advice with freezer

[cretan]

a freezer i have been working on has been giving me nothing but problems.low side pressure goes through the roof while in defrost.Then compressor trips overload.1.5 H.P. compessor.semi-hermetic on 404a.sytem has a expansion valve.my question is this...since there is a expansion valve,should there be a liquid reciever?should there be a CPR valve?The system has neither.

[Samarjit Sen]

Hi Cretan,

Your unit has a hot gas defrost system, and as such you should provide a Crakcase Pressure regulator on suction line. Please ensure that between the cpr and the suction valve of the compressor there is no other controls or instruments.

The TEV you are using must be with an equiliser line as your coil must be having a distributor. Please check if there is a connection for hot gas inlet in the distributor. If not either change the distributor with a hot gas side connection, or puncture the line from the TEV to Distributor and connect the hot gas line.

By not having the cpr , hot gas is entering the coil and is increasing the suction pressure .

If there is any further problem please do let me know.

[Samarjit Sen]

By the way Cretan, do you have an air cooled condenser. If so it is better to have a reciever although this has nothing to do with your suction pressure going up at the time of defrost. It is always adivesable to have a liquid reciever with air cooled condenser. The water cooled shell and tube condenser does not require as it also acts as a reciever.

[Electrocoolman]

Make it a pump down system to prevent high suction pressure during defrost.

[old gas bottle]

errrrrrrrr how will it hot gas defrost if you put it on a pumpdown ? thats for heaters, bit more info needed here,is it hot gas or heaters? if the suction pressure rises dramaticley the discharge should drop accordingly,run amps should not be high either,CPR,s are allways a good idea but will need setting propely to allow the system to get enough heat build up,or is thing on heaters and tripping when it comes off defrost due to lack of dripdown time or whatever ?.

[Samarjit Sen]

Since the suction pressure shoots up during the defrost, the system has got to be with hot gas system.

[cretan]

By the way Cretan, do you have an air cooled condensor

yes,air cooled. System is a heat element defrost. system did not come with a CPR valve. can I add a CPR valve to this sytem?Does it matter if the txv is internally or externally equallized when using a CPR valve ?

or is thing on heaters and tripping when it comes off defrost due to lack of dripdown time or whatever ?.

compressor really grunts and groans after defrost.Pressure were 400psi on high and 60 on low after defrost.ran for 20 seconds then clicked off on overload.......

[lana]

Hi there,

If you have electric defrost then do a pump down before the defrost cycle.
Internally or externally equalized TEVs have nothing to do with the CPR.
I suggest you pump down the system then check the high pressure. It must decrease during pump down. If it rises then the condenser is too small. Definitely Install a receiver.
After defrost there should not be high discharge pressure. I always use electric defrost and never had this problem.

Cheers

[chillyblue]

Hi

Lana is right, also when the defrost is finished you should let the heaters cool down by setting a drip down time, this should be long enough (approx 5mins). During drip down nothing should be operating, no fans, compressor or heaters.
If the unit has an electronic controller and the evaporator has forced air, set a fan delay time of one minute to allow the evaporator to cool down, set the fans stop temperature so the evaporator coil temperature drops to zero before the fans start running.

The idea is to reduce the amount of load on the compressor after a defrost as much as possible.

With electronic controllers there are many things you can do to avoid fitting new componenets to your system.

[Samarjit Sen]

Cretan, if you have a hot gas defrost system, then you must have a cpr. A cpr is always advantageous, as it controls the sudden thrust on the suction pressure.

Using a TEV with internal or external equiliser will depend on your Evaporator Coil. In case the coil has a distributor then it is better to have a external equiliser TEV. In case there is no distributor but only one connection to the coil then an internal equiliser TEV will serve the purpose.

You may visit www.sporlon.com for detail information on TEV and cpr.

[cretan]

thanks for your help everybody.Made some changes and system seems to be working much better.