Flooded Evaporators & HVAC Components: Function & Selection
Flooded Evaporator: Function and Operation
A flooded evaporator maintains a reservoir of liquid refrigerant on the coil. This is regulated by a float valve. As the refrigerant evaporates, it is absorbed by the compressor. The liquid level in the evaporator drops, causing the float valve ball to lower. The ball is attached to a moving pivot that opens the valve, allowing more liquid refrigerant to enter. The float valve thus controls the liquid refrigerant level, closing when the desired level is reached.
Evaporator: Definition and Purpose
An evaporator is a thermal heat exchanger used to produce cold through evaporation. It absorbs heat from the surrounding medium, causing it to cool.
Coolers: Enhancing Refrigerant Subcooling
Coolers are used to increase the subcooling of liquid refrigerant.
Defrosting Evaporators: Addressing Ice Buildup
When evaporation occurs below 0°C, ice can form on the coils due to condensation of moisture in the air. This reduces evaporator performance, increases power consumption, and can potentially damage the compressor.
Methods for Defrosting Evaporators
Defrosting melts the frost that forms on the evaporator. Several methods exist:
- Natural Defrost: The system stops long enough for the refrigerated enclosure to warm up above the melting point of ice.
- Water Defrost: Water is sprayed on the evaporator tubes to melt the ice.
- Brine Defrost: A brine solution is sprayed on the evaporator tubes, lowering the freezing point and melting the ice.
- Electric Resistance Defrost: Electrical resistance heaters are placed around the evaporator tubes and are periodically activated to melt the ice.
Key Factors for Evaporator Selection
- Required power
- Evaporation temperatures needed to cool the enclosure
- Project’s relative humidity
- Type of refrigerant fluid
- Enthalpy of cooling at the inlet and outlet of the evaporator
Expansion Devices: Features and Purpose
Expansion devices create a pressure drop and spray the coolant to regulate the flow rate. They are placed at the entrance of the evaporator.
Capillary Tubes: Advantages and Disadvantages
Capillary tubes offer low cost, simple assembly, reliability, and good performance. They also facilitate easy engine start. However, they can be affected by changes in refrigerant overheating in the evaporator. They are primarily used in low-power machines. Key characteristics of a capillary tube include its length, inner diameter, refrigerant type, evaporation temperature, outdoor temperature, and cooling efficiency.
Pressure Valves: Mechanism of Operation
A pressure valve has a membrane attached to a stem. Several forces act on it: upward evaporation pressure, downward atmospheric pressure, and downward spring pressure.
Thermostatic Expansion Valves: Regulating Superheat
Thermostatic expansion valves use the pressure signal at the evaporator outlet, acting on the bottom of the membrane, to regulate superheat. A pipe connects the chamber below the diaphragm to the evaporator outlet. This pipe should be connected to the suction pipe after the bulb and before the siphon. It should never be left plugged or installed without connecting the hole. This ensures that the pressure on the valve membrane is the evaporator outlet pressure, not the inlet pressure.
Four-Way Valves: Application in Heat Pumps
Four-way valves are used in heat pumps. Their operation is controlled by a solenoid coil mounted on the valve.