Vapor Compression Cycle Cooling
The term refrigeration, as part of a building, the ventilation systems and conditioning, generally refers to the vapor compression system, when a chemical substance, alternately changes from liquid to gas (evaporation, thereby absorbing heat and providing a cooling effect) and gas from liquid (condensate, thereby releasing heat). This cycle actually consists of four steps:
As shown in Fig. 1.1, steam-compression refrigeration system consists of four components that comply with the four steps of the refrigeration cycle. Compressor raises the pressure of the initially low pressure refrigerant gas. Condenser heat exchanger, which cools the gas high pressure, so that it changes in the liquid phase. Expansion valve controls the pressure ratio, and, thus, consumption, between high and low pressure regions of the system. The evaporator heat exchanger that heats the liquid at low pressure, which leads to a change in phase from liquid to vapor (gas).
- Compression: Low pressure gas-refrigerant is compressed, thus increasing its pressure on the mechanical energy to spend. There is a corresponding increase in temperature along with the increased pressure.
- Condensation: high pressure, high temperature gas cooled by outside air or water, which serves as radiator and condenses to a liquid under high pressure.
- Expansion: high pressure liquid flows through the outlet of expansion valve, thereby reducing the pressure. A small portion of the liquid flashes of gas due to pressure reduction.
- Evaporation rate: low pressure liquid absorbs the heat from the air or water and evaporates in the form of a gas or vapor.
Low pressure vapor flows compressor, and the process repeats.
Thermodynamically, the most common presentation of basic refrigeration cycle is manufactured using pressure-enthalpy diagram, as shown in Fig. 1.2. For each refrigerant, phase-change line represents the conditions of pressure and heat (enthalpy), at which it changes from a liquid to a gaseous state, and Vice versa. Thus, each of the steps of the vapor compression cycle can be easily applied to demonstrate the real thermodynamic processes, as shown in Fig. 1.3.
Paragraph 1 is the conditions of the compressor inlet. Compression of gas increases its pressure from Pj to P2. Thus, workthat makes the compressor adds heat to refrigerant, raising its temperature and slightly increasing its heat content. Paragraph 2-is the state of the refrigerant leaving at the inlet of the compressor and condenser. In the condenser, the gas is cooled, reducing it from enthalpy h2 h3.
Paragraph 3 paragraph 4 represents a reduction in pressure, which occurs during the expansion. Due to the small percentage of evaporation due to the reduction of pressure, temperature and enthalpy of the remaining liquid is also slightly decreased. Point 4, then, represents the state of the inlet of the evaporator. Paragraph 4 of part 1 is heating of the liquid, increasing its enthalpy h1 h4, completed the phase transition from the liquid to a gaseous state at the point 1.
Refrigerant whose properties are known, pressure-enthalpy diagram can be built and efficiency of the vapor compression cycle are analyzed by establishing high and low pressure systems. (Note that in Fig. 1.3 is the suitable cycle, but in practice, there are a variety of treatments dictated by the second law of inefficiency.)..