Vapor Compression Cycle Technologies

There are four basic components used in the vapor compression cycle: a compressor, a condenser, an expansion valve (or throttling device), and an evaporator, each corresponding to one of the four cycle processes. The relationship of these components in the vapor compression cycle is shown in Figure 37-1.

In all practical applications, the actual vapor compression cycle performance is always lower than the ideal cycle. This is the result of several factors, including friction losses, heat exchanges between parts of the system, and pressure drops in suction and discharge lines. Additionally, the refrigerant vapor charge enters the compressor with a small amount of superheat (to avoid potential damage to the compressor through the entrance of slugs of liquid refrigerant that are not completely vaporized during compression), and the liquid refrigerant entering the expansion valve is usually subcooled a few degrees. Also, since compressor suction and discharge are actuated by pressure difference, the process requires the actual suction pressure inside the compressor to be slightly below that of the evaporator and the discharge pressure to be above that of the condenser.

Vapor compression cycle systems can be categorized in several ways, including by compressor type, condenser type, and evaporator type. Vapor compression systems consist of a compressor, driver, liquid cooler (evaporator), condenser(s), refrigerant control device, and a controller. They often include a receiver(s), oil separator, intercooler and/or subcooler accumulator, and, in some cases, dual liquid pumps. They may use air-cooled, water-cooled, or evaporative condensers and refrigerant-to-air or refrigerant-to-liquid evaporators.

As detailed in Chapter 35, since it is generally impractical to circulate large volumes of refrigerant throughout a facility, chillers that use refrigerant-to-liquid evaporators are almost always used for large central air conditioning systems with multiple terminal units and for small to large process loads. They are also sometimes used in smaller units. Vapor compression systems using refrigerant-to-air type evaporators are called direct expansion (DX) units. These systems provide direct cooling as the refrigerant absorbs heat directly from the medium (i.e., air) being cooled. DX systems typically range in capacity from fractional tonnage up to a few hundred tons (1,000 kWQ and use high-pressure refrigerants such as HCFC-22.

DX units have two basic configurations: packaged and condensing systems. Packaged, or rooftop, units (RTUs) are self-contained units with all four vapor compression components in one housing. They are typically roof-mounted and directly connected to ductwork or to a diffuser and distribute chilled air directly to a conditioned space. These are air-cooled units that range in capacity from fractional tonnage window units to several hundred tons, with extensive ducting systems and controls.

Balance point betweenhigh side and low side

Expansion valve

Balance point betweenhigh side and low side

Expansion valve

Fig. 37-1 Relationship of Vapor Compression Cycle Components.

Condensing, or split, systems are units that consist of a compressor and driver with an interconnected air-cooled condenser mounted in a self-contained housing. Instead of direct ducting, a liquid and suction (vapor) line set sends and receives refrigerant from these components to an air handling unit (AHU) or, in some cases, multiple AHUs.

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