Grunert Reefer Overpressure When Engine Room Is Hot

[Fluenta]

Hello,

We have been having problems with our reefer system as long as we have had the boat. It is a Grunert system with a Bitzer compressor driven via a belt by a 12V DC motor. The system is water cooled and drives 2 Crosby holding plates in the freezer and one in the fridge. The whole package sits in the engine room sharing space with a nice big Perkins.

The system works fine at anchor and under sail but when the engine room is warm (after a few hours of motoring ) the overpressure switch shuts down the system. Even a few hours after motoring the system still overpressures. Confirmed overpressure as can force the system to run by bypassing the overpressure cutoff for troubleshooting.

The water pump works to capacity and was replaced anyway. The condenser coil is substantial and was just cleaned. The compressor is new. The engine room has a blower that always is run with the engine. The engine is not overheating. Refigerent level is good (checked by two different techs).

We have had four (!) different companies look at the system with no joy so seeking suggestions from the forum.

Suggestions so far but not yet implemented:

- wire in a switch so that under power the water cooling could be run independently prior to running the compressor. This would only cool the refigerant in the condenser though.
- suggested that the original shop in Seattle may have mis adjusted the tx valves or the superheat. The Seattle shop switched the system from R12 to 401A. With the problems we were having we switched back to R12 when we put the new compressor in at San Diego in case the the 401A was the problem with the overpressuring.
- insulating the refrigerant lines in the engine room.

Thoughts ! Suggestions ?

Intention is a South Pacific crossing next month so no time to rip the old system out and install something new (not to mention the cost !).

Mucho gracias,

Max

[Richard Kollmann]

A water cooled belt driven compressor/condensing unit should not be affected enough to increase high pressure from engine room heat environments.

Things that can cause high pressure to trip over pressure switch:
Faulty high pressure switch.
Bad electrical relay or wire connection.
Air in refrigerant.
Water turblance under boat near circulating pump inlet.
Refrigerant circulating cooling water sharing a through hull fitting with engine.
Too much refrigerant in receiver and sight glass too full.

Refrigerant line coming out on seawater condenser warmer than 115 degrees F would indicate poor condenser cooling. Grunert used 230 psi non adjustable high pressure switches and switches that are adjustable. Because a well designed marine water cooled system with correct amount of pure refrigerant rarely sees pressures above 200 even in the tropics I would first re-dehydrate system and refill with pure refrigerant and not fill sight glass as full as it is now.

I would not ever bypass high pressure switch. It would be a good idea to connect a high pressure gauge permanently until high pressure is confirmed as cause of compressor stopping. At what pressure does the high pressure switch actually stop compressor? Pressures of up to 250 psi and under should not cause an explosive affect.

I have seen good water flow many times through condenser and still poor heat transfer do to growth of marine life in condenser. Grunert at one time used a homemade condenser like Seafrost. In tropical waters with a condenser smaller then 1 ½ tons like these homemade ones compressor output pressure needs to be reduced by lowering RPM or reducing refrigerant volume. Running your system at higher pressure than nacessary is also wasting energy.

[Fluenta]

Much thanks for the thoughtful reply. My new pressure switch is set at 125 psi so will adjust it upwards a bit to see if that helps. Will also check the temps on the refrigerant coming out of the condenser.

Just ran the engine for 90 min to change anchorages with no problem but put the reefer in timer mode so that it ran continuously.

Thanks,

Max

[Richard Kollmann]

In tropical climates with an adequate size condenser I set high refrigerant pressure switch for water cooled refrigeration condensers to 175 psi. If this is an auto reset model switch the differential would be set to 40 psi.

[Fluenta]

Originally Posted by Fluenta

Much thanks for the thoughtful reply. My new pressure switch is set at 125 psi so will adjust it upwards a bit to see if that helps. Will also check the temps on the refrigerant coming out of the condenser.

Just ran the engine for 90 min to change anchorages with no problem but put the reefer in timer mode so that it ran continuously.

Thanks,

Max

Also, although the reefer ran fine when I forced it to run on timer mode during and before the period of motoring, after it sat in the hot engine room for awhile at anchor it now refuses to run due to the overpressure. Tomorrow will adjust the overpressure cutoff switch.

[Fluenta]

Tested today with the high pressure cutoff set to approx 175 psi (12 KGB/cm2). Still overpressures when the engine compartment is hot.

[Fluenta]

Originally Posted by Richard Kollmann

A water cooled belt driven compressor/condensing unit should not be affected enough to increase high pressure from engine room heat environments.

Things that can cause high pressure to trip over pressure switch:
Faulty high pressure switch.
Bad electrical relay or wire connection.
Air in refrigerant.
Water turblance under boat near circulating pump inlet.
Refrigerant circulating cooling water sharing a through hull fitting with engine.
Too much refrigerant in receiver and sight glass too full.

Refrigerant line coming out on seawater condenser warmer than 115 degrees F would indicate poor condenser cooling. Grunert used 230 psi non adjustable high pressure switches and switches that are adjustable. Because a well designed marine water cooled system with correct amount of pure refrigerant rarely sees pressures above 200 even in the tropics I would first re-dehydrate system and refill with pure refrigerant and not fill sight glass as full as it is now.

I would not ever bypass high pressure switch. It would be a good idea to connect a high pressure gauge permanently until high pressure is confirmed as cause of compressor stopping. At what pressure does the high pressure switch actually stop compressor? Pressures of up to 250 psi and under should not cause an explosive affect.

I have seen good water flow many times through condenser and still poor heat transfer do to growth of marine life in condenser. Grunert at one time used a homemade condenser like Seafrost. In tropical waters with a condenser smaller then 1 ½ tons like these homemade ones compressor output pressure needs to be reduced by lowering RPM or reducing refrigerant volume. Running your system at higher pressure than nacessary is also wasting energy.

Richard,

I can't get a set of gauges until back in a Puerto Vallarta but will definetly gauge this.

The refrigerant levels were reduced awhile ago by a refigeration engineer to reduce the pressure and to try to sort this issue. The issue happens only with a hot engine room but not dependent on the boat moving. The reefer has a dedicated thruhull. Pressure switch was changed out to an industrial adjustable one.

I have the rest of the suggestions checked off as far as I can except the electrical possibilities. When playing with the high pressure cutoff switch I noticed that there was about a 0.5v drop from the batteries to the cutoff switch. I haven't metered out the rest of the system but curious how electrics would cause an overpressure in the refigerent. I can understand how it may cause the motor to run hot and blow a cct breaker but not the overpressure. Certainly not to say I disagree but rather I cannot figure out the scenario.

Much thanks,

Max

[Richard Kollmann]

What is the temperature of high pressure line after it leaves condenser near filter dryer after unit has run for a wile but not yet stopped by high pressure switch? Warm say, 115 degrees F or much warmer to hot. At 115 degrees F pure 134a will have a pressure of 158 psi in liquid line. All refrigeration process heat is supposed to be removed by condenser's cooling mediums seawater thermo conductive transfer. The question is there a refrigerant problem or a seawater cooling problem or an electrical control problem. Temperature of refrigerant liquid line after receiver will confirm or eliminate contaminated refrigerant and problems with condenser, but will not eliminate problem of too much refrigerant in receiver. This size unit and type water cooled system with pure 134a refrigerant will rarely see pressure above 190 psi. The same air cooled system will have its high pressure safety switch set at 250 psi.

To eliminate a false reading from tripping High Pressure switch you must be able to read gauge pressure just before switch disconnects power to compressor.

Forget about engine room heat unless a direct metal to metal or water to water connection between engine and refrigerant pluming exists.

[Fluenta]

Originally Posted by Richard Kollmann

What is the temperature of high pressure line after it leaves condenser near filter dryer after unit has run for a wile but not yet stopped by high pressure switch? Warm say, 115 degrees F or much warmer to hot. At 115 degrees F pure 134a will have a pressure of 158 psi in liquid line. All refrigeration process heat is supposed to be removed by condenser's cooling mediums seawater thermo conductive transfer. The question is there a refrigerant problem or a seawater cooling problem or an electrical control problem. Temperature of refrigerant liquid line after receiver will confirm or eliminate contaminated refrigerant and problems with condenser, but will not eliminate problem of too much refrigerant in receiver. This size unit and type water cooled system with pure 134a refrigerant will rarely see pressure above 190 psi. The same air cooled system will have its high pressure safety switch set at 250 psi.

To eliminate a false reading from tripping High Pressure switch you must be able to read gauge pressure just before switch disconnects power to compressor.

Forget about engine room heat unless a direct metal to metal or water to water connection between engine and refrigerant pluming exists.

Thanks will check temps. Quick note that we are on R12.

[sww914]

Max, I have a set of gauges you can borrow if you need to. Steve on Landfall.
I'd buy some of that lovely pink foam and duct tape at Home Depot or maybe the construction supply store up on the hill above La Cruz and build a box around the compressor to insulate it from the engine room heat to some degree. Then I'd re-route the blower ducting to draw cool air into the new compartment. I'm no refrigeration expert but I'm the king of half-assed workarounds.
BTW, my gauges were only about $45.00 at the (very good) refrigeration store in the El Faro shopping center over by Mezcales.
BTW- 322-104-1863

[Richard Kollmann]

Sorry, with Freon R12 instead of 134a pressure at 115 Degrees F pressure would be 147 psi instead of 158 psi.

[Fluenta]

Quick update from Nuku Hiva, French Polynesia ... Found that we can make the reefer work when the engine room is hot by jury-rigging power to the water cooling pump and having it run for a few minutes without trying to run the compressor. After a few minutes and resetting the overpressure cutoff the reefer will generally run normally.

When I get a few minutes I will put a switch in so I can jury rig it without having to open the engine compartment and an LED to indicate when there is an overpressure condition.

Appears when the reefer is not running it gets too hot from the engine room temperature. If the reefer is running it continues to run fine despite the engine room temperature.

Thanks,

Max