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Chapter 6 The Refrigeration System
Refrigerant States • As refrigerant flows through the condenser a high-pressure superheated vapor changes to a high-pressure saturated liquid (liquid and vapor) before changing completely to liquid. The outlet line of the condenser can be called the liquid line. Discharge line of the condenser is smaller because liquid takes up less space. • As refrigerant flows through the TXV, a small percentage of liquid can vaporize as the pressure is reduced. This is known as flash gas and the more subcooled the refrigerant is by the condenser the less there will be.
Refrigerant States Con. • The subcooled refrigerant then flows through the evaporator. A fan circulates the warm cabin air through the evaporator fins. As heat is absorbed the refrigerant changes states from a low pressure subcooled liquid to a low pressure superheated vapor. • The compressor is drawing from the low side (evaporator) and pushing into the high side (condenser). This intensifies the heat.
State of Refrigerant in a TXV System Concentrate on states during location.
Temperature • Air temperature should drop 10 F as it passes through the evaporator. • During normal operation, hoses on the high side of the system should feel warm or hot, hoses on the low side should feel cool or cold. • Any drastic temperature change in the system (hot-cold) can indicate a restriction.
Capacity (Tons) • 1lb of ice at 32F to 1lb water = 144 BTU • 144 BTU x 2,000lb = 288,000 BTU’s in 24hrs • 288,000 BTU / 24hrs = 12,000 BTUs/hr =1 ton of capacity • Less than one ton capacity is rated in BTU’s • ½ ton capacity would be called 6000BTU’s • A normal truck AC system would be rated about 1.75ton capacity. • House hold A.C. system 1.5 ton rating
Electromagnetic Clutch • When current flows to the field coil the armature assembly is pulled magnetically into the rotating belt assembly causing the compressor crankshaft to rotate. • When de-energized the armature and pulley assemblies separate causing the compressor to stop.
Why Have Compressor Controls • Low system pressure can indicate a loss of refrigerant and oil which can seize the compressor. • With low ambient temperature the pressure will be too low causing the same problems as stated above + the oil thickens resisting flow. • High pressure can cause clutch slippage or blow out hoses and o-rings.
Thermostatic Switch(Cold Switch) • Prevents evaporator freeze up (32F) from ice on fins • Senses the temperature of the evaporator with its own bulb and capillary tube inserted in the center of the evaporator. • When the temperature drops the diaphragm pushes the switch open stopping the clutch. • Once the evaporator melts (water runs out drain tube) the switch then closes re-energizing the clutch.
Two Types of Switches • Preset temperature, two temperature presets one for on one for off. • Adjustable on and off points can be changed for operators preference.
Thermostatic Switch on H-Block and Low Pressure Switch Thermostatic switch prevents the evaporator from freezing up, while the low pressure switch prevents compressor operation with low refrigerant levels.
Pressure Cycling Switch • Used on cycling clutch orifice tube systems • Located on the accumulator • Senses system pressure, on the low side, and opens when below 25psi (evaporator flooded) and closes again at 35psi to restart the compressor. • Freeze protection device for the system • Also acts as a low pressure switch.
Pressure Cycling Switch Usually mounted on a Schrader-valve for easy replacement.
Low Pressure Switch When located on the low side it acts the same as a pressure cycling switch.
Low Pressure Cut-Off Switch • Prevent the compressor from running when the refrigerant pressure is to low. • Low pressure can be caused by low refrigerant levels which could cause compressor failure from lack of oil circulation. • Also low refrigerant can indicate a leak and prevent moisture and air from being pulled into the system. • Can be located on high or low side.
High Pressure Cut-Off Switch • Located on high side (receiver-drier) to prevent system damage due to over pressure. • High pressure can be caused by overcharging or a plugged condenser that can not dissipate heat properly. • Disengages clutch when switch opens
Binary Switch(High and Low Pressure) Usually located on the receiver-drier with predetermined settings. (28psi, 385psi)
Trinary Switch • Combines high and low pressure switches to disengage the compressor. • The third function is to engage the engine fan or open shudders when the high side pressure raises above 230 psi in normal operation. A fan timer is also usually used to prevent continuous cycling. (45-120 sec) • Usually located on the receiver drier. • Usually installed on a Schrader-valve
High Pressure Relief Valve (440psi)Located on Drier or Compressor
Summary • All of the previously mentioned functions should be checked when service intervals occur. • The manufacturer of the A.C. system decides which switches will work best on their system. Therefore every system will be configured with some differences.
