REFRIGERATION - 3

Thermostatic Expansion Valve (TXV)

The TXV regulates the amount of refrigerant to the cooling coil. The amount of refrigerant needed in the coil depends, of course, on the temperature of the space being cooled.

The thermal control bulb, which controls the opening and closing of the TXV, is clamped to the cooling coil near the outlet (tail coil), and before the back pressure regulating valve if installed. The substance in the thermal bulb varies, depending on the refrigerant used. The expansion and contraction (because of temperature change) transmit a pressure to the diaphragm. This causes the diaphragm to be moved downward, opening the valve and allowing more refrigerant to enter the cooling coil. When the temperature at the control bulb falls, the pressure above the diaphragm decreases and the valve tends to close. Thus, the temperature near the evaporator outlet controls the operation of the TXV.

Flash gas formed in the liquid line of a refrigeration system due to low refrigerant may cause expansion valve pins and seats to erode. A leaking expansion valve could result in excessively low temperature to the space regulated.

Evaporator

The evaporator consists of a coil of copper, aluminum, or aluminum alloy tubing installed in the space to be refrigerated. Aluminum tubing with copper fins are used in ammonia systems. The liquid R-12 enters the tubing at a reduced pressure and, therefore, with a lower boiling point. As the refrigerant passes through the evaporator, the heat flowing to the coil from the surrounding air causes the rest of the liquid refrigerant to boil and vaporize. Refrigerant temperature in an evaporator is directly related to refrigerant pressure. After the refrigerant has absorbed its latent heat of vaporization (that is, after it is entirely vaporized), the refrigerant continues to absorb heat until it becomes superheated by approximately 10°F. The amount of superheat is determined by the amount of liquid refrigerant admitted to the evaporator. This, in turn, is controlled by the spring adjustment of the TXV. A temperature range of 4° to 12°F of superheat is considered desirable. It increases the efficiency of the plant and evaporates all of the liquid. This prevents liquid carry-over into the compressor (flooding back).

Excessive circulation of the lubricating oil will cause the evaporator temperature to increase. The main cause of slugging is improperly adjusted thermal expansion valve.

Defrosting of evaporator coils of a multi-box, direct expansion type refrigeration systems, and ice machines can be accomplished by passing hot vapors from the compression cycle through the coils. A re-evaporator is one way to overcome the possibility of a large slug of liquid refrigerant entering the compressor suction when hot gas defrosting a refrigeration system.

Compressor

The compressor in a refrigeration system is essentially a pump. It is used to pump heat uphill from the cold side to the hot side of the system. The heat absorbed by the refrigerant in the evaporator must be removed before the refrigerant can again absorb latent heat. The only way the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it. Because of the relatively high temperature of the available cooling medium, the only way to make the vapor condense is to compress it.

When we raise the pressure, we also raise the temperature. Therefore, we have raised its condensing temperature, which allows us to use seawater as a cooling medium in the condenser. In addition to this primary function, the compressor also keeps the refrigerant circulating and maintains the required pressure difference between the high-pressure and low-pressure sides of the system. Many different types of compressors are used in refrigeration systems. The designs of compressors vary depending on the application of the refrigerants used in the system. The figure below shows a motor-driven, single-acting, two-cylinder, reciprocating compressor.

Compressors used in refrigeration systems may be lubricated either by splash lubrication or by pressure lubrication. Refrigeration compressors require a lubricant with a low pour point, and low wax content to keep any oil leaving the compressor from  congealing in the evaporator. Splash lubrication, which depends on maintaining a fairly high oil level in the compressor crankcase, is usually satisfactory for smaller compressors. High-speed or large-capacity compressors use pressure lubrications systems.

The sudden reduction of pressure occurring within the crankcase of a refrigeration compressor during starting causes the release of refrigerant from the oil/refrigerant mixture. Foaming of the oil in a refrigeration compressor crankcase is caused by refrigerant boiling out of the lube oil. The oil in the sump of a secured refrigeration compressor is heated to reduce absorption of refrigerant by the oil. Excessive oil foaming in the crankcase of a refrigeration compressor at start up can cause compressor damage from improper lubrication. The oil level in a refrigeration compressor, the most accurate reading is obtained immediately after shutdown following a prolonged period of operation. The refrigerant has had time to separate from the oil.

Refrigerant entering the compressor should be superheated vapor. Its possible to have liquid refrigerant returned to the suction side of a compressor due to a faulty or improperly adjusted expansion valve.  A flapper valve, also known as a beam valve, is frequently used in refrigeration compressor discharge valves, and is designed to pass liquid slugs. Some systems have devices installed in the compressor suction line to boil off liquid refrigerant returning to the compressor, such as liquid separators, liquid accumulators, economizers, and heat exchangers

Capacity Control System

Most compressors are equipped with an oil-pressure-operated automatic capacity control system. This system unloads or cuts cylinders out of operation following decreases in the refrigerant load requirements of the plant. A cylinder is unloaded by a mechanism that holds the suction valve open so that no gas can be compressed.

Since oil pressure is required to load or put cylinders into operation, the compressor will start with all controlled cylinders unloaded. But as soon as the compressor comes up to speed and full oil pressure is developed, all cylinders will become operative. After the temperature pull-down period, the refrigeration load imposed on the compressor will decrease, and the capacity control system will unload cylinders pressure accordingly. The unloading will result in reduced power consumption. On those applications where numerous cooling coils are supplied by one compressor, the capacity control system will prevent the suction pressure from dropping to the low-pressure cutout setting. This will prevent stopping the compressor before all solenoid valves are closed.