Technical
to the expansion valve obtaining the superheat quickly and maintaining the superheat in a safe manner to protect the compressor from any low superheat risk .
The performance characteristics between an electronic expansion valve ( EEV ) and thermal expansion valve ( TXV ).
The other technology we can consider is pulse width modulation expansion valves ( PWMs ). These would also be considered electronic expansion valves but the operation is quite different . The EEVs would use a method of continuous refrigerant flow where PWM valves would be electrically actuated and this operation would be start and stop control .
The PWM valve still has the ability to control the superheat for an evaporator coil by changing the relationship between on and off time . For example high demand would by long on and short off and low demand would be short on and long off . This brings us to the point that with PWM valves we are adding poor refrigerant feed to the evaporator that would lead to less stable superheat and subsequently less stable suction pressures that the compressor would have to combat .
One of the benefits of the PWM valve is that during a power failure the PWM valve would close and would save the system from any liquid migration from the liquid line to the suction line . Having liquid refrigerant in the suction line would pose a threat to the compressor during start-up .
With the EEV we have a solution for such an event where we use capacitors to store electrical energy . During the power failure the controller will take this power and use it the close the EEV to prevent any liquid migration .
A secondary use for an EEV is for custom applications that are not necessarily superheat control . Using an EEV in conjunction with a PLC we can have the valve modulate for other needs such as oil control , liquid injection , vapour injection and tank pressure modulation . These applications can be found in today ’ s new BLDC technology as well as CO₂ transcritical solutions for high pressure valves and flash gas valves . RACA
A comparison between and electronic expansion valve ( EEV ) and a pulse width modulation expansion valves ( PWMs ).
EEVs are the most advantageous solution , adding value in terms of energy savings in both the immediate and long term , and thus savings in total operating costs .
QUOTE OF THE MONTH
“ Gratitude unlocks the fullness of life . It turns what we have into enough , and more . It turns denial into acceptance , chaos to order , confusion to clarity . It can turn a meal into a feast , a house into a home , a stranger into a friend .”
– Melody Beattie
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RACA Journal I January 2021 www . hvacronline . co . za