View Full Version : Low, Medium or High back pressure compressor?

03-02-2014, 12:16 AM
Hello all,

I'm trying to spec a condenser section for a small (internal diam 1.85m x 3.55m x 1.35m ) fridge trailer I'm building. And looking for some advise on what sort of compressor would be best suited?

Basically I built one and I rent it out as a portable cold store, mostly for weekend events and cold store break downs. Most of the time the required temperature is around +2 for fresh food and drink. But on the odd occasion I get asked for a freezer, so now I'm building a second trailer what should I do?

Do I spec a frozen LBP unit capable of -18C and run it at +2 most of the time, what damaged could it do to the compressor if any, running outside of its parameters? Or spec a over duty MBP unit and on the odd occasion set it to run at -18C.

The first trailer has a MBP open type condenser section and the unit is well over duty as the unit came up second hand at the right time and right price so I made it fit rather than spec'd it for the cold store. But it does all work well for how it gets used.

Any advise or ideas welcomed.


03-02-2014, 12:41 AM
think you just answered your own question?

03-02-2014, 11:25 PM
But if I want to be able to set the unit between -18 and +2 is there a option or which configuration is less likely to cause damage to the compressor?

Or should I just stick with a chiller and not think about trying to provide a freezer option?

03-02-2014, 11:55 PM
Select for highest load condition, MBP and add an oil cooler or compressor cooling fan for low temp., stops frying compressor with low SST.

The Viking
04-02-2014, 12:01 AM
There are options, there are always options...

Bypass valves, suction pressure regulators, oil coolers to name a few. This would mean designing the system as a freezer and "degrading" it to operate as a refrigerator but the running costs will be higher and the design as well as the controls will be more complicated.
Might be worth looking in to how they do it in the Transport sector, there is probably a good reason why there is less than a handful of established manufacturers there but their kit normally do what you are asking for.


04-02-2014, 12:43 AM
I have a transport refrigeration back round, working on Carrier and the usual direct drive suspects. Maybe that's why they all go through compressors.

Like you say lots of different things can be added, if possible I like to keep it simple though, less to go wrong.

What about a MOP expansion valve?

04-02-2014, 01:37 AM
Generally transport equipment is build like brick out house for multi- operational modes. Compact and lots of air over compressor, that has deep and finned sumps etc., for heat dissipation and oil cooling. The whole compressor has finning in castings.
MOP TXV are OK but select for highest loading. Low temp conditions equals low vapour volumes and less cooling effect at compressor, hence need for extra cooling for compressor and particularly oil that cools compressor.

charlie patt
04-02-2014, 07:35 PM
for what our trailers run, they dont run long enough do do any long term damage i converted me first one in 2003 useing a caj2446z freezer comp the reason i used this was the biggest comp i could fit that would run well with a 16/13 amp supply i always fit a cpr to protect comp and the trailer runs chill and freeze i would always fit freezer kit as its a possible rental option if its out everyweekend overal that adds up to a total of 15 weeksrental so not a major issue we build one or two a year now and never fitted a comp yet, it really depends on your market our spot rentals are mainly chiller but when we have a commercial freezer go down its a real godsend

04-02-2014, 08:35 PM
Thanks for that reply charlie patt, I was figuring the same, the hours it'll run per year are minimal compared to a static cold store. Think I'll go with that then.