to meet the balancing standards of 90.1- 2010 (and 2013).

Hydronic Balancing Part 2: Making the Most of System Diversity

May 15, 2015 / JMP

By Chad Edmondson

Practically any commercial or institutional building has a certain amount of diversity within its cooling

load, meaning that peak loads will never occur simultaneously in all sections or zones of a facility. By

mapping out the individual load patterns of these sections, engineers can adjust the mechanical

design to reduce the overall amount of installed cooling capacity. This means incorporating variable

flow, which necessitates precise hydronic balancing.

To illustrate this point, consider this simple example of a central chilled water system at a college with

four basic groups of buildings and identical peak loads:

Building Peak Load

Dorms 1000 tons

Cafeteria 1000 tons

Library 1000 tons

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Gym 1000 tons

Clearly, the load pattern of these buildings will vary and at no time will there be concurrent peak

loads in all four buildings. (Students can’t be in more than one place at a time!) In other words,

there is diversity within the system. This gives the designer the opportunity to design the system so

that the cooling water is directed only where it is needed.

Let’s say that the design engineer has done a complete cooling load calculation and has determined

that the peak block load at any given time is 3000 tons. Block load is the instantaneous maximum

heating and cooling load for a calculated point in time for the entire building, including all envelope

and internal load components of the heating and cooling load calculation.

The engineer determines the peak block load based on the diversity factor that he or she has chosen

for the system given the anticipated load patterns of the system. In our example, the engineer

would have chosen a diversity factor of .75 because the diversity factor is the peak block load (3000

Tons) divided by the total connected load (4000 Tons).

Here are some very general rules of thumb for diversity in buildings:

.85 for systems up to 25 tons

.80 for systems from 25 tons to 100 tons

.75 for systems larger than 100 tons.

Figure 1


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If this particular system were designed in the old style with 3-way valves to provide constant flow

through the chillers (Figure 1), peak block load would not matter because constant flow systems do

not take advantage of diversity. You would have 2400 GPM of constant flow going to the dorms,

cafeteria, library and gym at all times. And the system would require 4 chillers at 1000 tons each

instead of just three.

However, we can get by with significantly less cooling capacity and less GPM by taking advantage of

the diversity within the system and incorporating variable speed pumps. (Figure 2).

Figure 2

Notice that the system now includes variable speed pump controls and 2-way valves instead of 3-way

valves. As a result we’ve trimmed most of the excess out of the system. We’re also doing the same

job with less equipment:

One less chiller, and 1000 fewer tons

One less chiller pump

One less cooling tower

One less condenser water pump

Reduced flow (7200 GPM vs. 9600 GPM)

Smaller pipe main (18‖ vs. 20‖)


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Obviously there is a lot to be gained both in terms of equipment cost and efficiency, but balancing is

more critical than ever. Why? Because although you have reduced the overall system flow, the peak

flow requirements for each section have not changed. They were 2400 GPM before, and they are

2400 GPM now, so balancing must be carefully integrated into the system to assure that the

maximum flow can be obtained if needed. In doing so we not only help ensure successful operation

the system, but also meet the requirements of ASHRAE 90.1 for balancing.

Hydronic Balancing Part 3: How To Use The System Syzer

May 28, 2015 / JMP

By Chad Edmondson

Virtually every aspect of hydronic balancing is based in one fundamental law:

As you double the flow through the piping the pressure drop increases by the square. In other

words, the pressure drop increases by four times what it was.

This law is expressed in the following equation:

Hydronic balancing law equation.

Understanding this relationship between flow and pressure is everyone’s first step toward designing,

installing, or commissioning a balanced hydronic system. It also allows you to take advantage of anynumber of tools the industry has made available for the purpose of system balancing like Bell &

Gossett System Syzer.

system-syzer-calculator-for-hvac-contractors-and-commissioning-agents

What Is the System Syzer?

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In its most basic form, the System Syzer is a simple plastic side wheel that lets you quickly determine

the pressure drop in a hydronic system assuming you know the pressure drop at one given flow – or

the Cv. The Cv or ―C sub V‖ of a system component is the flow rate in gallons per minute that results

in a pressure drop of 1 psi (or 2.31 feet of head). All components in a hydronic system have a rated

Cv; manufacturers make this information available. Note: Hydronic valves are always rated based on

flow through a fully open valve.

The System Syzer, whether in its slide wheel form or as an application on your i-phone or android,

not only assists in balancing, but also troubleshooting. Either is available for free from Bell & Gossett.

To make sure you understand how the System Syzer works, let’s solve a simple problem using the old

fashion slide wheel shown here:

Notice Scale 5 on the bottom half of the slide wheel. This scale is based on the above formula, and

therefore gives you all of the pressure drops for any given flow. Remember – for any piping and/or

equipment, if you know its pressure drop at a given flow (GPM), then you can calculate its pressure

drop at any other GPM. Therefore, if you know the Cv for the component (published by themanufacturer), then you also have your starting point.

Example:

Let’s say we have a base-mounted end suction pump with a combination valve on the discharge. We

know that at 701 GPM (the Cv provided by the valve manufacturer) we have a pressure drop of 2.31

feet through the valve.

Pressure Loss

What is the pressure loss through the combination valve if the flow rate increases to 1000 GPM? To

find out we simply go to the slide wheel and line up the values of what we know —701 GPM (on the

white scale) with 2.31 feet of head (on the upper blue scale).


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closeup systemsyzer

Without moving the slide again, we can now read off the pressure losses at every GPM on the

scale. At 1000 GPM there would be 4.6 feet of head loss through the valve. If we choose, we can

chart the pressure losses through this valve at other flows – just by reading off the values of the slide

wheel while it is in this same exact position.

GPM

Pressure Loss

701

2.31 feet

1000

4.7 feet

1200

6.8 feet

1400

9.2 feet

2000

19 feet

Keep in mind that this handheld version of the System Syzer is designed for typical chilled and hot

water systems with a specific gravity of 1 and a specific heat of 1. The electronic versions that you

download to your phone and/or computer let you incorporate many other variables such as PVC piped

systems, non-water systems, and a greater range of pipe sizes. They also have metric and Spanish

language conversion.

Any System Component Can Be a Flow Meter!

Given what we now know about the relationship between flow and pressure, it may have already

occurred to you that you can turn just about any other component in system into a flow meter – just

by knowing the pressure drop through it at a given flow. For the sake of accuracy, the inlet and

outlet pressures readings through a component (chiller, heat exchanger, etc.) should be taken with

the same gauge, as two gauges might not be calibrated exactly the same.


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Nevertheless, simply knowing these two values you can determine the pressure drop and at other

flow – or the flow at any other pressure drop.

Hydronic Balancing Part 4: How to Develop a System Curve

June 11, 2015 / JMP

By Chad Edmondson

What is a system curve and how is it used to develop a balanced hydronic system?

The ―system curve‖ is a graphical representation of the head losses and gains of a particular piping

system that result from changes in flow. And it’s all based on what you already learned if you read

our previous blog on Hydronic Balancing Part 3: How To Use The System Syzer:

As you double the flow through the piping the pressure drop increases by the square. In other

words, the pressure drop increases by four times what it was.

When used in combination with the pump curve, a design engineer can determine the system head

and flow long before the system is installed and the pump is turned on. It’s all based on the same

math:

Why System Curves Matter

Pump curves represent the energy that is put into a system; system curves represent what the

system takes out. A system will operate at the point at which these two curves intersect, as long as

nothing else changes in the system (such as a valve being closed or partially closed).

Design engineers want the system and individual circuits to operate at specific flows to satisfy the

space heating and cooling requirements while staying within the operating range of the

components. That’s why the system curve is important. It let’s the engineer know where and how

much design adjustment or ―tweaking‖ of valves wi ll be needed so that once the system is balanced

and the pump is turned on it will operate in a correct and efficient manner. It’s not a matter of

crossing your fingers – it’s a matter of knowing exactly how much resistance is in a given piping

system and matching it precisely with the flow characteristics of pump.

How to Plot a System Curve

A system curve is developed by using Scale 5 of the System Syzer, just as we discussed in the

previous blog.

Let’s say we have determined the design flow and head for our system to be 2200 GPM at 100 feet of

head. (These values would be based on the critical circuit.) Knowing this, we choose a pump capable

of generating this much head and flow and we take the following steps to develop our system curve

and determine the operating point of our system:

Step 1 –

Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

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Step 1 – Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

Step 1 – Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

Step 2 – Without changing the position (or settings) of the System Syzer, read off and

record the head at various other flows.

Step 2 – Without changing the position (or settings) of the System Syzer, read off and record the

head at various other flows.

Step 2 – Without changing the position (or settings) of the System Syzer, read off and record the

head at various other flows.

Step 3 – Plot these values to develop the system curve


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Step 4 – Overlay the system curve atop the pump curve for the selected impeller trim to

see where the lines intersect.

Step 4 – Overlay the system curve atop the pump curve for the selected impeller trim to see wherethe lines intersect.


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Step 4 – Overlay the system curve atop the pump curve for the selected impeller trim to see where

the lines intersect.

Wherever the system curve intersects with the pump curve is where that pump will operate under full

load conditions, when all valves are open and the system is at full flow design conditions. Remember,

the ―system‖ is everything from the pump discharge flange to the pump suction flange. Here it willstay unless the resistance within the pipe system changes – i.e. a valve changing position. As two-

way valves open and close, the system curve will change accordingly and thus where it intersects

with the pump curve. Ideally the pump will have been selected to weather the demand range and

safely ride the pump curve as demand changes.

By understanding how to plot the system curve we can correctly balance a pump at system started-

up!

Hydronic Balancing Part 5: Types of Balancing Products

June 24, 2015 / JMP

By Chad Edmondson

We know we have to balance our hydronic systems to meet the ASHRAE 90.1-2010 Standard. The

next question is what balancing technology should we use. For the most part, ASHRAE leaves that up

to the designer. Here are the typical options:

circuit-setter-calibrated-balancing-valves

Circuit Setter

Calibrated Balancing Valves. These have been around for a while and are what most people

commonly refer to as ―circuit setters.‖ Calibrated balancing valves are designed for pre-set

proportional system balance. This system balance method involves pre-setting the valves to achieve

optimum system flow balance (at minimum horsepower) using the manufacturers performance

curves. This straightforward method is based on the fact that if you know the pressure drop through

the device and its Cv (the flow rate in GPM through the device that results in 1 psi pressure drop),

then mathematically you can determine the flow.

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Ball Valve

Ball Valve

Standard Ball or Butterfly Valves. These devices, along with pressure gauges or test plugs, allow

the control contractor to measure a pressure drop across the coil or heat exchanger and then

determine and adjust the flow based on the manufacturer’s performance data.

Flow Limiting Valve

Flow Limiting Valve

Automatic System-Powered Flow Limiting Valves.

Although these valves are often referred to as ―automatic‖ flow control devices they are actually flowlimiting valves. These valves can be set to reliably limit flow through a give circuit; however, if the

flow drops beneath this value, there is no actual control. These valves can provide better flow control

over a manual balance when a variable speed system is operating at part load.

Pressure Independent Control Valve


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Pressure Independent Control Valve

Pressure-independent Flow Control Valves.

These valves combine all the attributes of a balancing valve, control valve, and a differential pressure

regulator into one valve. An integral pressure regulator automatically compensates for fluctuations insystem pressure to stabilize flow rate through the heating or cooling coil. When the actuator is

installed, it will adjust flow in response to heating or cooling demands. The valves eliminate the need

for any Cv calculations and maintain full authority over the entire flow range of the valve.

Ultimately, the type of pumping system you have will determine the type of control device that is best

suited for your application. Stay tuned for more on that in our next blog!

HYDRONIC BALANCING PART 6: WHAT KIND OF PUMPING SYSTEM DO YOU HAVE?

July 09, 2015 / JMP

By Chad Edmondson

Balancing contractors and facility operators would have a much easier time balancing a hydronic

system if they were present during the system design process. Unfortunately that is rarely the case

so there is usually a certain amount of detective work that comes with balancing. The biggest part of

that is getting a handle on the overall pumping system. You can’t effectively balance a system

without understanding the overall flow dynamic. For that reason, we always recommend making a

basic sketch of the system before the balancing process begins.

Pumping systems typically fall into one of five types, which are all noted below. Once you know what

the system looks like in a single snap shot, you are in a far better position to balance it. What you’llfind is that the system is likely to bear a striking resemblance to one of Figures 1 through 5.

Figure 1 shows a basic primary-secondary pumping system, with constant flow through the chillers

and a separate secondary pump serving the system load. In this system the primary flow is isolated

from the secondary flow by virtue of the common (decoupler) pipe shown in green between the two

loops. The chillers will be individually balanced for a constant design flow whereas the building flow

will vary based on load.

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Figure 1: Basic primary-secondary pumping system"

Figure 2 shows a slightly more complex pumping arrangement known as Primary-Secondary

Tertiary. The good news about this type of design is that it can be easy to balance, as each

building/load has its own pump with a decoupler pipe located between the secondary loop and each

of the tertiary loops. This means that changes in one zone will not affect changes in another so

balancing becomes less complex. This type of system is also easy to add on to in the future.


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Figure 2: Primary Secondary Tertiary pumping system

You might determine that you have a system like the one shown in Figure 3 where there is a single

zone remotely located from the others. Note that Zones A and B are pumped by the same pump,

while Zone C has its own dedicated pump. Each individual pump will have to be balanced and a 2way valve added to the Zone C return line.


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Figure 3: Primary - Secondary - Tertiary Hybrid pumping system

Figure 4 shows a Primary-Secondary Zone pumping arrangement where, although there are two

distinct loops and only one common pipe, we have separate pumps serving each zone. This type of

design keeps horsepower down, but adds some additional control complexity, as each zone (pump)

must be balanced. Also, since the pumps are in parallel, their performance curves must becompatible.


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Figure 4: Primary - Secondary Zone Pump

Figure 5 shows a system without any secondary or zone pumps. All of the flow is established by the

primary pumps, which vary flow through the chillers according to system demand. A motorized

control valve is needed to maintain a minimum flow through the chillers. If designed correctly, this

type of system not only has lower installed cost, but also lower operating cost. Balancing however

can be difficult as there are no common pipes to isolate flow between the various zones. That’s why

pressure independent control valves are often seen in variable primary applications.


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Figure 5: Variable Primary Flow System

In any of the above cases a quick sketch of the pumping system will give the balancing contractor orfacility operator the ―big picture‖ perspective that is needed when it comes to balancing.

Hydronic Balancing Part 7: When to Trim the Pump Impeller

July 11, 2015 / JMP

By Chad Edmondson

Balancing isn’t just about adjusting valves. Sometimes (very often in fact) it is about evaluating the

performance of the pump(s) under real world operating conditions.

Remember what ASHRAE 90.1 has to say about Hydronic System Balancing:

―Hydronic systems shall be proportionately balanced in a manner to first minimizethrottling losses;

then the pump impeller shall be trimmed or pump speed shall be adjusted to meet design flow

conditions.‖

— ASHRAE 90.1

But how does one determine if a pump impeller on an installed pump needs to be trimmed?

First, it’s important to understand that an installed system almost never matches what is in the

original drawings. Pumps may be oversized and head losses may be different from what was

originally calculated by the system designer, depending on how the contractor piped the

system. Therefore it is important to determine where a pump is operating based on the actual

system curve, not the theoretical curve.

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To better understand this, let’s say we have a single pump system with a design point of 650 GPM at

76 ft. of head.

Example System Selection: Model 4BC Pump - Design Point: 650 GPM, 76 ft of head

In other words, this pump has been selected to deliver 650 GPM to the critical circuit. The designengineer did his system head loss calculations and determined that we needed exactly 76 ft. of head

to pump this system. Based on these criteria, he selected the following pump:


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Model 4BC Pump - Design Point: 650 GPM, 76 ft

However, once the pump is installed the owner reports excessive noise in the piping. This is our first

clue that actual operating conditions are not quite as anticipated, so a little detective work is in order.

Out of Balance

First, we must determine how much head and flow the installed pump is generating. Using the same

pressure gauge, we take reading at the pump suction and discharge and discover that the pump is

generating 65 ft. of head. Right away we notice that something is not quite right. This pump was

picked, after all, to deliver 76 feet of head. We consult pump curve and see that the corresponding

flow for 65 Ft. of head is 850 GPM, not the 650 GPM design flow. We’re over-pumping the system

and that has resulted not only excessive noise, but also wasted energy.

Our system is not balanced. We are generating more flow than we need, and as a result we are out

of compliance with ASHRAE 90.1 and we’re wasting energy.


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Model 4BC Pump - Operating Point: 850 GPM, 63 ft

Throttle, Trim or Replace?

We could put a Band-Aid on the problem and simply throttle the pump back so that reduce flow backto 650 GPM, but ASHRAE says we’re not supposed to do that either. Remember, we want to

minimize throttling because throttling wastes energy – and money. A more appropriate solution is

trimming the impeller or perhaps even replacing the pump.

Continuing with our example, we now know our pump and our system are not exactly a match made

in heaven. Sure – we can throttle the pump back and even save the owner a little money over what

he or she is paying now, but the real question is how much more money could we save if the pump

was a better match for the installed system.

With a triple duty valve, we can force the system back to its intended operating point on the

curve. In this case, that would reduce our operating cost (based on .06 kW) from the previous

annual operating cost (AOC) of $8000.00 to $7400.00. Seems like a win, but is it?

What if instead we make our adjustment to the pump instead of artificially adding more resistance to

the system? We can determine the outcome of this solution simply by creating a system curve for

the system that we actually have rather than what was predicted/intended by the design

engineer. To do that we use our known operating points of 850 GPM at 63 ft. of head and our

System Syzer to plot the points of our actual system curve. (You can review how to plot a system

curve here.)

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Step 2: Graph System Curve with System GPM (Q) and System Head Loss (h)

Using these points, we can plot a new system curve (our real life system curve) onto the pump

curve. Keeping in mind that we only need 650 GPM to serve this system, we simply draw a vertical

line on the pump curve upwards from the 650 GPM to see where it crosses with our system curve.

As you can see in pump curve shown below, the intersection occurs just slightly above the curve for a

7-¼‖ impeller – or approximately 7 ½ inches, a far better match for our system than the 9- ½‖impeller we currently have.

Notice also the drop horsepower from 17 bhp (how the system was originally running – no trim, no

throttle) all the way down to 7 ½‖HP. Now our annual operating costs are SIGNIFICANTLY less --

$3600.00 versus $8000.00. That’s a far greater improvement over the $600.00 we would save simply

by throttling valve.


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Impeller Trimming with Design Point, Operating Point, and Desired GPM = 650

If our system head just was slightly less, the intersection point with the system curve might occur

below this particular pump, in which case we would probably want to replace the whole pump.

Finally, there is one other solution. We could install a variable speed drive on the pump to slow itdown – thus changing its performance curve. In this case, however, a trim is all we need to balance

the pump with the system, and meet ASHRAE 90.1.

ASHRAE Passes Standard 188-2015, Legionellosis: Risk Management for Building Water

Systems

August 06, 2015 / Chad Edmondson

By Chad Edmondson

We interrupt this regularly scheduled series on hydronic balancing to announce that ASHRAE has

officially published Standard 188-2015, Legionellosis: Risk Management for Building Water Systems.

It’s a timely bit of information given our current discussion about balancing, even though it is directed

at domestic water rather than hydronic heating and cooling.

Among other things related to the prevention of Legionella, Standard 188 states:

―All water systems shall be balanced and a balance report for all water systems shall be provided to

the building owner or designee.‖

The keyword here is ―all‖ water systems.

What Does Balancing Have To Do With Legionella?

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Why has ASHRAE decided to address balancing in a standard that is written for the purpose of

Legionella prevention? The reason has to do with domestic hot water recirculation systems –

particularly large systems with multiple returns coming back to the boiler.

If these return lines are not balanced it is possible that a period of ―no flow‖ might occur in one or

more of the return lines. Often referred to as ―dead legs‖, these stagnant areas in the pipe increasethe risk for Legionella growth because scale and biofilm tend to collect there, creating a safe haven

for Legionella to grow. Remember -- Legionella can grow and multiply in water temperatures beyond

its typical survival range if it happens to be residing in a cozy bit of scale. That’s why it is important

to keep the water moving – even during periods of no demand.

Dead legs can be avoided by installing an automatic balancing valve on each return line to ensure

that some amount of flow is always maintained through each line —under all demand conditions.

Time to Get Serious about Domestic Water Balancing

Now that Standard 188 has been passed, it is likely to become an ANSI standard, which will no doubt

be accepted into local codes. It’s just a matter of time.

So if you are designing or installing any kind of domestic recirculation line now or in the near future

don’t forget to balance. As we have discussed in the past, Standard 188 shifts the responsibility of

Legionella prevention to building owners and operators. As such, it will leave them more vulnerable

to lawsuits resulting from a Legionella related incident.

Actuators for Chilled Water Valve | DDC Commercial Systems

Richard Ashworth

September 6, 2008

Commercial HVAC

Actuators for Chilled Water Valve – These chilled water actuators control the flow rate for a

chilled water system in a data center. There are various sequence of operations for chilled water

systems and the sequence of operation is usually always different from one chiller plant to another

chiller plant. It depends on the components in the loop, the application the chilled water system is

supplying cold water for, and what the demand of the system requires for the chiller plant. Some

chilled water valves control two-way valves while others control three-way valves. A three-way valve

can either be a mixing valve or a diverting valve but the actuators controls the flow in either type of

application. Other actuators modulate a valve based on demand. The actuator usually receives its

command for position for control from the DDC system or another type of control system. In this case

these actuators are controlled by DDC. In the sequence of operation the chiller plant will have a valve

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line-up usually in a valve matrix that was compiled by the original design engineer and this valve

matrix shows the default position of the valves which are controlled by the actuators. Some

applications in the piping that are controlled by the actuators include:

Primary – Secondary system where a primary loop is attached to a secondary loop via a

decoupling loop. The primary loop is constant volume while the secondary loop is variablecapacity.

Variable Flow Primary loop only has one loop and the flow varies according to demand and

pressure set point. For better flow control a variable flow primary loop system will have a

bypass loop for better control of loop pressures. In the bypass and on the loads you will

find actuators that control chilled water flow.

Other applications where the control actuators can be found is in a free cooling sequence of

operation where the chillers are shut down and bypassed completely to take advantage of

free cooling when the temperatures are optimal for a free cooling application.

The actuators control the amount of chilled water going to the evaporator coil by automatically

moving a valve. Chilled water systems provide air conditioning typically to large buildings. These air

conditioning systems, or cooling systems, use cold water which is piped through a coil in a large air

handler. The control actuator can modulate allowing only a certain amount of cold water to reach the

evaporator coil.

Actuators for Chilled Water Valve – Depending on the cooling supply air set point will depend on

where the control actuator will modulate the valve in the piping. Chilled water systems offer an

economical way of cooling large commercial buildings. These air conditioning systems are a great

alternative to direct expansion constant volume air conditioning systems. There are constant volume

air conditioning systems which use chilled water but most constant volume air conditioning systems

use direct expansion. Many chilled water systems are used for VAV systems where there are many

zones on a large air handling unit. Both constant volume air conditioning system and VAV zoning air

conditioning systems can be either direct expansion or chilled water systems.

Actuators for Chilled Water Valve

Control actuators, in chilled water systems, offer control of the flow of chilled water which is routed to

the evaporator coil. This photo shows a new installation of a control actuator and piping. The control

actuator is modulating and on a three way valve. The valve will modulate depending on the supply air

temperature of the air handling unit which provides air conditioned air to the space. The piping was

installed with several unions so that if the valve malfunctioned it could easily be replaced.


26/33

The air handler being served by this control actuator and piping serves a large commercial kitchen. In

the summer it is critical for the air conditioning system to remove the heat produced by the ovens

and grills so that workers remain safe and comfortable. The air conditioning increases the workers

productivity and morale. The piping feeds chilled water to an evaporator coil inside the air handler.

The blower in the air handler moves the air from the kitchen across the coil where the heat in the air

is absorbed into the coil and the water. The conditioned air is then ducted back into the kitchen at a

cooler temperature than when it left the kitchen through the return duct.

Chilled water actuators give chilled water systems the ability to precisely control chilled water systems

and precision control results in added efficiency of the chilled water system. The chilled water

actuators are typically controlled by the building automation or DDC system that give the chilled

water actuators added control and precision control.

To learn more about HVAC and chilled water systems click here .

WHAT IS DECOUPLER LINE IN CHILLED WATER SYSTEM AND HOW IT WORKS?

The typical application I have ran into is a chiller loop that is primary and runs a constant flow

through the barrels and those chillers have dedicated pumps to maintain the design flow through the

chillers. The other loop is what I would refer to as the house loop or the secondary loop and that loop

will also have dedicated pumps, these days many are on VFD'S (Variable Frequency Drives) Pumps

throttle based on the connected load demand.

The de-coupler is the piping connecting the primary to to the secondary and is usually set by a critical

distance or so many pipe diameters apart. Do a search on Primary and Secondary Hydronic

Applications and you can see a diagram better describing my garbled explanation.

I just pipe them, I don't design them.

Pipe it:

To add to what was said, there is the primary pump and loop, which circulates water through the

chiller at a constant speed. Then there is the secondary loop to the building, which has it's own

pump, and typically VFD. They share a common pipe, or decoupler pipe, which basically connects

between the supply and return of these two loops. As the secondary loop pump increases speed and

requires cold water, it will pull the cold water out of the primary loop. As it pulls more and more

water out of the primary loop, the flow through the decoupler pipe is less and less. It will reach a

point where the building return is equal to the primary pump GPM, and there will be no flow in the

decoupler pipe. So in essence, there will be no circulation, but all return water going into the chiller.

In fact, the decoupler pipe can actually flow backwards, depending on the GPM of each pump and the

mixing of building return with chiller supply. That description was only one chiller. Make contact with

your local pump representative and schedule a lunch and learn with some diagrams. This stuff is too

complicated for one paragraph. Best regards.

CHILLED WATER PRIMARY/SECONDARY VARIABLE FLOW SYSTEMS.

Multiple chiller plants use primary/ secondary variable flow configuration. There are two loops in this

set up. One is plant side loop. It includes chillers, primary pumps and decoupler line. It makes a

complete circuit. Primary pumps are low head pumps. They are selected to meet head of this circuit

only. This circuit is called primary loop. Second circuit is the system loop or secondary loop. It

comprises secondary pump, piping network in the building and all air side equipment. Secondary

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pump covers friction losses of piping system, cooling coils, control valves, all fittings and return piping

up to cross over bridge. Secondary pump is variable speed pump. Primary pump is constant speed

pump. When air conditioning loads in the building drops, it requires less water to meet building

needs. Providing same volume of water at these conditions is wastage of pumping energy. Therefore,

secondary pump varies its speed according to building requirements. Pressure differential transducer

is installed at the remote leg of piping circuit. When number of control valves close due to drop in

cooling demand, pressure in the pipe lines increases. Control valves on air side equipment are two

way types. These reduce flow of water to the coil. Less flow to the coils increases pressure in the

lines. Increased pressure is sensed by differential pressure sensor in the lines. This signal is

transmitted to controller which reduces secondary pump speed to maintain required pressure

differential in the lines. When flow of water in the secondary circuit increases, water flows in the

crossover bridge in reverse direction. It mixes with supply water from chiller and increases supply

water temperature to the cooling coils. Warmer water is unable to meet air handling

unit requirement. Due to this, control valves open more under these conditions. Desirable direction in

decoupler line is from primary supply to primary return. In this case primary water flow exceeds

secondary water flow. When excess water is equal to a chiller capacity, one chiller is taken out of

circuit or destaged. For this purpose flow meter is installed in decoupler line. For chiller staging,

bidirectional flow sensor is used to ascertain direction of flow. When this sensor reads flow from

secondary return to secondary supply, it starts a new chiller. It means there is more demand of

cooling in the building. Size of decoupler line is kept equal to pump header size. It is long enough to

minimize sharp bends before and after flow meter and water flow sensor. Diameter is kept large to

minimize pressure drop through this line

Thomas Hartman, P.E., The Hartman Company

In many large cooling systems, the chilled water distribution system

poses a much more immediate problem to overall cooling system performance and

efficiency.

Last month I outlined an approach, designers and facility managers can use to evaluate the cost

effectiveness of incorporating new "all-variable speed" technologies into new or existing chiller plants. All-variable speed technologies offer substantial energy use reductions and also extend the life of the

plant's chillers while lowering their maintenance requirements. However, in many large cooling

systems, the chilled water distribution system poses a much more immediate problem to overall

cooling system performance and efficiency. Because many chilled water systems fail to attain their

design delta T, loads at the end of the distribution system may be starved at peak periods, while at

the same time the chiller plant is not able to utilize its full design capacity. Many "fixes" worsen the

problem by raising the chilled water supply temperature to loads which reduce their latent cooling

capacity and result in an endless stream of complaints. At some facilities, the inability to solve

nagging distribution problems has undermined the integrity of the entire central plant and new

approaches for cooling are being considered by unhappy end users.

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Truly effective solutions to such problems are relatively

straightforward, and extending all-variable speed principles to the

chilled water distribution system facilitates such solutions. So this

month, I will expand the discussion started last month and outline

how all-variable speed technologies can be most effectively

extended to upgrade chilled water distribution systems for better

performance.

Distribution System Problems

Though chilled water distribution systems vary enormously in size

and configuration, the problems associated with these systems are

quite universal: low delta T, inability to fully load chillers,

inadequate flow in sections of the distribution system, and excessive pumping pressure requirements

at peak cooling demand conditions. Some of these problems plague nearly all chilled water

distribution systems. Figure 1 shows a typical primary-secondary variable flow distribution system. In

smaller systems the primary loop and the secondary distribution pumps may all be located in the

plant. In a large building complex, the primary (or a secondary) loop may extend throughout the

campus and secondary (or tertiary) distribution pumps are typically located in the individual buildings

served by the distribution system. Actual configurations may have more or less distribution circuits

and usually will have multiple pumps at each pumping station. However, the lessons discussed here

are generally scalable and are easy to apply to a wide variety of distribution systems.

Figure 1: Typical Chilled Water Distribution System Configuration.

In Figure 1, the primary chilled water pumps (PCHWP1 - 3) are nearly always constant speed pumpsand the secondary chilled water pumps (SCHWP1 - 3) are variable speed pumps. The primary pumps

are cycled on and off with the chiller each serves, and the speed of the secondary pumps is

modulated to meet a differential pressure setpoint as measured at the end of the distribution circuit

each serves. A decoupling line shown in the lower right end of the figure permits flow in either

direction at the end of the primary circuit since the "stepped" primary flow will nearly always be

different than the continuously variable secondary flow. This system is widely employed, but has two

inherent problems that lead to low delta T and poor performance:

1.

When primary flow is greater than secondary flow, low delta T in the primary circuit results

from the recirculating primary chilled water through the decoupling line and directly back to

the chillers. The lower than expected return chilled water temperature makes it impossible to

Tom's May article - the third in

the series: Optimizing All-

Variable Speed Systems with

Demand Based Control

Tom's March article - the first in

the series: All-Variable Speed

Chilled Water Distribution

Systems: Optimizing Distribution

Efficiency

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fully load the on-line chillers because the primary pumps are fixed flow. This wastes energy

and if it occurs at peak conditions, it robs the plant of capacity.

2.

Whenever secondary flow exceeds primary flow, flow reverses in the decoupling line and is

mixed to degrade the supply water temperature. This reduces the cooling capacity of the

loads in distribution circuits closest to the decoupling line. The result is greatly increased flowin those circuits and reduced delta T, which also robs the system of its full capacity

capabilities.

Because primary and secondary flow is almost never exactly balanced and actual delta T always

varies somewhat from design, one of the two problems is almost always at play in such systems, both

of which can reduce the design delta T of the system and both of which make it difficult to operate

the system effectively at full capacity. A number of solutions have been proposed to correct this

problem, but such "cures" often destroy the system's ability to meet the cooling load requirements.

One popular method of correcting low secondary circuit delta T problems is shown in Figure 2.

Figure 2: Diagram of a Typical Delta T "Enhancement"

While the Figure 2 diagram, or some variation of it, is often touted as a cure for low delta T, it much

more often has disastrous effects on system operation. The idea is that the diverting valve on each

secondary (or tertiary in some cases) circuit return (sometimes a mixing valve is used on the chilled

water supply) will modulate some return water back to the pump anytime the return temperature is

below design. It is reasoned that the elevated supply temperature will raise the return temperature

and ensure that the design delta T from the circuit is maintained at all times. However, this fix rarely

has the desired results. When air is the medium being cooled, return chilled water temperature is

much more affected by entering air temperature than chilled water supply temperature. Raising the

chilled water supply temperature thus has little effect on return chilled water temperature, but it does

profoundly reduce cooling coil capacity, especially latent cooling capacity. As the supply chilled water

temperature rises, load valves open further and flow in the circuit increases dramatically, often

without a significant increase in the return water temperature and usually with a reduction in cooling

effect. Thus, when the scheme shown in Figure 2 is installed on a distribution circuit, one poorly

operating load in the circuit can severely compromise the capacity of all loads in the circuit. In large

systems it is also possible at times to have the flow reversal such that return chilled water from the

mains travels to the supply header through the diverting or mixing valve. Thus the Figure 2 "fix", and

the many schemes that are similar to it, do not fix system operation at all. Instead, it is a "poison pill"

to chilled water distribution systems.

Getting Real About Low Delta T

So what is the solution to low delta T problems? To configure a successful solution we must recognize

what helps and hinders delta T. Delta T problems are sometimes caused by the designs themselves

which may include added bypasses and three way valves scattered through the system to keep water

moving at low load conditions. Solutions that involve mixing return water with supply water


30/33

undermine the thermodynamic efficiency of the system, destroy the capacity of the coils to meet their

loads, and add further to low delta T problems. To solve the types of distribution problems that lead

to low delta T, the design or retrofit needs to follow these rules:

1. Eliminate all possibility of direct mixing between chilled water supply and return:

This means eliminating all decoupling lines and three way valves. In this era of networkedDDC and variable speed control, pumping systems no longer need to be decoupled.

Furthermore, modern chillers accommodate varying flows over substantial ranges without any

loss of efficiency or operational stability. By selecting equipment wisely, it is not difficult to

design "all-variable speed" chilled water generating and distribution systems without any

mixing so that every bit of supply water must pass through a load before returning to the

plant and supply chilled water at design temperature is available to all loads at all times.

2. Employ a direct coupled distribution system: This means that when multiple pumping

circuits are employed such as in the Figure 1 system, they need to be connected directly in

series rather then isolated with the use of decoupling lines. Primary/Secondary systems

become "Primary/Booster" systems in which "all-variable speed" pumping stations are

operated in series. Such systems are extremely effective and can save capital cost when

compared to decoupled Primary/Secondary schemes because Primary/Booster configurations

can incorporate built-in backup without the need for redundant equipment.

3. Focus delta T attention at each and every load: Once decoupling lines and three way

valves have been eliminated, the only source of low delta T problems is overflow through

individual loads. Overflow can occur because of improperly sized valves and varying pressure

differentials across valves. It can also occur when the air side of cooling coils becomes

clogged or other maintenance failures take place. A simple means for preventing overflow is

to install a temperature sensor on the return water line at each load and to use thistemperature as a limit for the control valve serving the load. When the return water

temperature falls to approach the design leaving water temperature for the coil, the valve is

limited from opening further. This step eliminates the problem of low delta T at the load and

gives the designer a little more flexibility in sizing valves for each load. The simple logic that

limits the valve operation can also be employed to notify the operator that a maintenance

problem may be affecting the operation of the load.

Configuring The Solution

Figure 3 shows a system incorporating the above points that ensures every load will be satisfied and

guarantees that the design delta T is maintained at all times.


31/33

Figure 3: "All-Variable Speed" Chilled Water Distribution System Configuration with

Network Controls

Notice how similar Figure 3 is to Figure 1. Because there are no decoupling lines in Figure 3, it is

called an "all-variable speed series Primary/Booster system." Here are how the rules listed above

have been implemented to convert the conventional Primary/Secondary system to a Primary/Booster

and solve the problems typically associated with distribution systems:

1. Eliminate all possibility of direct mixing between chilled water supply and return:

Notice in Figure 3 that the decoupling line in the primary header has been removed and the

primary pumps have been converted to variable speed control. With a DDC network

coordinating the primary and secondary (now called booster) pumps, the pumping systems

no longer need to be decoupled. Modern chillers easily accommodate the varying flows overwide ranges (depending on chiller manufacturer), so varying the flow throughout the entire

system as conditions change works very well. The primary pumps operate with their

respective chillers to maintain a neutral pressure in the primary distribution header as

measured by a differential pressure (DP) sensor shown at the end of the primary distribution

header. Operation of the booster pumps is described below.

2. Employ a direct coupled distribution system: The schematic in Figure 3 is now a series

distribution system because the booster circuit pumps are directly in series with the primary

pumps. In smaller distribution systems, one set of pumps can often be eliminated making the

system a primary only system. In addition to eliminating the possibility of mixing supply with

return chilled water, this direct coupled configuration can save capital cost when compared to

decoupled Primary/Secondary schemes because Primary/Booster configurations

accommodate built-in backup without the need for redundant equipment. Consider that if a

booster pump fails, the primary pumping speed can be adjusted to operate at a higher

pressure and provide some level of pressure differential to any of the booster circuits until the

failed pump can be repaired. Thus, there is often no need for redundancy at the booster

pumping stations.

3. Focus delta T attention at each and every load: This is probably the most important

area of improvement. Consider that in Figure 3 control of the booster pumps has changed. In

primary/secondary systems it is most common to control the pump in accordance with adifferential pressure setpoint. However, when a network control system is employed to


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connect the system with loads served, the network enables a much more efficient and

reliable method of making certain all loads in the circuit are satisfied with a minimum of

pumping power. Network control of the booster pumps eliminates the need to maintain a

fixed static head in the circuit at all times. Instead direct service of the loads calling for

cooling is accomplished with a new network enabled control called "demand based control,"

the details of which will be covered in a later article. There is one other feature of the Figure

3 system that is crucial to this upgrade. Notice that each load in Figure 3 now employs a

temperature sensor on its return chilled water line and that temperature sensor is coupled

with the operation of the valve. This temperature sensor acts as a limit on the valve

operation. Under normal circumstances, the valve is modulated in accordance with

requirements of the space served by the load. However, if the return chilled water

temperature falls to the design return chilled water temperature limit for the load, it acts as a

limit to the operation of the valve such that the return chilled water temperature is not

allowed to fall further.

Other Design Considerations

There are some hydronic issues that must be addressed with large series pumping systems. The

potential for water hammer is increased because without decoupling lines, flow through the entire

system will change if a rapid change of flow occurs through any large load. However, simple steps will

ensure that water hammer will not be a problem. First, the all-variable speed distribution system

shown in Figure 3 should employ electrically actuated modulating valves. Large valves usually employ

90 second to 360 second motors. This means that the valve cannot abruptly change flow to cause

water hammer. If other considerations make water hammer still a possibility, the potential can be

further mitigated by using distributed expansion tanks. With this, a small expansion tank can be

installed at each booster pumping station. When correctly piped, in addition to providing temperature

expansion protection for each booster circuit, the distributed expansion tanks will act as buffers to

absorb potential pressure spikes between the booster circuit and the remainder of the system.

Another potential issue with the configuration shown in Figure 3 is the need to ensure some level of

minimum flow anytime a chiller is operating. Without any decoupling or bypass, the flow will drop to

zero if all the valves close. The simplest solution is to shut the last remaining on-line chiller down

when the flow falls below a predefined minimum threshold. Consider that in many comfort cooling

applications, this low flow condition will be reached when the outside temperature is very close to the

point at which outside air economizers alone can provide the supply air temperature setpoint. By

shutting down the mechanical cooling, the supply temperature may rise slightly, requiring some

additional fan power to meet the cooling load, but the overall system may still operate much moreefficiently than by keeping a nearly unloaded chiller on line.

If chiller operation at low loads is necessary, then it is a simple matter to add a small bypass valve in

the primary circuit that is normally closed but opens at low flow conditions to maintain a minimum

flow rate.

Evaluation Costs and Savings for an All-Variable Speed Upgrade

While the savings that can be expected from upgrading an existing decoupled primary - secondary

distribution system to an all-variable speed primary - booster system may not be eye catching, the

costs for such an upgrade are also often quite modest. Adding variable speed drives to the constant

speed primary pumps, closing bypasses and implementing new network control can be accomplished

quite economically in many systems. So, even though the energy economics may not be enormously


33/33

attractive at first blush, the potential benefits to the overall cooling system are. The main driving

forces for retrofitting to an all-variable speed distribution system are:

1.

The desire to recapture chiller plant capacity that is being lost to low delta T problems, or

2.

The desire to correct distribution flow problems resulting from the low delta T that make it

difficult to properly cool some areas of the facility under certain conditions.

These are big problems for some facilities, and converting to an all-variable speed primary/booster

chilled water distribution system can usually lead to enormous improvements at relative low costs.

However, such a conversion needs to be very carefully analyzed and designed to be sure the retrofit

does not introduce new hydraulic problems into the system and that the chillers and other plant

equipment maintain adequate minimum chilled water flow at all times. Cost is not generally a major

consideration for a primary/booster upgrade, but a careful design process should be!

Summary & Conclusion

Low delta T problems are very widely experienced by chilled water distribution systems in operation

today. Many of the fixes that have been suggested to mitigate low delta T offer no solution at all,

only more problems. However, reconfiguring such chilled water distribution systems as

primary/booster "all-variable speed" systems without decoupling lines and with return chilled water

temperature limits on each load will absolutely guarantee an end to low delta T problems.

Furthermore, such a system can alert operators to potential problems at loads that are under

performing so that these problems can be corrected before they adversely affect the comfort of the

spaces served. With a carefully developed design, an economical upgrade is often achievable that will

greatly improve overall cooling system performance.

Additional information on technologies discussed in this article is available at www.hartmanco.com .

Comments and questions may be addressed to Mr. Hartman at [email protected] .

References

1. The Hartman Company, 2001, "The Hartman LOOP Chiller Plant Design and Operating

Technologies: Frequently Asked Questions," March

2. Hartman, T, 2001, "Getting Real About Low Delta T in Variable-Flow Distribution Systems,"

HPAC Engineering April.

3. Hartman, T., 1996, "Design Issues of Variable Chilled Water Flow-Through Chillers," ASHRAE

Transactions, June.

4. Hartman, T.B. 1999, "Network Based Control of Fluid Distribution Systems," Renewable And

Advanced Energy Systems For the 21st Century, Lahaina, Hawaii.5. Kirsner, W., 1996, "The Demise of the Primary-Secondary Pumping Paradigm for Chilled

Water Plant Design," Heating/Piping/Air Conditioning (HPAC) November.
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