Accomplish Maximum Green Results in Commercial Buildings #7 9.8.10

Nearly everyone agrees that multiple small boiler systems provide optimum efficiency when compared to a single larger boiler system in commercial buildings. These savings are often enhanced by pumping systems that vary

system flow to meet load demand. For maximum energy saving results primary/secondary pumping is the right choice, as explained below:

1) To achieve optimal energy savings, lowest seasonal energy consumption and most precise load tracking primary/secondary pumping should be selected in lieu

of primary loop systems only. With primary/secondary pumping the boilers not operating to meet the immediate load will not be consuming or losing energy. Because the system load is rarely at maximum, one or more boilers will be in

standby mode most of the time. 2) Standby flue and jacket losses occur anytime a boiler has heated water

flowing through it, as is normally the case in primary loop systems. This is true for all types of boilers. Heat losses are virtually eliminated in standby boilers utilizing primary/secondary pumping. A significant additional savings is in

electrical operating energy. Standby boilers not only save heat energy losses, they consume minimal electrical power. This is not the case with primary loop systems that typically pump water continuously through all boilers.

3) Most boiler manufacturers that promote primary loop pumping have boilers that contain relatively high water volumes. The only way to stop these continuous standby and electrical loss is to change to a primary/secondary

system, or stay with the total primary loop design and isolate each boiler. Then it can be shut off when not needed. Isolation, if accomplished, is done by adding costly, (spring loaded) often problematic, automatic water valves for

every boiler. These typically cost $5,000 or more per boiler. Primary/secondary

boilers are self-isolating, so this additional first cost and ongoing

maintenance cost is completely eliminated.

4) On the rare occasion when primary loop systems boilers are isolated as described above, the stored Btus contained in the water will still be lost on every

off cycle. The stored water content of high mass low recovery boilers is in the order of 30 to 40 times that of low mass high recovery boilers. The heat contained in the stored water is typically 500 Btus per gallon. A one million Btu

low mass boiler may contain only one gallon where as a high mass boiler may hold 30 or more gallons. Thus the low mass boiler will contain only 500 Btu’s and the high mass will contain 15,000 (30 x 500 = 15,000) which is lost each time it

goes into standby.

5) With both primary/secondary and full primary loop pumping, the system pump runs any time there is a building demand, often 24/7 during the heating

season. For primary/secondary systems, the system pump is sized for the system loop only. One does not need to upsize that pump for the pressure drop

in the boilers and boiler piping. That is handled by the individual boiler pumps. The primary system pump will be the only one running continuously. The secondary pumps run only when their specific boiler is called on to fire. This

reduces total first cost as well as operating cost.

6) When a full primary loop design is selected, the system pump will always be providing flow through all the boilers, including those on standby. This allows a

manufacturer to specify extremely low and impractical flow turndown if they wish, even to the point of “Zero flow capability”. This is not really

possible or recommended for use in any legitimate manufacturer’s instructions, including those who get it specified. It should be pointed out that equal or greater turn down can be achieved by utilizing primary/secondary systems. The

variable volume system pump can operate at the lower flow rates associated with only the boilers currently on line. As an example, two Raypak XTherms are capabable of 24:1 flow turndown, or 36 with 3, 48 with 4, etc. Who needs

more?

7) Because each boiler is pumped separately, with primary/secondary pumping, boilers in standby mode can be isolated from the system for routine

maintenance, service or repair without interrupting the operation of the rest of the heating system

8) To further illustrate the various savings attributable to primary/secondary

pumping systems, an actual specified design that was recently converted from primary loop to primary/secondary is summarized below. The potential savings in both first cost and continued operation of the primary/secondary design were

recognized. The savings calculation was based on lower hardware cost and bonus future energy savings, without even considering the savings from eliminating isolation valves. They weren’t specified on this job. First and

operating savings justified any piping drawing alterations needed, and the system selection became the optimum --- Primany/Secondary Pumping with

Raypak High Recovery Boilers.

Example: The initial design specified two high mass boilers plumbed in parallel. The

specified primary only system pump was 5 horsepower. Each boiler was to be fitted with costly automatic isolation valves that, along with the boilers, added to the total system pressure drop. Even with the isolation valves the stored energy

in the heated water in the boiler is lost each time the boiler is shut down and isolated..

By going to low mass high recover copper finned tube boilers with primary/secondary pumping the system pump no longer needed to pump

through the boilers and isolation valves. This resulted in a reduction for required system pump to 3 horsepower. Each of the two high recovery boilers incorporated a ½ horsepower boiler pump and a 1/8 horsepower variable speed

injector pump. The injector pump typically runs at a reduced speed and using less than 1/8th horsepower. The ½ plus 1/8 horsepower pumps add up to 5/8

horsepower capacity per boiler. The revised design pumping requirements then become 3 horsepower for the

system and 2 sets of pumps at 5/8 horsepower per set for a combined total of 3 + (2 X 5/8) = 4 1/4 horsepower versus 5 for the original design. So even at peak load the pumping horsepower is reduced from 5 to 4 ¼. Keep in mind that

for most of the season only one boiler will be required with the associated pumping capacity of 3 + 5/8 = 3 5/8 verses 5 with the original design.

Additionally high recovery low mass vertical boilers substituted are certified for 94% or more thermal efficiency versus 92% for the originally specified high mass boilers.

The revised design significantly reduced first cost from the original projections. Continuing operational energy savings will accrue each year in electrical energy

from reduced pump horsepower. There is also, along with the electrical savings, less heat energy loss from stored water each time a boiler shuts down and is isolated from the system. The higher thermal efficiency of the copper finned tube

boilers is a green bonus.

This is just one real world example. Larger systems with more boilers will demonstrate even greater savings. The two drawings on the following attachment show the original piping and pumping design and the revised

primary/secondary pumping design.

SThurlkill

Polygon