Effect of Varying Evaporator and Condensing Temperature

While air conditioning systems operate with a refrigerant evaporating temperature of about 35 to 45°F (1 to 7°C) to produce 44°F (7°C) chilled water or 55°F (13°C)

supply air, process refrigeration systems often operate at much lower temperatures. Industrial fluid chillers can supply glycol mixtures, brines, or other specialty process fluids as low as -60°F (-51°C), with refrigerant evaporating temperatures 5 to 10°F (3 to 6°C) lower than leaving fluid temperatures. In most cases, the power requirements per ton (or kWr) are higher for process refrigeration applications, sometimes dramatically. For example, under design conditions for a conventional refrigeration system, assuming 105°F (41°C) condensing temperature, performance figures at corresponding evaporating temperatures may be similar to the values shown in Table 33-7.

Evaporating

Representative Power

Temperature

Requirement

40°F (4°C)

0.80 hp/ton (0.17 kWm/kWr)

0°F (-18°C)

1.66 hp/ton (0.35 kWm/kWr)

-40°F (-40°C)

3.59 hp/ton (0.76 kWm/kWr)

Table 33-7 Relationship of Evaporating Temperature to Power Requirement.

Table 33-7 Relationship of Evaporating Temperature to Power Requirement.

To achieve a large differential between evaporating and condensing temperature, process refrigeration equipment and refrigerant choices frequently differ from those selected for air conditioning duty. Commonly used process systems include screw compressors and two-stage compound reciprocating compressors. Commonly used refrigerants include ammonia and HCFC-22. While uncommon for air conditioning applications, the thermo-dynamic characteristics of ammonia (NH3), or R-717, make it attractive for lower temperature refrigeration applications. It has a boiling point of about -28°F (-33°C) and a freezing point of about -108°F (-78°C).

Figures 33-4 and 33-5 show the effect of suction temperature and condensing temperature on capacity and performance for a vapor compression system featuring a screw compressor. As suction (evaporating) temperature is increased, the mass flow through the compressor increases due to a decrease in refrigerant-specific volume. Concurrently, the compressor's volumetric efficiency increases due to the decrease in compression ratio at constant condensing temperature.

In Figure 33-4, for example, at a suction temperature of 40°F (5°C), system capacity is about 600 tons (2,100 kW.), while at a suction temperature of 5°F (-15°C), the capacity is only 280 tons (985 kW.). Notice also that the power requirement at 40°F (5°C) is about 560 bhp (418 kWm), while at 5°F (-15°C) it is 460 bhp (343 kWm). Both capacity and COP increase at higher suction temperatures. Also notice that while total power requirement decreases with decreasing suction temperature, power

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