What is the purpose of a flooded evaporator?

[expat]

As I always understood it an evaporator should be fed from the bottom. The evaporating liquid, boiling off, would have become vapour at the upper end of the evaporator during it's strugle against gravity. This would seem logical as inversely the fluid in liquid form would be drawn down to the exit at the bottom of the evaporator by gravity increasing the chances of flood back in low load conditions.

I post this question about flooded evaporators as I'm wondering if it doesn't have something to do with this and that perhaps I have a misconception somewhere.

What is a flooded evaporator then, is liquid supposed to come out of it???

[Billy Ray]

Hi,

Flooded evaporators are generally found in large industrial installations.

The evaporator is liquid laden, oposed to direct expansion evaporaors which are liquid/gas mixturre.

The cooling effect is latent only i think, no superheating of vapour in the evaporator (atutared vapour is drawn of the top).

The industrial guys on this forum can explain in more deatil.

Billy Ray

[Billy Ray]

spelling mistake, thats saturated vapour drawn of the top of the evap.

To you industrial guys-

Am i correct in my analaysis??

Billy Ray

[US Iceman]

Well, to make sure we are discussing one item and not something different let's concentrate on gravity flooded air-cooling evaporators (not to be confused with liquid overfeed air-cooling evaporators).

I say this because you have to make a distinction on the type of evaporator and the liquid feed method.

What is loosely called a flooded evaporator when a shell and tube construction evaporator is used is essentially pool boiling. The tubes are immersed in a pool of liquid refrigerant which boils when heat is applied or the pressure is reduced (by a compressor).

A gravity flooded evaporator can be either a tube and plate fin (air-cooling evaporator) or a brazed plate heat exchanger or plate and shell heat exchanger (for cooling liquids).

The reason they are called gravity flooded is that liquid is heavier and flows into the evaporator by gravity. The heat transfer surfaces are essentially "flooded" with liquid refrigerant. As liquid boils, the resulting vapor formation rises up and out of the heat exchanger.

Since liquid and vapor are forming in the evaporator, the vapor volume leaving the evaporator tends to "drag" liquid out of the evaporator too. If operating properly this process occurs at the saturation temperature of the refrigerant for the pressure maintained in the evaporator by the compressor. There should be NO superheat!

The reason for why this type of heat exchanger might be selected is the higher capacity available for the same amount of heat transfer surface. If you use the heat exchange surface for superheating the vapor you are not evaporating liquid, hence you get slightly less cooling capacity.

[expat]

Thanks for that fellas. Just siffting through a couple of books to straighten my thinking out.

[brian_chapin]

Thanks for that fellas. Just siffting through a couple of books to straighten my thinking out.

If you can afford them - the RETA books on industrial refrigeration (Volumes 1&2) are excellent resources.