In these days of varying climate conditions and energy costs, most home owners want to enjoy comfort conditions from their ac unit in their residence with the best energy efficiency available.
The expansion valve is a critical component of the air conditioning unit to achieve the above requirements. This is the demarcation point between the high side and low side of the refrigeration system and is crucial to the best operation of the air conditioning system. The expansion valve controls the refrigerant flow in the cooling system
What role does an expansion valve play in an air conditioning system?
In laymen’s terms the expansion valve is the point in the cooling system where the compressed refrigerant dramatically reduces in pressure which then makes drop in temperature. This cold refrigerant is what does the hard work in producing the cooling effect of your air conditioner.
The expansion valve controls the refrigerant flow in the air conditioning system. The refrigeration flow is paramount as the correct flow will maintain the desired superheat. The correct superheat will maintain the best co-efficient of performance hence the best energy efficiency of your air conditioner.
How does an air conditioner expansion valve work?
The air conditioner’s expansion valve’s role is to maintain the desired superheat.
In the case of an Electronic Expansion Valve this is achieved by the various sensors in the air conditioner controlling the flow of refrigerant. The sensors are resistors than take their readings (ohms) and correlate these to a temperature.
A Thermostatic Expansion Valve relies on the sensing of the suction line temperature combined with mechanical pressures in the valve body to modulate the flow of refrigerant.
The liquid line entering the expansion valve is transporting sub cooled refrigerant which comes from the condenser coil. The expansion valve monitors the amount of liquid entering the evaporator. Whilst in the evaporator coil the refrigerant absorbs the heat passing over the evaporator coil. The effect of this heat absorption is that the liquid refrigerant changes state to a vapour and becomes superheated. This superheated vapour then travels to the compressor to commence the refrigeration cycle again.
What are the different types of expansion valves
There are various types of expansion valves in use in hvac systems. In the most basic of systems this may just be a capillary tube. Thermal expansion valves were used almost exclusively until the advent of the electronic expansion valve which are used extensively in most modern air conditioning units. Both of these valves control refrigerant flow
The electronic expansion valves have definite benefits over the other expansion valves in the ac unit. Some of these benefits include:
- Repsonds faster to load change and controls the refrigerant flow to suit.
- Control of the refrigerant flow is more precise than other expansion valves.
- Injects the correct amount of refrigeration into the evaporator to meet the requirements at any given time
- Energy savings over other forms of expansion valves due to the electronic sensing and modulating of the valve
Common signs of expansion valve problems
Temperature Fluctuations
Temperature fluctuations can be a sign of a faulty expansion valve as the valve is not allowing the correct amount of refrigerant to flow to the evaporator, hence the superheat is incorrect to maintain the required conditions in the air conditioning unit and the conditioned space. This can lead to fluctuations of cold and and warm air in the conditioned space
Compressor running continuously
The compressor may run continually due to the incorrect operation of the expansion valve and this will lead to higher energy usage and the possibility of excess wear to the compressor.
Icing of the Evaporator Coil
A failed or faulty expansion valve can lead to icing of the evaporator coil. This is an undesirable situation for your air conditioning unit . Icing the evaporator coil will not give the desired heat transfer across the coil. This can lead to liquid refrigerant flowing back to the compressor. Compressors are not designed to pump liquid refrigerant and can lead to costly mechanical failure of the compressor and failure of the ac unit.
Warm Room Conditions
A faulty expansion valve will affect the operation of the hvac system with the potential to make the system stop operating completely. Various safety devices (high pressure and low pressure) will operate and prevent the air conditioner from operating. This is to prevent any further damage being done to the system.
Noisy System
A failure of the expansion valve can lead to undue noise in the air conditioning system. As the valve cannot control the refrigerant flow as required, these can be in the form of hissing sounds as refrigerant is trying to pass through the expansion valve.
Safety devices tripping
Various safety devices (high pressure, low pressure, various sensors) may operate and prevent the air conditioner from operating. This is to prevent any further damage being done to the system.
What are common causes of a bad expansion valve
Thermostatic Expansion Valves (TXV)
- Loss of pressure in the power head.
- Blocked inlet screen due to contaminants in the refrigeration system
- Internal failure of the sprng mechanism
- Incorrect placement of the temperature sensing bulb
Electronic Expansion Valve (EEV)
- Failure of the step motor
- Failure of the sensors controlling the operation of the expansion valve
- Failure of the printed circuit board to send the correct signals to the step motor to modulate the flow rate of the refrigerant
- Mechanical failure of the internal components of the valve thus not allowing the correct operation
How can you fix a faulty expansion valve
Prior to any repairs or replacement of an expansion valve, a correct diagnosis is requried to ascertain the fault. This diagnosis can include some or all of the following:
- Recording suction pressure
- Recording discharge pressure
- Checking of the resistance of any sensors associated with the expansion valve.
If a Thermostatic expansion valve is used, various components can be changed:
- The power element which has the temperature sensing bulb attached
- The internal pins that push against the internal diaphram to open or close the valve
- The internal spring
With a Electronic Expansion Valve, the only component that can be changed (in some instances) is the step motor. On most cases the entire valve needs to be changed once it is ascertained that is has failed
Key takeaways
- The expansion valve is an integral and vital apart of the air conditioning unit
- The correct operation of the expansion valve has an important role in the energy efficiency of the air conditioning system
- Regular maintenance of the air conditioning system is vital to ensure the correct operation of all components including the expansion valve in the hvac system
- The expansion valve needs to work in harmony with all the other components of the air conditioner to ensure comfort conditions in association with energy efficiency.
