View Full Version : TXV vs CPR

21-03-2015, 04:00 AM

What will happen if I install aMOP txv that employs aCPR

Glenn Moore
22-03-2015, 08:35 AM
Why do you need an MOP valve and a CPR they both do a similar function but the CPR is the better control as it can be adjusted to suit the actual compressor suction limitation ?

23-03-2015, 08:37 AM
And ...MOP valves are for restricting heavy load conditions in one or multiple circuits ( such as an Ice bank) that will easily overload a compressor and condenser on start up. MOP 45...90 etc

CPR valves and a suction accumulator are commonly used to limit the higher suction pressure on a low temperature room after every defrost or thermostat cycle. ( Fast recovery.)

The difference? CPR's act as an Evaporator Pressure Regulator (EPR) while NON MOP valves flood the Evaporator....raising suction pressure and corresponding boiling point.....and the thermal response from the TX thermal element.

Crankcase pressure regulators only see incoming pressure.
I only detailed to some extent on Glenn Moore's response above.

PS: Viking's Questions in another thread had me beat :eek:

Glenn Moore
23-03-2015, 06:52 PM
To fully clarify the difference between MOP expansion valves and Crankcase Limit Valves or (CPR's).
Crankcase limit valves are fitted in the suction line close to the compressors suction intake port. This means as soon as the compressor starts the Crankcase valve will immediately throttle to limit the suction gas pressure entering the compressor to the pressure limit of the compressors motor full load current. As the suction pipe volume is fairly small the valve controls this max pressure limit into the compressor almost immediately thereby the compressors motor never goes into an overloaded condition thereby safeguarding the motor windings. These valves should be used when there is one compressor with multiple evaporators in the system. Once the evaporating pressure has dropped below the set point of the Crankcase valve the valve will be fully open and the system running on full duty

MOP expansion valves have a limited charge in the bulb, by limiting the fill charge inside the bulb various MOP points can be manufactured. MOP =Maximum Operating Pressure or Motor Overload Protector !
Danfoss for instance have several types of MOP Charges such as type NM range -5 to -40C evap range , NL range -10 to -40C range and B range -25 to -60C
So you have to select the correct MOP valve range to match the evaporating limitation of the compressor motor fitted to the system. MOP valves are used normally on one to one systems ie one compressor and one evaporator this is due to the fact that if multiple evaporators are used with one compressor, the combined gas volume of the evaporators ,even though the MOP valves are closed may cause the compressor motor to run in an overloaded condition for some time before the compressor can pull the pressure in the evaporators down to a level where the motor is no longer in an overloaded condition.

MOP also suffer from charge reversal , where if the bulb becomes warmer than the power element then the charge migrates and the valve closes.
Also when using multiple MOP valves in a system it is necessary that all superheat settings are identical on the valves, if the superheat springs are set differently then when the compressor starts the suction pressure falls down to the valves MOP point, the valves with a low superheat setting will open causing the suction pressure to rise and stop the valves with a higher superheat setting from opening, So you end up with some evaporators working well some poorly and some not at all.
So on multiple evaporator systems a Crankcase limit valve with universal charge expansion valves is the ideal solution.

23-03-2015, 10:14 PM
Thanks guys. I have been refreshed on how a MOP and CPR works....but none of you have answered my question. Can they be installed together??

Glenn Moore
23-03-2015, 11:07 PM
Yes you can use both, but whats the point in doing that. The TEVS MOP point will be at a fixed pressure condition and the CPR will be at a similar pressure condition. How they inter react depends on what setting you make on the CPR . I expect massive pressure swings as the 2 devices try to control the system. If this is a low temperature plant the extra pressure drop in the suction line will cause a lose of capacity

23-03-2015, 11:23 PM
As per Glenn.
Recently installed a large cold store with multi MOP TXs as well as a CPR as concerned with suction pressure after defrost. CPR was not necessary and I removed it from system and reduced suction line losses.

24-03-2015, 04:51 AM
hi magoo
as you know in large system .
we have multi compressor with muli Evap. each compressor has own unloading system that protect them also in starting from over loads .
mainly we control separators pressure so we have not high pressure in suction lines.
during defrost times with hot gas - return liquid goes to separator .
CPR is very useful for those compressors has not got unload system in starting time in low room temperature Also if you want to run your room with different temps or your room temp( EVAP. temp) is higher than compressor running rang.

24-03-2015, 06:13 AM
Hi The CPR releases a constant pressure to the Evaporator whereas a TXV is dependant upon the temperature of the Evaporator outlet where its thermal bulb had been installed,so whenever the temp raises up the bellow of txv expand thus allowing refrigerant to flow through it but when the temp of evaporator decreases the bellow contracts thus disallowing the refrigerant to flow through it.However txv are used in system were load varies but cpr are used were load is constant.

25-03-2015, 04:04 PM
Carrier Transicold and TK use both.CPR have lower value than MOP.Reason is cooling after defrost mode