An alternative test procedure and mixing strategy

Chapter 3: STANDARDS, MEASUREMENTS AND

H. An alternative test procedure and mixing strategy

H1. Why bother with alternative test rig designs?

In this section, we describe a test facility which not only can provide the stringent requirements of constant coolant temperatures and flow rates to the evaporator and condenser, but also is economical to op- erate in terms of equipment costs and electricity consumption. This facility

Hot End Cold End To condenser

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cond, incond From condenser

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cond, outcond

To evaporator

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evap, inevap

From evaporator

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evap, outevap

Bypass

To Cooling Tower

From Cooling Tower

Mechanochemistry of Materials

Cool Thermodynamics

dispenses with the need for the conventional storage of large volumes of hot and cold water at the required rating conditions prior to the com- mencement of a test run. A second advantage is that our alternative test rig allows for faster turnaround times for a given rating test. We’ll delineate the operation of a water-to-water cooled chiller, but the prin- ciples involved can easily be extended to other cooling media, as well as to other types of chillers and heat pumps. Furthermore, the facility described here is not merely a proposal; it has been built, tested and verified as an accurate, inexpensive alternative to conventional chiller test installations [Bong et al 1989, Bong et al 1990].

H2. The basic idea for simplifying the procedure

A mixing tank is used to produce the required temperatures at the en- trance to the evaporator and condenser of the test unit (chiller or heat pump, as in Figures 3.6 and 3.7). The essence of this procedure is to neutralize the heating and cooling capacities that are produced by the condenser and evaporator of the test unit. The mixing of these streams is performed in a manner such that only the desired temperature lev- els are returned to the evaporator and the condenser. Based on temperature measurements at pre-selected locations, the mixing processes are executed by computer-controlled modulating valves using a simple Proportional-Integral-Differential control strategy. For effective control of extreme conditions, as required by application ratings, the mixing tank is designed with bypass pipelines so that part of the chilled water and/or warm water can circumvent the mixing process.

H3. The mixing process for a chiller

One end of the mixing tank is cool and the other is warm. The cool and warm ends are where water returns from the evaporator and condenser, respectively. Two streams of water from the cooling tower are used for neutralizing the cooling and heating capacities. The mixing process proceeds from the cool end to the warm end which, for con- venience, are termed the upstream and downstream ends, respectively. Any excess heat from the mixing process is then returned to the cooling tower where it is rejected to ambient. Figure 3.6 illustrates the mixing of the cool and warm streams in the tank for a chiller rating test. The mixing tank allows us to set the temperature of the water returning to the evaporator to be lower than that of the cooling tower. The control valve (at the cool end) is set to act in reverse so as to supply

less cooling water to the cool end. In contradistinction, the tempera-

ture of the water returning to the condenser can be raised by the control valve at the warm end, where the valve control strategy can act in either

the forward or reverse direction based on the feedback from the measured temperatures. A bypass of the condenser water to the cooling tower may be necessary if Tin

cond is lower than that of the temperature of the cool-

ing tower. Figure 3.3 is a temperature–time trace from a chiller rating test performed with the alternative test rig in accordance with ARI Standard 590.

H4. Mixing process for a heat pump

Rating tests for a heat pump require higher supply temperatures of coolant water to both the evaporator and the condenser of the test unit. We retain the same mixing tank as for the chiller rig, but proceed from the warm end to the cool end (instead of vice versa). Typical param- eters for a rating test with a heat pump are illustrated in Figure 3.7.

With this new mixing strategy, the condenser can be supplied with water at a temperature above that obtainable from the cooling tower. Similarly, the evaporator can be supplied with water at a temperature above, as well as below, that derived from the cooling tower. The control valve at the warm end operates in the forward mode when the temperature of water returning to the condenser is higher than that of the cooling tower. In rating tests where Tevapin is set above that of the cooling tower,

bypass water from the condenser outlet is reduced to a minimum. Con- versely, should Tevapin be set lower than that of the cooling tower water,

the bypass from the condenser outlet is allowed to open, and the cor-

Figure 3.7: Schematic illustration of the mixing process for a heat pump rating test. Cold End Hot End To evaporator

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evap, inevap From evaporator

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evap, outevap

To condenser

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cond, incond

From condenser

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m