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
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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
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On Mon, 25 Aug 2008 14:23:14 -0500, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:
SNIP

Yeah, that would be much better than the -40 C stuff, and besides it's easier to get..
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Ooooh, sneaky _and_ ornery; I like that in a person. ;)
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Steve Lusardi wrote:

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
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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

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says...

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.
--
Cheers,

Bruce in Bangkok
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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

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

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says...

I think you'll find it worth the money. It takes you from wanting to have a fridge through each and every step in sizing, design, building, measuring heat loss, cooling requirements depending on water temperature. Just about everything you have been asking about. There is even a step by step overhaul section on most common compressors.
And, the guy is a good enough writer that you can almost read the book for entertainment. He has also written a book on electrical systems that is interesting and factual.
The author has been living aboard and cruising for quite a few years now so everything he recommends is based on experience and I found both the refrigeration and the electrical books worth paying for.
--
Cheers,

Bruce in Bangkok
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On Tue, 26 Aug 2008 13:45:53 +0200, "Steve Lusardi"
<snip>

Steve, they must have saw you coming on this one. Just in case the deal falls through and you can try again take a look at this search:
http://www.abebooks.com/servlet/SearchResults?an lder&sortby=2&tn=Refrigeration+for+Pleasureboats
There were several new copies for under $20 when I visited.
An even better place to go looking for the best price is here:
http://www.bookfinder.com /
Give these places a whirl next time you need an odd book :)
--
Leon Fisk
Grand Rapids MI/Zone 5b
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On Wed, 27 Aug 2008 14:42:54 -0400, with neither quill nor qualm, Leon

Don't forget www.ISBN.nu and www.ebay.com . I get lots of 1-cent books shipping for $2.36, the total price essentially covering Media Mail s/h on eBay (and Amazon $4 total.)
-- Smokey the Bear's rules for fire safety should apply to government: Keep it small, keep it in a confined area, and keep an eye on it. --John Stossel in _Myths, Lies, and Downright Stupidity_
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Steve Lusardi wrote:

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
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Jon, Effectively, I am building an old fashioned ice box and these plates will be atached to the walls of the well insulated box. The air flow inside the box would be convective, just like in your freezer at home. All the plates would be in parallel. These cold plates are holding plates and I suspect they are filled with brine, as they were probably used as a refrigerator. In order to use these plates in a freezer efficiently, the solution will have to be changed to one that changes state at a lower temperature. You stated that the expansion valve must have the same freon on both sides of the diaphragm. I hadn't thought of that, but it makes sense. I know I must use the most appropriate freon for the task, which means that I will have to buy new expansion valves to match that choice. Good info. I understand the difficulty in calculating the heat load, but I don't think the ice box itself has much to do with it. I would think the volume of the cooled fluid medium is the 90% answer. The rate that medium has in warming up is not very relevent, because the system will not be operating then. It is also unreasonable to think that these holding plates will freeze solid at he same time, but if each has its own control valve it shoudn't matter. Steve

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Steve Lusardi wrote:

They CAN be refilled, but that's not for the amateur. Good info. I understand the

OK, I gather the compressor will only be run a couple times a day, and the brine (or whatever state-change fluid) needs to keep it cold between runs? Is that the basic idea?
Very concentrated brine can probably be made up with something like calcium chloride that will freeze out at some really low temperature. I don't know if you can get to -40, but definitely well below 0 F. The cooling capacity of these brines drops as the water content drops, though. Oh, yeah, litium bromide would definitely work! But, it might be way too hazardous to use near food. If there was any leakage, you'd get REALLY mellow on all that lithium.
Anyway, I'm not a pro at this, but I think you will have real trouble making it to -40 with an R-22 system and any appreciable heat load. It will eventually get there if you can keep the doors shut, and there aren't any air leaks.
One other thing I discovered some years ago is that some foam insulations get saturated with ice. You know this has happened when a chunk of foam suddenly weighs about 40 Lbs! It is HELL to get the water back out of the foam, too.
Jon
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this.

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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
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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
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message

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would
That's not nearly enough information. You need to know the heat load at pretty much all operating conditions. You also need to know the condensor operating conditions. For a job of this nature, I doubt you'll ever gather enough good info to do it.
Lacking that information, and doing it simply by 'overkill', you'll have to follow Jon's advice, and keep the compressor running any time either TEV is open.
You can do that by adding a low-side switch that monitors the evaporators' exit pressure, and turning on the compressor any time it goes higher than your desired preset.
LLoyd
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On Aug 26, 7:24 am, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:
in

The problem is that air conditioning suction pressure needs to be much higher then freezer suction pressure. As indicated in my post something like a POA valve is needed so the air conditioning evaporators operate at a higher pressure then the freezer evaporators. You still can use thermostatic expansion valves to control a constant super heat of the refrigerant in each evaporator.
I have not worked this industry since 1977, but my recommendation is to keep the two systems separate down to dual compressors, and condensers. Each sized for the application, and operating temperatures. ignator
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"Steve Lusardi" wrote:

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
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