Refrigeration cycle and components

NAVEDTRA 14075 §6-1

NAVEDTRA 14075 §6-1 — The vapor-compression refrigeration cycle Refrigeration moves heat from a cold space to a warmer one, against its natural direction, by repeatedly evaporating and condensing a refrigerant in a closed cycle. It works because a liquid absorbs a large amount of heat (its latent heat) when it boils, and gives that heat up again when it condenses; by controlling the pressure, the refrigerant can be made to boil at a low temperature in the space being cooled and to condense at a higher temperature where the heat is rejected. The vapor-compression cycle has four essential components connected in a loop: the compressor, the condenser, the metering (expansion) device, and the evaporator. Following the refrigerant around the loop: low-pressure liquid refrigerant boils in the e…

NAVEDTRA 14075 §6-2

NAVEDTRA 14075 §6-2 — Compressors The compressor is the heart of the refrigeration system: it draws low-pressure vapor from the evaporator, compresses it to a pressure high enough that the refrigerant will condense at the available cooling-water or air temperature, and so keeps the refrigerant circulating and the heat moving. Most marine refrigeration and air-conditioning plants use reciprocating compressors, in which pistons draw vapor in through suction (reed) valves on the down-stroke and force it out through discharge valves on the up-stroke; they may be open (driven by an external motor through a coupling or belt, with a shaft seal) or hermetic/semi-hermetic (motor and compressor sealed in one housing). Larger air-conditioning plants often use rotary screw or centrifugal compressors,…

NAVEDTRA 14075 §6-3

NAVEDTRA 14075 §6-3 — Condensers and receivers The condenser rejects the heat that the refrigerant absorbed in the evaporator plus the heat added by the compressor, condensing the high-pressure vapor back into a liquid so the cycle can continue. Marine plants commonly use water-cooled shell-and-tube condensers: high-pressure refrigerant vapor fills the shell around a bundle of tubes through which seawater is pumped, and as the vapor gives up its heat to the cooler seawater it condenses on the outside of the tubes and drains to the bottom of the shell. Some smaller or air-conditioning units use air-cooled condensers with a fan blowing air over finned coils. Condenser performance directly controls the high-side pressure: if the condenser cannot reject heat well — because the tubes are foule…

NAVEDTRA 14075 §6-4

NAVEDTRA 14075 §6-4 — Metering devices and refrigerant control The metering device separates the high side from the low side and controls how much liquid refrigerant enters the evaporator, dropping the high-pressure liquid to the low evaporator pressure so it can boil and absorb heat. The most common control on marine plants is the thermostatic expansion valve (TXV), which meters liquid into the evaporator in proportion to the cooling load: a sensing bulb on the evaporator outlet measures the temperature of the leaving vapor and modulates the valve to hold a constant superheat, admitting more refrigerant when the load is high and the outlet is warm, and less when the load falls. By maintaining superheat the TXV keeps the evaporator fully used while ensuring only vapor — not liquid — reach…

NAVEDTRA 14075 §6-5

NAVEDTRA 14075 §6-5 — Evaporators and cooling coils The evaporator is where the refrigeration system does its useful work: low-pressure liquid refrigerant boils inside the evaporator coil and absorbs heat from the air, water, or brine around it, cooling the refrigerated space or product. Because the refrigerant is boiling at low pressure and therefore low temperature, heat flows into it from the warmer surroundings; the refrigerant leaves the evaporator as a low-pressure vapor, slightly superheated, on its way to the compressor suction. Evaporators are built in two general arrangements. In a dry-expansion evaporator the metering device feeds just enough refrigerant that it is fully evaporated by the coil outlet; in a flooded evaporator the coil is kept filled with liquid to a level set by…

NAVEDTRA 14075 §6-6

NAVEDTRA 14075 §6-6 — Refrigerants and safe handling A refrigerant is the working fluid that carries heat around the cycle, chosen to boil and condense at convenient pressures over the temperature range the plant must serve, and to be stable, relatively non-toxic, and non-corrosive. Older marine plants used R-12 and R-22 (fluorocarbons) and some used ammonia (R-717) for large refrigeration; because the ozone-depleting fluorocarbons have been phased out, systems now use replacement blends, but the operating principles are the same. Ammonia is an excellent refrigerant thermodynamically but is toxic and flammable and has a sharp warning odor, so ammonia plants demand strict safety precautions. Whatever the refrigerant, moisture is its enemy: water in the system freezes at the metering orific…