Oil return / refrigerant velocity in brazed plate evaporator?

Hello All,

Not been on this forum for a while as have been working on other things.
I'm building a small water-to-water heat pump which uses a brazed plate heat exchanger as the evaporator.
I would like to know if anybody has had problems with oil return to the compressor when using a brazed plate heat exchanger, or can offer any guidance on the required refrigerant velocity within the heat exchanger?
The evaporator will be operating with at least 11K of superheat at the outlet so a reasonable proportion of the upper part of the evaporator will contain vapour, the velocity of which is likely to be quite low (less than 0.5m/s). This would too low for a suction line.
Given that the channels in a brazed plate heat exchanger are much smaller than the suction line, does this mean I can get away with lower velocities?

Any help much appreciated.

James.

P.S. The system uses R134a and PVE oil.

I'd be concerned where the local velocity, for a vertical surface, drops below 0.5 m/s.

You may want to chat with your evaporator designer/selector & make sure that this requirement is met, to prevent oil backing up in the evap.

Perhaps there would be a way to trap the excess oil off the base of the evap & transport it back to the compressor?

Lie the BPHX on its side,

Quote:

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Originally Posted by Magoo

Lie the BPHX on its side,

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Hello,
I thought about that, hower this paper:

http://fire.nist.gov/bfrlpubs/build97/PDF/b97097.pdf

put me off the idea as they found a drastic reduction in evaporator performance when in the horizontal position.

Removing the oil from the bottom of the evap is a possibility, however this system has to fit into a very small space envelope so I'm trying to keep it as simple and compact as possible.

James.

I used it a lot.
if you are using in vertical position and it is better you supply form bottom (expansion valve ) and from top goes to suction separator ( suction accumulator) and then goes to compressor. to return of of oil in this position you have to put small pipe 1 mm2 after expansion valve inside of brazed plate and other end goes to outlet pipe of brazed plate to suction separator ( this small pipe carry all oil might in low side of plates ) .
if you are going to use in horizontal -- outlet pipe of plate should be deep inside plate .

had a lately problems with oil return in milk tanks with 15 condensing units.
The solution i came up to was to add between 1% to 2% of propane to the system. It did the trick and all units work perfect for a few months now.

Did you remember to add oil to oil separator and liquid accumulator if you have any of these in your system?

Quote:

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Originally Posted by chemi-cool

had a lately problems with oil return in milk tanks with 15 condensing units.
The solution i came up to was to add between 1% to 2% of propane to the system. It did the trick and all units work perfect for a few months now.

Did you remember to add oil to oil separator and liquid accumulator if you have any of these in your system?

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chemi-cool,

You havn't been on holiday in Vietnam working on containers lately have you?

Dear chemi,

are you sure about that.
I have not heard before this is my first time.
we vacuum system as much as as we can , then you are saying we put propane in our system???
please give us more information about that.

Quote:

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Originally Posted by mbc

Dear chemi,

are you sure about that.
I have not heard before this is my first time.
we vacuum system as much as as we can , then you are saying we put propane in our system???
please give us more information about that.

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In my case, R-507 is used, flooded evaporators and SB38 compressors. I have added extra oil, about 1.5 KG so receiver and liquid separator will not hold too much oil. Still there was problems of oil return, [evaporating temp is -5C ] I did not want to loose any compressor so stopped all units, collected some colleagues and we were trying to find a solution. Someone suggested it as propane is known for a good oil carrier.
Every circuit has 6 KG of R-507 so I added 150 grams of propane and the oil returned beautifully to the crankcase.
I'm not worried about explosion - I'm not there!! :)

R290 plus R134a mixtures employed a lot; some of the wonderfluids used 10-years and more ago, as drop in for car air conditioners to replace R12 contain a lot of propane....Also some small commercial and domestic type cabinet units.

Check your lubricant compatibility, and "soft part" and drier fill compatability (though that would be a rare issue) but at these temperatures transport of lubricant should not be a big issue. You will have to manage concentration and should the fraction of R290 get over a couple of percent it may effect superheat and it will want to work a fraction high on the Discharge Pressure side. And you should label the circuit as containing some R290. Just so the next service type doesn't start brazing up a leak with a little residual refrigerant left behind....

Thanks for your replies, some very interesting suggestions.