How do you design/build, know if the right application?

[paul_h]

I've been in this game for a while now. Never really studied much theory, down here you do some practical work, learn safety, then you're off on the tools. I've read the books and think I've got some infofrom the, but I never have to apply the theory a lot.
I never really worked with interesting stuff, never seen techical data or manuals written by the suppliers etc.
Most of my work is electrical/electronic. I get manuals with fault codes, I know what pressures and temps to get, and thats it.
I know how to set superheat on TX valve, and to chuck them out if they play up.
So I'm generally a fixer, ie. leak repairer, wiring fault finder.

What to do if you need more than that? (one of the reasons why I joined this forum).

I got sent to a small yacht recently, it has a small condensing unit (model unknown but similar size to a two door glass display cabinet if you're wondering).
It ran two cabinets, the first was a ~100L bar fridge, the second was a tub built into the boats fibreglass on the rear deck, around 40L
Both had universal thermostats hooked up the liquid line solenoids, with the condensing unit cycling on LP control.
Both had small static coils, 3/8 tubing covered by a steel plate. The rear deck tub coil was about 40cm by 50 cm (the tub was about 50cm W x 50 cm H x 30cm D).
And that coil was only on one side, the thermostat was secured to an ajacent fibreglass wall.

The boat owner only recently bought it, and was told the rear deck tub was a freezer.
His problem was it never cycled off, ie. the rear deck tub never got to it's -15C set point.
The condensing unit was OK, the charge seemed OK, the TX valve was hunting (-9 to -16C at the bulb, 60kpa suction pressure)
The TX was a danfoss TF2, R12 model, #01 oriface.

I guess my question is I don't even know if the box with that coil could even get to -15C
Is it just a case of a faulty valve, or bad equipment design. How to find out?
In my personal opinion I couldn't see -15C setpoint happening, this is just a fibreglass tub, the bottom of with the tub in the generator/engine room.
The cold plate was only on one side, the fibreglass wass only 5mm think with no other insulation. The condesing unit was a fair bit newer than anything else, so had been relatively replaced recently. The refrigerant/oil smelt rank, like this unit was about to burn out too.

I've got no idea on whether someone just turned the thermostat down on that tub and pronouced it a freezer or not, hence causing these problems. And I, nor the new owner know if that's the case.

[sparrow]

Hi Paul firstly I would like to comment on your experience to date, I will be honest and say without a doubt that your threads are always very informative and enlightening. I have enjoyed reading many facts submitted by yourself and would like to point out that there is no substitute for experience.

I thought about this problem you have I definitely think that the load being placed upon this little tub is all too much, co efficient of transmission, the u factor, what would it be the fibre glass next to the engine compartment, no wonder the compressor oil looked and smelt as it did.

Could insulation be applied to the outside of the tub?

What do you think? Sparrow

[Bones74]

Hi

We design a bit, and I highly recommend the Modern refrigeration book. It's dry but I believe in chapter 15, might be wrong there is a section on sizing and heat loads. Great read. It should help out the mystery behind sizing. As to your issue, Has this system been contractor redesigned? Curious, before everyone tries to figure this one out.

[Contactor]

Hi Paul

What are the stats on the solenoids doing, also is the lp being used for temperature control? I'm no expert, just trying to figure out the controls.

[paul_h]

Hi Paul

What are the stats on the solenoids doing, also is the lp being used for temperature control? I'm no expert, just trying to figure out the controls.

Thermostats shut the solenoids. So when one is down to temp, no more refrigerant to that system. When the other gets down to temp it closes it's solenoid, no more refrigerant to the compressor, so the compressor cycles off.
The LP at the condensing unit is a safety device, and also a cycle off device for when both cabinets get to temperature and shut their solenoids.
There's not a problem with this control system, just explaining it for you as it's a common system for refrigeration, even if it's a single evap setup.
It's basically a "pump down system", thermostat controls liquid line (or suction line in some horribly designed systems I work on) solenoid valve, the condensing unit LP switch performs double duty as an off switch for regular cycling, and as an LP safety.

[Contactor]

Ok thanks. Someones been chopping around then I reckon, maybe didn't finish the job... sounds a bit like a domestic fridge with a cold box on top.. are both the tx valves the same?

[philjd26]

hi paul,
i probably way of but you could get the compressor kw/model and tev size/model give it to a wholesalers and they usually have a sizing chart to work off!they let you know straight away whats capable of.. too me it might be an orifice problem..

cheers phil

[750 Valve]

Paul an R12 freezer is not a really great performer... this is why r502 came about - R12 and its replacements are for all intensive purposes medium temp refrigerants. Unless you want to run in a vacuum R12 on a static coil won't keep much lower that -18C. MP66 was delveloped as an R12 replacement for freezers, you may get -22 odd out of it in this application. back in the day when R12 freezers were the go HACCP wasn't even thought of and I doubt there were food safe stooges running round with digi temps (no digi temps either!). Likely it is a seafood freezer and won't require -20 operation so check out the freezing point of some seafood products and go from there. Also I've seen this application a few times before - usually with a milk and ice cream well but same theory and it really helps to run the MT load on EPR, it keeps the SST more constant as the epr will continually bleed a bit of pressure thru rather than cycling on and taking the sst above the freezer sections requirements. This allows the LP to be set for low temp load cut out/ cut in and the MT section will always be satisfied. To get the KW its rated at try with the TXV specs (TX2 @ say -25sst) and comp specs (check with mfg catalogue for -25 sst). To get the actual KW its doing is a little harder - it can be done using air on, air off conditions and enthalpy values, a bit hard for a static coil setup though.

I don't know what TAFE SA has to offer as far as engineering but most other states have a post trade HVACR engineering diploma. Its very informative and helps to fill in these blanks that are often left from trade school, its usually 2 nights a week for 2 years - can be a real pain to get to but is very rewarding. If you can't get into TAFE or they don't do an engineering course in SA let me know, I can swing some of my old texts your way.

Coolpack is a great program to learn a bit on, at first you may need to use the tutorials, its a free program and you can get it from here

[expat]

I doubt if the origional purpose of the tub was a freezer.

My guess is it's a fridge for putting your catch in.