The expansion valve regulates the amount of compressed liquid refrigerant moving into the evaporator. It removes pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a gas in the evaporator.
Thermostatic Expansion Valve
There are two types of expansion valves commonly used on air conditioning systems:
- Internally-equalized valve
It receives warm high-pressure liquid refrigerant and leaves the expansion valve quite cold. Keep in mind that the expansion valve does not lower heat. It only decreases the pressure. Thus, the heat molecules can spread out more as it is moving out of the valve and becomes very cold.
- Externally-equalized valve
Operation of the externally-equalized valve is the same as the internal type except that evaporator pressure is fed against the underside of the valve diaphragm from the tailpipe of the evaporator by an equalizer line. This balances the temperature of the expansion valve when it is doing the phase conversion.
In order to properly select Expansion Valves, the following items should be considered:
- Required valve capacity should be based on the actual system operating conditions rather than the normal valve capacity rating.
- When there exists an appreciable pressure drop between the valve outlet and the evaporator outlet, i.e., above 0.02 MPa (0.2 kgf/cm2), or when a pressure drop type of refrigerant distributor is used at the evaporator inlet, the valve should have the external equalizer feature for best performance. Otherwise, it will increase a static superheat (valve opening temp.), restricting the refrigerant flow and causing the reduction of system capacity. As for R134a, 0.01 MPa {0.1kgf/cm2} pressure drop will increase the static superheat approximately 1°C.
- Internally equalized valve may be used with the evaporator which has a negligible pressure drop, i.e., below 0.02 MPa (0.2 kgf/cm2).
EQUALIZER
An internal or external equalizer should be selected depending on pressure drop between valve outlet and evaporator outlet. Internal equalizer increases superheat in the refrigeration system of which evaporator has some pressure drop, and the increase of superheat decreases the effective area of evaporator.
Select internal or external equalizer depending on refrigerant, pressure drop and evaporating temperature. The Guide Table for Equalizer indicates the pressure difference of refrigerant corresponding to 1°C temperature. The external equalizer valves should be used when the pressure drop exceeds the value of pressure difference indicated in the Table.
SETTING OF SUPERHEAT
Superheat Adjuster of Expansion Valve adjusts the superheat by which the valve begins to open from the fully closed condition, and this superheat is called Static Superheat.
- S.S.H.: Static Superheat
- O.S.H.: Operating Superheat (Superheat necessary for valve and refrigeration system operation)
- S.H.C.: Superheat Change (Superheat which keeps the valve opening at the optimum balance point for refrigeration systems)
S.S.H. ＝ O.S.H. − S.H.C.
To change the adjustment, remove the seal cap and turn the adjusting spindle. Turning the spindle clockwise to compress the spring decreases flow and raises superheat and turning the spindle counter clockwise to loosen the spring increases flow and lowers superheat.
CHARGE & MOP (MAXIMUM OPERATING PRESSURE)
G–Charge
Gas charge used generally on air conditioning gives pressure limiting, but loses control if valve body becomes colder than sensing bulb. A gas charged valve should be installed in a location where the valve body can be warmer than the bulb to prevent condensation of the charge in the powerhead.
L–Charge
Liquid charge provides accurate control when valve body becomes colder than sensing bulb; hence a liquid charged valve may be installed in any location regardless of temperature. The charge, however, does not provide maximum operating pressure (pressure limiting) for motor overload protection.
C&CL&CY–Charge
Cross charge and Cross Low Temp. charge used generally on low temperature range application will not lose control even if valve body becomes colder than sensing bulb. A cross charged valve may be installed in any location regardless of temperature. Cross charge (C) for normal refrigeration (higher than − 40°C temp. range) and Cross Low Temp. charge (CL and CY) for low temp. refrigeration (CY… − 70 to − 40°C with R22 for Type ATX valves).
S&SA&SL–Charge
The charge provides accurate control even if valve body becomes colder than sensing bulb and further it provides MOP (pressure limiting) for motor overload protection. S–charged valve may be installed in any location regardless of temperature.
ORDERING Expansion Valves
- Normal Pressure and Maximum Pressure
- Normal Temperature and Minimum Temperature
- Detailed Application
- Refrigerant
- Valve Location
- Capacity (Condensing & Evaporating Temperature)
- Length of Capillary Tube
- External or Internal Evaporator
- Pressure Drop at Evaporator
- MOP (Maximum Operating Pressure)
- Two Stage Compressor System or not
FREQUENTLY ASKED QUESTIONS
The two common types of expansion valves used in air conditioning systems are internally equalized valves and externally equalized valves. Internally equalized valves receive warm, high-pressure liquid refrigerant and leave the expansion valve quite cold, whereas externally equalized valves have a separate connection to the evaporator outlet to sense the pressure.
If an expansion valve is not properly sized for the HVAC system, it can lead to inefficient operation, reduced system capacity, and increased energy consumption. An undersized expansion valve may cause the system to operate at higher pressures, while an oversized valve may result in reduced system performance and potential refrigerant leakage.
The expansion valve regulates the refrigerant flow rate by controlling the pressure drop across the valve. As the valve opens, the pressure drop increases, allowing more refrigerant to flow into the evaporator. Conversely, as the valve closes, the pressure drop decreases, reducing the refrigerant flow rate. The expansion valve must be carefully selected to ensure the correct refrigerant flow rate for the specific HVAC system.
When selecting an expansion valve, consider factors such as the type of refrigerant used, system operating pressures, evaporator design, and desired superheat setting. Additionally, consider the valve’s flow characteristics, pressure drop, and capacity to ensure proper system operation and efficiency.
The expansion valve plays a critical role in maintaining the overall efficiency of an HVAC system. A properly sized and selected expansion valve ensures that the refrigerant flows at the correct rate, allowing for efficient heat transfer in the evaporator and condenser. Inefficient expansion valve operation can lead to reduced system performance, increased energy consumption, and potential system failures.
