Degrading Chilled Water Plant Delta-T: Causes and …

1 THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY, FOR INCLUSION IN ASHRAE TRANSACTIONS 2002, V. 108, Pt. 1. Not to be reprinted in whole or inpart without written permission of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA , findings, conclusions, or recommendations expressed in this paper are those of the author(s) and do not necessarily reflect the views of ASHRAE. Writtenquestions and comments regarding this paper should be received at ASHRAE no later than January 25, Chilled Water plants are designed to main-tain a relatively constant delta -T, the difference between returnand supply Chilled Water temperature. But in almost every realchiller Plant , delta -T falls well short of design levels. The resultis that flow and load do not track, usually requiring that addi-tional chillers be brought on line to maintain flow requirementseven though none of the chillers is fully loaded.

2 Both pumpenergy and chiller energy increase accordingly. Many designand retrofit measures have been tried to resolve the problem,but they are sometimes expensive and not always this paper, the author argues that while many Causes ofdegrading delta -T may be eliminated, in most plants it is notpossible to avoid Degrading delta -T under all operating condi-tions. Several design and operational techniques are presentedboth to minimize Degrading delta -T and to design plants to beefficient despite Degrading most variable-flow Chilled Water plants , it is assumedthat delta -T, the difference between return and supply chilledwater temperature, will remain relatively constant. Becausethe load is directly proportional to flow rate and delta -T (Equa-tion 1), if the delta -T is constant, it follows that flow rate mustvary proportionally with the load.

3 Most variable-flow systemsare designed based on this assumption and usually fail toperform well if the delta -T does not stay relatively constant.(1)In almost every real chiller Plant , delta -T falls well shortof design levels, particularly at low loads. The result is higherpump and chiller energy usage. Many papers have been writ-ten on the subject of low delta -T syndrome (Kirsner 1996,1995; Lizardos 1994; Sauer 1989; Fiorino 1996; Avery 1997;Mannion 1988). Most are oriented toward how to keep delta -T high. This paper also will address Causes of Degrading delta -T along with mitigation measures, but it goes on to show whydelta-T degradation will almost always occur in Chilled watersystems and how to design around that eventuality to maintainchiller Plant efficiency despite Degrading ENERGY IMPACT OF Degrading delta -T Figure 1 shows a hypothetical chiller Plant servingseveral buildings in a large facility, such as a universitycampus, office complex, district cooling system, or industrialfacility.

4 The system is piped in a traditional primary-second-ary manner with some tertiary pumps at remote buildings. If the delta -T in this system is low, at least two problemsresult: increased pump energy usage and either an increase inchiller energy usage or a failure to meet cooling loads. The increase in pump energy is obvious. According toEquation 1, any reduction in delta -T must cause a proportionalincrease in Chilled Water flow rate. Pump energy, theoretically,is proportional to the cube of the flow rate, so any increase inflow will have a much higher increase in pump energy. In realQm cpT =QBtu h ()500 GPM T (IP units) =QkW()LPS T (SI units) = Degrading Chilled Water Plant delta -T: Causes and MitigationSteven T. Taylor, ASHRAES teven T. Taylor is a principal at Taylor Engineering, Alameda, , actual pump energy impact will be less than this theo-retical relationship suggests,1 but the impact is significantnonetheless.

5 The impact on chiller energy usage is more complex todetermine and will be a function of how the chillers arecontrolled. There are two basic chiller start/stop control strat-egies, one based on system flow rate and the other based onload. Ideally, the two strategies would be effectively the samesince flow and load should track in a variable-flow , when flow and load do not track, when delta -T falls,neither strategy works flow-based strategies stage chillers and primarychilled Water pumps in an attempt to keep the primary systemflow larger than the secondary system flow. In this way, thesecondary supply Water temperature is equal to the primarywater temperature leaving the chillers. Flow is often sensed inthe common leg or in the primary and secondary supplies fromwhich flow in the common leg can be deduced.

6 When flow inthe secondary exceeds the primary, as indicated by flow in thecommon leg moving from the secondary return toward thesecondary pumps, another primary Chilled Water pump andchiller are started. A pump and chiller are shut off when flowin the common leg exceeds that of one pump, with some addi-tional margin to prevent short load-based strategy measures system load or indirectindications of load such as return Water temperature. Chillersare started when the operating chillers are operating at theirmaximum capacity. Chillers are stopped when the measuredload is less than the operating capacity by the capacity of onechiller. So what happens when delta -T falls below design levelsand flow and load are no longer in synch? The flow-based control system will always make sureloads are met by starting additional chillers and pumps to keepthe primary system flow larger than the secondary flow.

7 Butthis means that chillers are not fully loaded when delta -T isbelow design. For example, assume the system was sized fora 14 F delta -T on both the primary and secondary sides. If thesystem were at 50% load but the actual delta -T was only 7 F,all the chillers and primary pumps in the Plant would have tooperate to keep the primary flow up. This wastes pump energyand chiller energy since the chillers would all be operating at50% of capacity, less than the 65% to 85% range where effi-ciency is typically maximized for fixed-speed load-based control system would not start a newchiller until the operating chillers were loaded. As delta -Tdegrades, secondary flow increases, causing Water in cubic relationship between pump energy and flow assumesthat pressure drop varies as the square of fluid velocity, anassumption valid only for fully developed turbulent flow insystems with fixed geometry.

8 In real systems, pressure drop inmost system elements varies less that this since flow is not fullyturbulent; velocities are more typically in the transitional regionbetween turbulent and laminar flow at design load conditions andin the laminar flow regime at low loads. Control valves open andclose, changing system geometry and, hence, its flow character-istics. Most variable-speed pumping systems maintain a mini-mum differential pressure setpoint, further changing the pressuredrop/flow relationship. Finally, motor efficiency and (to a lesserextent) variable-speed drive efficiency drop off at lower loads. Allof these factors combine to make part-load pump energy savingsless than what may be expected from the ideal cube-law 1 Typical Chilled Water Plant and distribution leg to flow from the secondary return back into thesecondary pumps.

9 This Causes the secondary supply watertemperature to rise, which in turn Causes coil performance todegrade, which in turn Causes control valves to open more todemand more flow, which in turn Causes ever increasing flowin the secondary and ever warmer supply Water , coils will starve, their control valves will be wideopen, and temperature control is lost. The system controllingchiller staging would be oblivious to these problems; it wouldnot start more pumps and chillers since the operating chillerswere not fully solution to these problems lies in first maximizingdelta-T as much as possible but then designing the Plant toaccommodate the low delta -Ts that will inevitably occur. Degrading delta -T: Causes AND MITIGATIONThe Causes of Degrading delta -T can be broken into threecategories: Causes that can be avoided by proper design or opera-tion of the Chilled Water system Causes that can be mitigated or resolved but throughmeasures that may not result in overall energy savings Causes that are inevitable and simply cannot be avoided Causes THAT CAN BE AVOIDEDI mproper Setpoint or Controls CalibrationProbably the most common cause of low delta -T isimproper setpoints on controllers controlling supply airtemperature off of cooling coils such as those in VAV systemsand other central fan systems.

10 When the setpoint of a coolingcoil is set too low, the controller Causes the Chilled Water valveto open fully since it is unable to attain the setpoint no matterhow much Chilled Water flows through the coil. Table 1 showshow even a modest drop in supply air temperature setpointfrom 54 F to 51 F can cause coil flow rate to more than doubleand delta -T to drop in half (see also example in Kirsner 1995).Setpoints are often adjusted downward by building engineerstrying to resolve a comfort problem quickly, although this isseldom the real source of the problem. In the author s experi-ence, coils are seldom undersized. Cooling shortages are moretypically due to undersized zone terminal boxes or ductworkto those boxes and to the inability of the supply fan to over-come the resulting high pressure drop to these similar effect can be caused by improper sensor orcontroller calibration.