CHILLER PLANTCONTROL

MULTIPLE CHILLER CONTROLS

By: Michael J. Bitondo,Mark J. TozziCarrier CorporationSyracuse, New York

August 1999

INTRODUCTION

In December of 1998, the American Refrigeration Institute(ARI) released a revised standard for water cooled chillers –ARI 550/590-98. One of the major changes in the stan-dard was made to the Integrated Part Load Value formula,or IPLV. The IPLV is a calculation of predicted chiller effi-ciency at the ARI Standard Rating Point. This efficiencynumber is an estimate of how efficiently a chiller will oper-ate at part load conditions, based on average criteria dictat-ed by the standard.

The revisions to the IPLV equation were designed to makeit a more accurate representation of actual field operatingconditions, such as geographic locations and buildingtypes. However, because the many assumptions in the for-mula cannot exactly match any one particular chiller instal-lation, it is still not the most accurate way to simulate anactual chiller system. In fact, the ARI Standard 550/590-98 white paper, published in ASHRAE Journal (the maga-zine of the American Society of Heating, Refrigeration andAir Conditioning Engineers) states:

“Because IPLV represents an average single chillerapplication it may not be representative of a partic-ular job installation. It is best to use a comprehen-sive analysis that reflects the actual weather data,building load characteristics, number of chillers,operational hours, economizer capabilities, andenergy drawn from auxiliaries such as pumps andcooling towers, when calculating the overall chillerplant efficiency.”

It is estimated that 86% of chillers are installed in sometype of multiple chiller application. It is therefore impor-tant to understand how typical chiller systems will operateas a whole, particularly since many engineers will onlyassume the evaluation of a single machine, even thoughthere are multiple machines in the system. Chillers willoperate very differently when placed in a system, asopposed to a single chiller application. This paper willexplain how typical multiple chiller systems operate andhow they are controlled.

When using multiple chillers to maintain building loadconditions, proper controls are critical to meet constantlychanging building requirements. The first step is to deter-mine the type of chilled water system needed to meet thebuilding load requirements. A chilled water control systemshould be provided, to both supervise and optimize theoperation of the chilled water plant. All elements of theplant must be considered, e.g., cooling towers, pumps, vari-able frequency drives (VFDs), heat exchangers and the con-trol valves used on the building’s air handlers.

CHILLED WATER SYSTEMSTWO CHILLERS – EQUAL TONNAGE

For the purpose of this discussion, we will examine twotypes of common chilled water applications.

Let’s first examine an application using two chillers of equaltonnage. In this application, each machine is designed,when it is operating at 100% capacity, to maintain 50% ofthe total building load. There is a primary and a secondarychilled water loop with a hydronic decoupler. The second-ary chilled water pumps are equipped with VFDs, to main-tain a differential pressure across the supply and return ofthe system; and two-way control valves are used on the loadside. Figure 1 shows a diagram of this system.

When the building load dictates a need for cooling, theplant control first enables one chiller, referred to as the leadchiller. This chiller has a pulldown timer program to allowit to cool the supply water before starting the second – orlag – machine. The lead chiller continues to ramp up tomeet the requirements of the load. As it reaches its fullcapacity, and building load is approximately 50% (based onsystem supply water temperature, return water tempera-ture, delta temperature or kW%), the chilled water plantcontrol system ramps the lead chiller down and enables thelag chiller. Ramping down the lead chiller before startingthe lag machine helps to avoid demand charges that canoccur when operating one chiller at full capacity whileenabling another. From this point on, if the building loadincreases, the two chillers ramp up together as a system, tomeet the building demand.

A load prediction calculation is incorporated into the plantcontrol to determine when the lag chiller can be disabled.This control routine calculates a reduced cooling capacitykW setpoint based on present chiller tonnage, capacity ofthe lag chiller to be stopped, and adjustable deadband toprevent short cycling.

In addition to the chillers themselves, the chilled waterplant control system must also control the chilled waterpumps (primary and secondary), condenser water pumps,cooling tower fans, and any other devices in the system,such as bypass valves and VFDs.

Typical control of the primary chilled water pumps (seeFigure 1) is as follows: the lead chilled water pump is start-ed when a call for cooling is received. As the building loadincreases and there is an additional call for cooling, the lagpump is enabled. It is also common practice to have abackup chilled water pump, in case either the lead or thelag pump fails.

Typical control of the primary condenser water pumps (seeFigure 1) is as follows: the lead condenser water pump isstarted when a call for cooling is received. As the buildingload increases and there is an additional call for cooling, thelag pump is enabled. It is also common practice to have abackup condenser water pump, in case either the lead orthe lag pump fails.

Looking at Figure 1, let’s assume the building load has two-way valves on the cooling coils. In order to maintain anacceptable differential pressure of the secondary water sys-tem, there are variable speed drives for each secondarywater pump. The speed of the pumps are controlled tomaintain a system differential water pressure as sensed bytransmitter(s) located at the end of the loop. When the sys-tem is in operation the lead pump will be enabled. Thispump will ramp up to its full speed as dictated by the sys-tem. When this pump reaches its full output speed, after atime delay, the lag pump will be enabled. The lag pumpwill ramp up and follow the lead pump to maintain the system differential.

THREE CHILLERS – TWO OF EQUAL TONNAGE,ONE OF LESS TONNAGE

Next, let’s discuss an application using two chillers of equaltonnage and a third chiller of less tonnage. The two largermachines are each sized to handle 40% of the total build-ing load. The third, smaller machine is sized for 20% of thebuilding load, and is used as the lead chiller (see Figure 2).

When the building load dictates a need for cooling, thesmallest tonnage chiller is enabled. As the building loadincreases above 20% of total building load, the first lagmachine is enabled. As this machine ramps up, the leadmachine (smallest tonnage) is disabled. If the load increas-es to greater than 40% of total building load, the other lagmachine is then enabled. The lead machine ramps downand ramps back up in conjunction with the lag machine(s).If the building load increases to greater than 80% of totalbuilding load, the lead machine is re-enabled to meet thebuilding load requirements.

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CONDENSER WATER SYSTEMS ANDCONTROLS

The condenser water system differs from the evapora-tor (or, cooler) system in a number of ways. The con-denser is an open-loop system, while the evaporator isclosed-loop. In addition, the condenser system is typ-ically constant flow, while the cooler loop may be variable flow.

Typically, the condenser system functions as follows:the chiller requests that the condenser pump and

cooling tower become active. If the chiller isequipped with an isolation valve, the position of thevalve (open/closed) needs to be verified before startingthe pump. Once flow has been established, and veri-fied by a differential pressure (or flow) switch, and allother safety conditions are satisfied, the chiller willstart. As the chiller loads up, the heat of the refriger-ation cycle will be rejected to the cooling tower. Asthe water continues to increase in temperature, thecooling tower fans are cycled on, to maintain thedesired temperature setpoint.

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As the condenser water temperature is reduced, thechiller has less work to perform and therefore, energyusage is reduced, increasing efficiency. A general ruleis: for every one degree drop in condenser water tem-perature, chiller efficiency will increase 2%. Whendecreasing condenser water temperature, minimum“lift” must be maintained. Lift is the amount of pres-sure differential required to get the refrigerant to flowfrom the cooler compressed by the compressor andinto the condenser. Insufficient lift results in therefrigerant “stacking up” in the cooler; excessive liftcauses the compressor to surge.

The cooling tower is limited in the amount of heat itcan reject. This is based on the design of the coolingtower and the outside wet bulb temperature – the dif-ference between these two variables is called the cool-ing tower “approach.” Typically, cooling towers canreduce the water temperature to within seven degreesof the wet bulb.

To optimize the efficiency and reduce the overall ener-gy costs of the condenser system, the following stepsshould be taken:

• Determine the lift requirements of the chiller. Thiswill dictate the lowest condenser water temperatureat which the chiller can operate.

• Determine the cooling tower design and approach.For a new installation, consider a larger tower and/orincreasing flow, to reduce the approach factor.

• Install a direct digital control (DDC) system to calculate and control the cooling tower and fans.

CONCLUSION

Understanding how multiple chillers interact andwork together in a chilled water system is critical foranyone involved in designing, specifying or purchas-ing chiller-based HVAC systems. Knowledge of theappropriate number and tonnage of chillers, as well ashow condenser systems and controls work, is a keyfactor in arriving at the best solution for a given application.

In addition, to thoroughly understand an application,a comprehensive review of the many factors con-tributing to chiller efficiency should be considered.These include geographic and climate conditions,building load characteristics, anticipated operationalhours, economizer capabilities and predicted energydrawn from auxiliaries such as pumps and coolingtowers. The Integrated Part Load Value (IPLV) for-mula, while helpful as a guideline, should not be reliedon to accurately represent a particular, multiple-chillerinstallation.

SOURCES

Mark J. Tozzi, Product Manager, Systems GroupCommercial Systems and ServicesCarrier CorporationPhone: 315-433-4910E-mail: mark.tozzi@carrier.utc.com

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