Impact of control mode and circuit type on return water temperature in cooling
Chiller efficiency is usually indicated by the Energy Efficiency Ratio (EER). In order to keep the EER of a chiller as high as possible at partial load, a key element is to avoid a degradation of the log mean temperature difference between the chilled water and the refrigerant. With a constant chilled water supply temperature as generally considered in cooling, this means that one should avoid all causes of decrease of the chilled water return temperature at partial load. As an indication of the importance of this point, the results of a simulation performed on a chiller manufacturer simulator for a chiller of 703 kiloWatts with water condenser temperatures of 29.5-35°C and chilled supply water temperature of 7°C indicates a 15% drop of the EER when the chilled return water temperature drops from 12.5°C to 10.5°C.
In order to determine the evolution of the return water temperature at partial load, one must look at what happens on the terminal unit side.
Let us first consider the case of proportional control on a variable flow circuit with fan-coil unit equipped with a twoway control valve and assumed to be properly balanced (figure 1a). Since the flow is progressively decreased at reduced load, the temperature difference through the unit (and thus through the circuit) regularly increases as displayed by the red
curve in figure 2 for a 7-12°C temperature regime. Hence, a stable and accurate proportional control of the cooling output of a terminal unit with a variable flow circuit benefits to the chiller EER.
Fig. 1 (a) Two-way variable flow circuit; (b) Three-way diverting circuit
Let us now turn our attention to 3-way diverting circuit also with proportional control (figure 1b). Such a circuit is often used in variable flow systems at the end of branches to maintain a minimum flow for the pump but also to avoid warming up of the supply water due to heat gains in the piping. With such a circuit, the evolution of the temperature difference through the unit at partial load is the same as for the 2-way circuit. However, when the 3-way control valve progressively closes, there is an increasing amount of flow that is bypassed and that cools down the global return of the circuit as
illustrated by the blue curve in figure 2. It is therefore clear that the use of this circuit should be kept to the strict minimum to ensure required minimum flow since it systematically deteriorates the return water temperature.
Fig. 2 Return water temperature evolution at reduced load with proportional control
for a 7-12°C design temperature regime and a room temperature set-point of 24°C
In the above cases, stable and accurate proportional control has been assumed. For cost saving reasons, it happens regularly that on-off control is preferred. It also happens regularly that proportional control deteriorates due to incorrect valve sizing leading to an uncontrollable on-off type of behaviour. Let us consider thus the two circuits of figure 1 with onoff control.
Part load conditions in a plant uniformly equipped with on-off control circuits can be monitored by counting the number of units that are 'on' at one moment in time. At 50% load, we should have in average 50% of the units 'on' and 50% of the units 'off'.
If all circuits are 2-way on-off circuits, when some circuits are 'off', there is less total flow and the pressure drops in the piping decrease with the square of the flow decrease. There is therefore higher available differential pressure at all points in the system, resulting in higher flow than per design in the circuits that are 'on'. Due to the nonlinear heat output characteristic of terminal units, the heat output of the units only increases moderately with the flow above design flow . Thus, with a higher flow than per design and a heat output that does not increase much, the temperature difference through units decreases at partial load with an on-off control system applied to 2-way circuits. Figure 3 depicts this effect for a model case. It can be observed for this model case approximately a 2°C drop of return water temperature which will
affect the chiller EER as discussed above. If all circuits are 3-way on-off circuits, when some circuits are 'off', the flow is bypassed in these circuits so that the total flow in the plant does not change. However, as flows are bypassed in the proportion of the number of units that are 'off', the return water temperature decreases linearly with the load in the system (see blue curve in figure 3).
Fig. 3 Return water temperature evolution at reduced load with on-off control
for a 7-12°C design temperature regime and a room temperature set-point of 24°C.
(Model case with 100 identical units, pump head of 150 kPa and 20 kPa in terminal units)
It is thus clear that amongst the different considered circuits, the 2-way variable flow circuit with proportional control
should be the preferred circuit provided that stable and accurate control is ensured by proper selection and sizing of the
control valves. Interestingly, this issue has been addressed in the renovation of the cooling system of two buildings of Hong-Kong Polytechnic University . Specifically, differential pressure controllers have been added at the inlet of branches of on-off controlled fan-coil units and across control valves of air handling units to prevent flow increase at partial load in fan-coil units and guarantee stable control of air handling units. The graph presented in figure 4 displays the measurements performed before and after the renovation work delivering a 16.5% reduction of chiller annual energy consumption.
Fig. 4 Measurements before and after application of differential pressure controllers on branches of on-off controlled fan-coil units and across control
valves of air-handling units