Design Issues of Variable Chilled-Water Flow …

1 SA-96-12-2 Design Issues of Variable chilled -WaterFlow Through ChillersThomas B. Hartman, ASHRAEABSTRACTV ariable-speed alternating current (AC) drive technolo-gies are of particular interestfor heating, ventilating, and air-conditioning (HVAC) designs because controlling motorspeed with Variable -frequency AC drives to achieveflow mod-ulation provides an opportunity to capture exceptionally highpart-load operating efficiencies. Since HVAC systems spendlong hours operating at part-load conditions, improvement inpart-load efficiency results in substantial energy Variable flow to Chilled-Water systems is particularlyattractive because Chilled-Water pumping has two associatedpower costs, directly as pumping power and also as a load onthe chiller plant.

2 The small temperature differentials associ-ated with Chilled-Water systems mean load-side stratificationis generally not a concern. but the small temperature differen-tials do raise concerns about heat transfer at reduced , typical Variable -flow Chilled-Water systems aredesigned with two Chilled-Water circuits: primary and second-ary. The primary circuit is usually a low-head circuit thatmaintains a constant Chilled-Water flow through the chiller,while the secondary Chilled-Water pump(s) provide variableflow to the loads based on their demand for cooling.

3 Becausethe primary circuit is low head and requires relatively lowpower, it is often reasoned that two-circuit configurationsinvolve only a small pumping energy penalty and only whenoperating at low loads. However, closer analysis uncovers thefollowing true first-cost penalty for employing two separate pumps,.a part-load chiller efficiency penalty from mixing bypassedsupply chilled water with the return chilled water , chiller capacity penalty of underutilizing the full chillercapacity during high cooling demands at conditions notprecisely congruent to the Design peak many building cooling applications, it is possible todesign a Chilled-Water supply and distribution system with onlya single Variable -flow circuit.

4 Such designs can avoid the prob-lems listed here. However; there are potential pitfalls that mustbe considered before such a system can be successful. This paperdiscusses the benefits and problems associated with a single-circuit Variable - Chilled-Water -ji ow system and offers a chillerplant control strategy that can provide safe, stable, and reliablechiller operation over the entire operating range employed intypical HVAC Chilled-Water plant Design utilizing variablechilled- water flow involves primary/secondary loops with sepa-rate pumps, as shown in Figure 1.

5 Typically, one low-headprimary loop pump for each chiller in the primary circuit providesaconstant flow through the chiller, while one or more higher headvariable-flow secondary loop pumps modulate to adjust second-ary Chilled-Water flow to meet actual cooling demand. The imbal-ance in flow between the primary and secondary circuits resultsin flow through the bypass piping circuit. While this configura-tion satisfies the objective of maintaining constant chilled -waterflow through thechiller, it may not achieve the highest chiller effi-ciency at part loads and can limit chiller capacity due to the factthat under almost all operating conditions, the flows in the twoloops are not achieve the full potential of Variable Chilled-Water flowin an environment of integrated HVAC equipment operatingunder high-performance control strategies, it is necessary torethink the physical configuration of Variable -flow chilled -watersystems.

6 Recent work (Hartman 1993) has shown that integratedcontrol strategies can be employed to operate Variable -flowchilled-waterdistribution systems at much higher efficiencies bycoordinating the pump speed directly to the load demands with-out employing pressure control. It is prudent also to analyze theThomas B. Hartman is a principal of The Hartman Company, Marysville, CHILLER j7 ConstantSpeedPrimaryPumpFigure I Primary/secondary Chilled-Water of the primary circuit before a particular configurationand control scheme is striking feature of Figure 1 is the necessity of havingtwo separately powered Chilled-Water circuits.

7 Designers shouldask themselves whether this is really necessary. The constant-flow primary circuit has become accepted Design practicebecause it is well known that below certain velocities of flowthrough heat exchangers, a switch to laminar flow may causesudden substantial reductions in heat transfer capacity. Thepurpose of the primary pump is to ensure such a condition nevertroubles the system. However, at very low cooling capacityrequirements, the heat transfer requirements are also greatlyreduced, and by monitoring the chiller load, chilled -watertemperature, and refrigerant temperatures, a properly integratedcontrol system can easily adjust the overall system operation ifwater flow becomes too low to provide efficient heat transfer ormay cause the chiller to approach operating limits If the controlsystem is operating with suitable high-performance control algo-rithms.

8 It can promptly make the necessary corrections to ensureefficient and stable operation of the entire system at all loadconditions. With this in mind, consider the simpler pipingconfiguration in Figure Figure 2, the chiller itself may be a Variable -speed unit,but in any case, it is one that offers a high turndown ratio and anVariable SpeedChilled water -lLOAD 41 LOAD 1- LOAD 2 --&I---- LOAD 3 --&I---wFigure 2 Single-circuit Variable -flow flow Chilled-Water coefficient of performance (COP) as the cooling loadis reduced. The required rate of flow through the chiller dependson the cooling load being delivered.

9 This is a good Design fitbecause the loads are connected with two-way valves such thatload-side flow also varies with load. In such a scheme, both thechilled- water flow and chiller capacity are adjusted to effectivelymeet all load conditions. A threshold cooling capacity limit isdefined below which the system does not operate, just as is thecase with present chiller systems. The potential benefits of asingle-circuit Variable -flow chiller system as shown in Figure 2are:.lower first cost and lower maintenance costs,.higher overall chiller plant operating efficiencies, flexibility in utilizing full chiller capacity at discussing these benefits in detail, let us consider thecritical Issues of such a Variable -FLOWSYSTEM CONSIDERATIONSC onfiguring a Variable -flow Chilled-Water system as shownin Figure 2 does not mean it will work adequately under all loadconditions without specific attention to the Chilled-Water flowover the wide range of potential operating conditions.

10 To ensureeffective and efficient operation of the Figure 2 configuration,several basic requirements must be met. First, the system mustnot be permitted to operate unless the cooling requirement isabove a minimum threshold load. The threshold cooling loadrequirement is the lowest stable chiller operating SA-96- 12-2 Next, the water flow through the chiller evaporator heatexchanger must always be sufficient to maintain evaporatortemperature within suitable limits.