Condenser outlet lower than receiver inlet?

[Kevin S]

Can anyone point me to the correct guide lines for refrigeration piping design in an application where my receiver inlet is higher than the condenser liquid outlet? I was always told this is a bad idea, but I am faced with a particular site where I have no choice. Help! Kevin S.

[US Iceman]

Hi Kevin,

My own opinion is there are no correct guidelines for the receiver being located higher than the receiver.

Is the anticipated condenser location that causes this situation one that allows the installation to be easier?

Or is it physically impossible to locate the condenser at a higher elevation?

[Peter_1]

We have to do it sometimes in the trucks we install.
I know it's not done according the books but we often have no other possibility.

And it will work Ken.

[Argus]

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You don't indicate what the lift is. In my opinion it will become a problem when you are talking about very large height differences where the pressure losses and the static head in the vertical column of liquid refrigerant combine to induce premature evaporation (flash gas) at the top end of the riser pipe.
The same will apply with evaporators at the top of high liquid line risers.

It?s not unheard of but in normal installations, a few metres elevation, say, you should not have any difficulties.


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________
roll blunts

[Peter_1]

You're right Argus about the height but I think Ken is mainly working on small OEM cabinets, isn't it Ken?

But you can't generalise this of course.

You can see on this receivers we're installing now that liquid also has to climb

[Peter_1]

This is the condenser we made

[Kevin S]

Thanks for the input. The lift is only about 2m in this case. I have been caught out myself in an application with a 8m rise in the liquid outlet of from the the reciever and because we had very little subcooling, we found massive flash gas at the texv inlet (found by installing a simple sight glass!). What I have been told in this instance, is that the delta P from the liquid will cause the liquid the "back-up" in the condeser and thereby reduce the heat transfer surface area of the condenser. Apparently the liquid needs to "drain" out of the condenser into the receiver. (I have even heard of a "vent' pipe being installed to allow the high pressure vapour to pass back to the condenser). lastly - I almost always see evap condensers used with ammonia being installed on raised platforms - surley this is done for the same reason?

[Peter_1]

If you have to rise, then you certainly need 'enough' subcooling to provide 100 liquid to the metering device.

Why not place a subcooler in the condensor?
The last bends have to go upwards once and the liquid will then be pushed out the condensor.

[windy]

I have had this situation before, and as pointed out providing the height is not too great and causing flash gas, we got around it by inserting a magnetic non return valve in the condenser liquid discharge pipe.

[US Iceman]

(I have even heard of a "vent' pipe being installed to allow the high pressure vapour to pass back to the condenser). lastly - I almost always see evap condensers used with ammonia being installed on raised platforms - surely this is done for the same reason?

The vent line or sometimes called an equalizing line is indeed used on larger systems. The purpose of this is to equalize the receiver pressure with the outlet pressure of the evaporative condenser.

The line also serves the purpose of venting the flash gas that is formed in the receiver (when the receiver is exposed to temperatures higher than the condensing temperature).

This line also allows the gas in the receiver to be vented back up into the condenser inlet line. As liquid flows into the receiver, the volume of liquid displaces an equal volume of gas. This gas must be vented at the same rate as the incoming liquid to prevent the receiver pressure from increasing.

If you look at small air-cooled condensing units, the liquid line from the receiver is almost always below the receiver inlet connection. This does work obviously since many systems are built this way.

If there is no subcooling of liquid, any rise in the liquid line piping can cause flash gas. A small change is not too bad. Several meters can make a noticeable impact as does the terminal evaporating temperature.

[mcgru-]

The lift is only about 2m in this case. I have been caught out myself in an application with a 8m rise in the liquid outlet of from the the reciever and because we had very little subcooling, we found massive flash gas at the texv inlet
...
is that the delta P from the liquid
...
surley this is done for the same reason?

yes, 10m height column of water is very close to 1 bar of pressure drop. Refrigerant has a greater density, so pressure drop will be greater +10..20%.
on the inlet side of condenser, we have a gas with density of about 50..100 lesser, than density of liquid state, so the pressure drop on inlet side of condenser will be about 10..20 mbar.
Setting condenser higher, we could use gravitation to press the liquid to the greature pressures, preventing flashing of the gas when ambient temperatures become higher and subcooling disappers.