electronic expansion valve

[GENERALCOOL]

Dear friends,

Have a question.

Regarding electronic expansion valves, Is it actually 100% prevent floodback or it is same as thermal expansion valves but is very faster and more accurate ?

Sincerely yours.

[Brian_UK]

I think it depends on how they are set up as you can adjust the closing of the valve.

[mbc]

Hi
If you set right all function it should do it .
But for more safe work it is better you have suction separator (suction accumulator).
Also careful pipe slopes
Danfoss is close as same as solenoid valves
But I am not sure about others

[Glenn Moore]

Hi
Electronic expansion valves if set up correctly are more energy efficient than Thermostatic expansion valves . But the same problems can occur with electronic valves (EEV’s) as Tevs
With thermostatic expansion valves when operating with a compressor with capacity control (unloading gear) each time a set of cylinders are energised to load the compressor, the suction pressure will suddenly drop to a lower pressure as more pistons start to pump. This sudden drop in suction pressure will force a Tev to go fully open for a short time as the suction pressure drops via the equalising pressure line which is a “closing force “ to the Tev, so with the compressor loading this sudden drop in pressure will cause the Tev to fully open with the potential of liquid slugging occurring which can damage or break valves and damage pistons and con rods .On these systems the Tev superheat setting is normally set to a higher setting to keep these liquid slug periods to a minimum reducing the risks of compressor damage.
So providing the Tev is set up correctly, the bulb is fitted correctly along with the equalising line then liquid flooding should not occur
With the electronic expansion valve there is no equalising connection, but a pressure transmitter/transducer plus a temperature sensor are part of the control set up.
The temperature sensor should be of high accuracy with a short temperature constant , and the pressure transmitter must be of a good quality and calibrated
The controller is normally of the PID type although the D function is not normally required just the P(Proportional) and the I (Integral)
In the electronic controller there are many parameter settings to tune the EEV to give the lowest stable superheat possible
The controllers also have differing modes of control :- the Danfoss controller has 2 types of control
1) MSS Control in this mode the controller constantly tries to lower the superheat setting to get the ultimate effect from the evaporator
2) Loadap this is tuned to the compressors capacity which can give extremely efficient part load capacity
Apart from these running modes there are other parts of the EEV,s control such as the start up sequence ,the running mode and the stability control around the set point reference.
So to sum up the TEV is a P ( Proportional) control which has 3 signals to make it operate :-
1) The power element which is the opening force
2) The bulb which is a closing force , combined with
3) The superheat spring also a closing force.
So the Tev will simply find a balance between the 3 forces. If the suction pressure varies for any reason this will affect the orifice position, if the bulb is loose on the pipe or has warm air blowing across it ,that could cause the valve to hunt , if the orifice is over or undersized this can cause problems.

With the EEV it is easy to have the wrong parameter setting , often the speed of the motor can cause problems ( steps per second ) total steps etc . The positive thing is when either the sensor or transmitter go faulty the valve is forced to close electrically. Providing the system and valve is clean and small amounts of oil (lubricant ) are circulating then the valve and stepper motor should operate correctly. If there is no solenoid fitted it is a good idea to fit a UPS to close the valve under loss of power for safety reasons.
Some faults with EEV’s I’ve seen include incorrect settings to the suction temperature sensor which caused flood back, seized stepper motors causing both high and low superheat . Mismatched valve to controller. Some stepper valves are direct driven ,some are gear driven . So the power from the controller to the valve has to be correct as the direct driven valves need a higher current to drive the motor as against the easier gear train type. This can cause slow reaction times from the valve.

So the tev is a simple valve which can be easily disturbed by the system pressure / temperature changes as its only balance stability is the spring , whereas the EEV has many parameters than can be tuned to overcome any system fluctuations,.
Therefore both the Tev and the Eev can cause problems if not set up correctly during commissioning.
Learn all you can about the Eev settings then you will gain in confidence to adjust to get improvements in system control and energy efficiency , all to often I have seen EEV’s removed from a plant purely because engineers do not understand how they should be set up , even Tevs are often fitted as out of the box with no thought as to setting them to match the system design

Note “P” regulation is the basic type of regulation which needs an offset to achieve control , similar to a thermostat on/off
“P&I” regulation gives a tight control during start up the P gets the control moving quickly to get to the reference setting, this will give an overshoot past the reference then the I comes in and removes the offset so the control runs along the reference line to give a very tight control
“D” regulation is mostly used in process type control where a large deviation between the reference value and measured value is large and the D gives the system a big kick to get the system under tight control very quickly ideal for close control systems,. Not really needed for controlling a stepper EEV

[hyperion]

Thanks Glenn, great explanation.