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HVAC Gurus!! Tiger? UserName?

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CreePinG_DeatH

Member
Joined
Aug 17, 2001
Ok looking around in the grainger website.... Is there a difference between BTU, Btu, Btuh and BTU/h? I don't wanna be looking for the wrong thing.

What is the best, or better refrigerant to aim for?

Does the Horsepower and such matter if the compressor still handles the 860 btu's I need?

What about High temperature compressors/low temperature compressors? Does that mean the operating temps of the units?
 

DoGMaN

Member
Joined
Aug 20, 2001
Location
.ca
Couple things too look for when buying a condensing unit. First check to see if it moves the requested power (BTU/h) at the specific temperature you are wanting (say -30*C). Probably the "best" refrigerant to aim for is R404a. Its still farily cheap and easy to find and is much better than R134a. Horse power doesn't matter much. As long as it can handle the requested load at an evap temp that you are happy with. 860BTU/hr is about 270W. While doing some research I found a couple Tecumseh units that would handle the loads of a computer very nicely. Model: AEA2410ZXAXC 1/4HP 115v60Hz 400W @ -40*C R404a

Probably a 1/6hp or 1/8hp would do.
Hope this helps.
 

Tiger

Member
Joined
Apr 6, 2001
Location
UK
D-man has been quite concise. There are a few things you should be clear on in you own mind before you start looking around. Firstly what your target temp is. Most air con units which have big compressors etc are not designed to pull low temps (-30C) If you get one of these bear in mind you might be involved in some serious re-design to get the target temps you may want. That's what we are trying to get to grips with in the Project X thread. Most domestic refrigeration units will handle most procs with ease provided we can crack this condenser/evaporator ratio riddle.
 
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CreePinG_DeatH

CreePinG_DeatH

Member
Joined
Aug 17, 2001
if 860BTU/h is around 270 watts, then I will most likely need a 8600 BTU/h compressor. I would like the temp of the water mixture to stay around -20*C I guess. Any ideas?
 

DoGMaN

Member
Joined
Aug 20, 2001
Location
.ca
What the hell are you trying to cool that you need 8600BTU? You'd be looking at like 3/4HP R404a condensing unit. Like $2000
 
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CreePinG_DeatH

CreePinG_DeatH

Member
Joined
Aug 17, 2001
Well I was told in a previous thread that compressor setups are only about 20%-25% effecient. So if I was going to use an 860BTU/hr compressor and think I was cooling 270 watts, I would only be cooling 270*0.2= 54 watts... that's not alot. Was I misinformed? I am looking to build a chiller that will cool about 500 watts.
 

DoGMaN

Member
Joined
Aug 20, 2001
Location
.ca
Most fridges are about 880BTU/hr, thats a full sized fridge with freezer. Even 500W is a huge heatsource. My XP 1800+ @1800Mhz 1.85v is only 83W add 2x 156W pelts and you are still at > 400W. This has some nifty info. The thing is, if it can't handle say 800Btu at an evap of -40 it probably can handle 800BTU at say -20 or -25. The unit I am using (in Project X) is a 1/5th HP Embraco R134a unit. Its rated at 129W @ -30*C. I figure I'll never see -30, but -15 to -20 is possible. I won't be using any pelts at first, just the cpu. If you are smart, you'll just get a freezer and use it as your chiller. All this DIY stuff is getting mad complex. :beer:
 
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CreePinG_DeatH

CreePinG_DeatH

Member
Joined
Aug 17, 2001
500 watts may be a large heatsource, but I plan on using this for a long time. I was thinkking about having a dial-in setting so that I can increase the cooling power as the need arises, whether it is caused by something as little as pushing the CPU too far, or using faster CPUs in the future.

But I still need to know what I should be looking for if I wish to cool 500 watts... as in what BTU/h rating is correct?
 

starscream33

Registered
Joined
Feb 26, 2002
I'm assuming by a dial-in setting that you mean a thermostat?

Sorry to break the news to you, but a thermostat doesn't change the cooling power it just turns on or shuts off the compressor at a given temperature. If you add heat and the compressor is off, then the thermostat will kick it on. In other words if the compressor is off, and you turn on your computer, adding heat, then the compressor will probably kick in. You won't be able to cool the computer more than the HVAC system can handle just by turning a knob.

I hope that didn't come across bad...that wasn't my intension.
 

starscream33

Registered
Joined
Feb 26, 2002
I'm assuming by a dial-in setting that you mean a thermostat?

Sorry to break the news to you, but a thermostat doesn't change the cooling power it just turns on or shuts off the compressor at a given temperature. If you add heat and the compressor is off, then the thermostat will kick it on. In other words if the compressor is off, and you turn on your computer, adding heat, then the compressor will probably kick in. You won't be able to cool the computer more than the HVAC system can handle just by turning a knob.

I hope that didn't come across bad...that wasn't my intension.
 
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CreePinG_DeatH

CreePinG_DeatH

Member
Joined
Aug 17, 2001
I know you can't cool beyond the maximum rating of the compressor, but you can cool less. I will be able to adjust the amount as the wattage increases.
 

Tiger

Member
Joined
Apr 6, 2001
Location
UK
Its probably wise to get into this issue of the compressor size at this point. While the the compressor is an integral part of the system it is not the compressor alone that determines the efficiency of the system.
Refrigeration system are designed around phase change. The basis of the whole system revolves around the Energy/pressure charactaristics of the particular refrigerant used.
On the heat loss side it is essential to determine what the maximum amount of energy loss can be achieved via the latent heat of condensation. By looking at the energy/pressure graph for the particular refrigerant you can determine the maximum amount of heat loss by getting the peak linear pressure. The next problem involves the volume of refrigerant being used. The larger the volume the greater the stroke of the compressor that is required. So the greater to volume of refrigerant the greater the power of the compressor. Most compressor manufacturers sell their compressors and condensers as a package for a particular refrigerant. So if the refrigeration manufacturer is using R134A then there is a specific range of compressors/condensers available in various sizes depending on the volume of refrigerant to be used. The pressure in the condenser is the same for all of them. So none of them is more powerful than the next.
Lets use an example. Say we take a 110W compressor based system that uses 100cc of 134A. The condenser pressure will be around 13 bar with a temp of around 50C and the evaporator will have a temp of around -10C. If we have a 220W system that uses 200cc of R134A the condenser will still have the same pressures and temps and the evaporator will still be -10C.
If we take the 220W compressor and attach it to the 110W piping system we will have a disaster. The pressure will be way too high and the vapour will not condense therefore no liquid refrigerant therefore no refrigeration.
Sorry for the long story but each system is a complex arrangement of various components that go to make a whole.
 
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CreePinG_DeatH

CreePinG_DeatH

Member
Joined
Aug 17, 2001
So tiger, are you saying that I should look for systems that are already assembled? Forgive my question, I am tired as hell right now so I might have missed the point of the post...
 

Tiger

Member
Joined
Apr 6, 2001
Location
UK
CreePinG_DeatH said:
So tiger, are you saying that I should look for systems that are already assembled? Forgive my question, I am tired as hell right now so I might have missed the point of the post...
That would be the wise route. One of the problems faced by most going this route is the actual size of the evaporator. I have a design for a two stage system for high wattage units. Basically the concept is to use two systems with the same end temp. Lets say you want -25C which is kind of average for freezer type units and a single domestic unit cannot cope with the given heat load. Using an existing under counter frdge case take both refrigeration units and install them in the same case. There is room in the bottom for two compressors. Place the one evaporator in a top tank that cools the returned coolant and build in an automatic overflow pipe to feed a lower tank where the second evaporator is housed. Link both comprssors to the single thermostat contolling the lower tank. This is all based on chiller type units. I wouldn't like to be paying the electricity bill though ;)