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Old 05-07-15, 10:47 PM   #21
jeff5may
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The main question here is: how much utility would you like to get out of your experimental unit? Will it be compact or spread out? Recirculation of hot water through the condenser or heating cool water? Indoors outdoors, or both? High pressure cutout or thermal cutout? All these questions will bear directly on your condenser specs.

Here is today's deal:
Surplus City Liquidators

Swep has an app that will model your conditions and tell you exactly how much heat the model in question will move. Go fish.

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Old 05-08-15, 03:07 AM   #22
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Ok, so you need to know some maths. I'll run an example for you.

I have a flat plate condenser on my ducted system at home. Here's how I sized it.

I have water available to me at 19C. I have a bore pump capable of giving me ~0.5L/s (30L/minute).

I'm supplying superheated vapor from the compressor at about 37C, and I want to condense as close to the water temp as possible to lower the compressor power draw. I've got 19C water, so I'll aim for an approach of 2C (Condense at 21C).

15,000 Watts into water at 30L/min will raise the 19 degree water to 26 degrees.
So, water in at 19C, vapor in at 37C, Water out at 26C and liquid out at 21C. My LMTD is ~12.
U value of a water cooled condenser is ~150 (rule of thumb and works pretty consistently across tube-in-tube or flat plate). My Watts/M2 of surface area is U*LMTD (or 12*150 == 1800). I want 15,000 Watts, so 15,000 / 1800 == 8.3 M2 of heat exchange area.

So my HX wants to be ~8M2, which is both big and expensive. Having said that, I bought it, built it and the results measure pretty close to the maths.

Now the only thing that was a thumb suck was the discharge temperature of the compressor. I honestly forget how I calculated that, but from memory I used coolpack (it was 4 years ago now).

The biggest factors are the U values. U for a water cooled condenser is roughtly ~150, and U for a water contact evaporator is ~800. I verified the evaporator by making one from a coil of pipe and building a shell & coil evaporator for a water chiller. Only ~600W capacity, but again the results measured close enough to the maths.

When I was sizing the condenser I sent the numbers to SWEP (heat load, incoming water temp) and they came back with numbers close enough to mine to validate my maths.

My next additions are a bigger compressor, a SLHX and a de-superheater to pre-heat some hot water. but as you can see with a 37C discharge temp there is not a massive amount of superheat to extract.
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Old 05-08-15, 10:17 AM   #23
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Quote:
Originally Posted by BradC View Post
Ok, so you need to know some maths. I'll run an example for you...
BradC,

Welcome back to the Forum! I really missed your experience and clear insights.

-AC
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Old 05-11-15, 12:52 AM   #24
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Personal experience:

I tried a plate hx with threaded connections. I used a thread sealer and an impregnated pipe 'string' (I use this in day to day plumbing with wáter and never get leaks).

No problem at all on the wáter side but the refreigerant side leaks on both connections, a very slight leak that is undetectable to testing with soapy wáter but became evident after about a month or 6 weeks of 24x7 use as there was oil seeping from both the joints.

Mini Project for the summer - braze the two joints.

If you are concerned about wanting to modify your system then why not braze a short piece of tube to the hx and then put on a flange nut fitting?

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Old 05-11-15, 02:21 AM   #25
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Most of you have seen these before.






Brazing good. Threads bad.

With the liquid line sitting a connectors radius from the bottom of the HX I get a nice pool of liquid refrigerant in the bottom of the HX to get some subcooling. As the HX holds 18L (or about 8.8KG of propane I don't need a receiver either.

<edit> Here is an OpenOffice spreadsheet I posted in 2012 that automates the calculations. The most complex one is the LMTD, but it makes sizing a heat exchanger pretty easy. I've used it quite a bit in recent times, but I'd forgotten I broke it out into a separate spreadsheet and put it up for critique.

http://www.fnarfbargle.com/private/1...y_HX_Calcs.ods

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Old 05-11-15, 07:56 AM   #26
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Quote:
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Here is an OpenOffice spreadsheet I posted in 2012 that automates the calculations...
I was able to open the file in Gnumeric, no problem.

Thanks, BradC!

-AC

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