Any refrigeration experts out there?

I wish to use a single large compressor to service 3 cold plates and 3 air conditioning evaporators, but I am not a refrigeration expert, so if my thinking is incorrect please let me know. Each one of these (users) will have their own mechanical temperature controlled evaporator valve. I assume the compressor should be cycled by head pressure with an overtemp sensor for safety. I would like to use a salt water cooled condensor on the high pressure side and a freon gas that changes state around -40 C. Please advise. Do I need an accumulator of some description on either side? If so, what? Each one of the users have 3/8" in/out tubes. How do I size the compressor? Any tips or books on this subject would be welcome. Steve

Reply to
Steve Lusardi
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"Steve Lusardi" fired this volley in news:g8uv3b$4hm$03$ snipped-for-privacy@news.t-online.com:

Yep, you need an accumulator (fairly large one) on the high-pressure side, and that's a good place to put the head-pressure switch.

However, I'd recommend a refrigerant that changes state at about -40F.

LLoyd

Reply to
Lloyd E. Sponenburgh

SNIP

Yeah, that would be much better than the -40 C stuff, and besides it's easier to get..

Reply to
Brian Lawson

You can't realy cycle the compressor off in an application like this. When the compressor is off, the liquid in the evaporator continues to evaporate, and very quickly the low-side pressure rises. Then, ALL the evaporators stop working. With thermostatically-controlled metering valves regulating the temperature of the separate evaporators, you can probably run the compressor all the time (except for fault conditions) and as long as the accumulator has sufficient volume, it should all regulate itself pretty well. The compressor will unload when there's little gas coming in. Are you really trying to achieve -40 (C or F, doesn't matter) with a single stage? That is quite pushing it. R-22 will do it, but at low pressure, so a larger compressor and evaporators will be needed.

You can't size the compressor until you know the heat flow. Knowing the tube size is nearly worthless. You need orifices in the expansion valves to prevent slugging the liquid in the evaporator all into the compressor at once. These set the maximum capacity per evaporator.

Jon

Reply to
Jon Elson

Ooooh, sneaky _and_ ornery; I like that in a person. ;)

Reply to
John Husvar

I'd probably crosspost a request to alt.hvac; those guys are very helpful when someone who isn't a trained hvac technician asks a question. Be sure and tell them you are a homowner, and that you have a done a few DIY projects (like a fence and a water faucet) in the past so they know you would be capable of the work. ;)

Jon

Reply to
Jon Danniken

Jon you are a mean person, hvac guys hate anyone to touch an a/c without a licence, especially a homeowner. They really wish this to become a death penalty crime. This group will chew him up if he does this, a bunch of arrogant (*&^%$) people there.

Scott

Reply to
jano

Half the problem is they think the refrigeration cycle is some kind of arcane magic . The other half is most of the sheeple think so too .

Reply to
Terry Coombs

Jon, Good info. The cold plates I have bought are used units and they all have the expansion valves with them. I have assumed they also have integral spray orifices. I suppose I could apply compressed air to determine approximate flow rates. How else would I test these, as they are not able to be disassembled? The cold plates are used marine units and I want to use them for a freezer, not a refrigerator, hence the -40 requirement. I am aware of the requirement to change the medium in the cold plate itself in order to reduce the temperature of the state change. The goal is to freeze the plates down once a day or less when in use. How do I estimate the heat load? The cold plates are aproximately 24" x 18" x 4". Steve

Reply to
Steve Lusardi

Somehow I get the idea that you are building a "boat installation" in which case try to lay your hands on a book named Refrigeration for Pleasureboats: Installation, Maintenance and Repair by Nigel Calder.

It gives you methods and formula to calculate heat and insulation values, condenser water flow, cold plate area and everything else you need to design and build a refrigeration system for a boat.

Even tells you how to build your own condenser using either copper or Cupro-nickle tubing.

Reply to
Bruce in Bangkok

Yes, they should. The orifice size should be able to be looked up from the part number. Also, thermostatic expansion valves have to have the same refrigerant in the sensing bulb as is used in the system. I suppose I could apply compressed air to determine approximate

What are you cooling? Just the plates? A tank of brine? Making a block of ice?

It gets messy. What is the starting temp of the plates when you begin to cool them? Will you be cooling only one at a time? Would you be keeping one or more plates cold while starting the cool-down process on a warm plate? That's the worst-case scenario. To calculate the heat load, you need to know the surface area of the tubing, and the thermal conductivity of the heat source to the cold plate. If you are hanging the plates in mid air and cooling the air in a room, the heat flow is very low. If the cold plates are attached to metal walls or immersed in circulating brine, the heat flow is maybe 100x higher. You really aren't telling us enough to give you any help.

Anyway, to get to -40 C (or F) you need an evaporator pressure (suction, low-side) of 105 kPa absolute, or just barely above atmospheric pressure. This will cut the performance of the evaporator, as well as making the compressor work a lot harder.

Jon

Reply to
Jon Elson

Reply to
Steve Lusardi

Bruce, Good assumption, that is exactly what I'm building and effectively what I'm looking for is a simple reliable design that fits my requirements. I see this as a good example of spiral developement philosophy. The more I learn, the more I know, the more I learn etc. That book is a good tip. It certainly can't hurt. Thanks. Steve

Reply to
Steve Lusardi

Jon you are a mean person, hvac guys hate anyone to touch an a/c without a licence, especially a homeowner. They really wish this to become a death penalty crime. This group will chew him up if he does this, a bunch of arrogant (*&^%$) people there.

Scott

Reply to
Steve Lusardi

Jon Elson fired this volley in news: snipped-for-privacy@wustl.edu:

(except

trying

expansion

Jon, I agree with most of what you said, but it's not true about cycling the compressor. That's what accumulators (high liquid and low gas) are for. Think about the automotive CCOT systems. They don't have expansion valves at all, but still cycle the compressor, based upon the high-side pressure at the accumulator.

Now, I'll agree that you may need some pretty large low-side capacity to keep the low-side pressure low enough, but it certainly can be - is

- done that way on many systems.

It's not a simple setup in terms of design criteria, but it's simple mechanically.

I wholeheartedly agree that -40 is a tough spec.

LLoyd

Reply to
Lloyd E. Sponenburgh

snipped-for-privacy@cctc.net fired this volley in news:1d295d6f-e8bf-4bc5-9503- snipped-for-privacy@m73g2000hsh.googlegroups.com:

Agreed. Most of them are there to pet their own egos.

LLoyd

Reply to
Lloyd E. Sponenburgh

I'm not an expert, other than maintaining my 2000 bushel cooler for 20 years, but a good choice for low temperature freon is R-502. its used for freezers. Hold on to your shorts when you buy some.

Karl

Reply to
Karl Townsend

Lloyd, These holding plates are filled with a fluid medium and they are around 18" x 24" x 4" . How much high side and low side tankage would be enough in cubic inches to self regulate do you think? Steve

Reply to
Steve Lusardi

Bruce, I ordered the book. It is $65. Must be good. Had to buy a used one. It is out of print. Steve

Reply to
Steve Lusardi

No, the compressor should NOT be cycled on head pressure, only safety devices for over pressure and under pressure should be used for head pressure. Also it can be done mixing air conditioning and refrigeration/freezer evaporators in one system, but you will need many more controls and mechanical systems. You want to have 2 compressors and keep the air conditioning separate from your cold plate application. This is because what ever working fluid you use (R22/502/134A) the evaporator boiling point is a function of suction pressure. For freezer applications, you will cause the air condition coils to freeze, or for air conditioning you will never get the cold plates to freeze. If you are hell bent on this, you will need to control the input refrigerant to the evaporators with electric solenoid valves, I assume you have a 12 volt battery system on your boat, I've seen 12 volt valves. Also I assume you are connecting the compressor(s) to a diesel engine. The output of the condenser will go into a receiver (not an accumulator). There are suction accumulators, which are used to prevent liquid slugging into the compressor. To control air conditioning suction pressure on such a system (one compressor) you will need to use pilot operated suction valves for the air conditioning evaporators, in the output of the evaporator so the suction pressure will not be at the same pressure as the cold plates.

The last time I was sailing on a Moorings bare boat charter, the cold plate system was replaced with a +12 compressor, and this worked so much better then the cold plate of previous sails, it would keep ice cream frozen, where the cold plate would totally defrost between engine recharge cycles, or be one frozen mass of sh*t. ignator

Reply to
ignator

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