hot gas defrost - top feed/bottom feed

[SimonBrown]

Hi

In some refrigeration literature I read that for bottom feed evaporator (air coils) hot gas defrosting is encouraged while air, water or electric defrosting is more compatible or logical with top feed evaporator.

Could you explain the background for this practice ..?

simon

[lana]

Hi

In some refrigeration literature I read that for bottom feed evaporator (air coils) hot gas defrosting is encouraged while air, water or electric defrosting is more compatible or logical with top feed evaporator.

Could you explain the background for this practice ..?

simon

Hi,
Do you mean DX evaporator of flooded?
In DX there is a distributor so I think you mean flooded. As far as I know hot gas is top feed. But I am sure ammonia guys can give you the best explanation.

Sorry to call you 'ammonia guys'. it is a matter of speaking .

Cheers

[US Iceman]

This has always been a fascinating topic for me.

I think a lot of this logic as you stated it is based on the current methods of coil circuiting. Almost all of the evaporators I am familiar with use an orifice to distribute liquid (from the liquid header) into each individual circuit tube.

With hot gas defrost the hot gas flows into the evaporator and begins to condense as the heat (of the gas) is rejected to the evaporator coil surfaces. When this condensate flows out of the coils it flows backward through the orifices (into the liquid header).

The orifices are only "effective" when liquid passes through them. If you tried to push gas through an orifice it would probably choke and restrict gas flow. That would not help defrosting at all.

On the other defrost methods (air, water, or electric) there is no condensate to deal with, so "they" say you can use top-feed on these applications.

I think a much better method is to use top-feed on all applications (including hot gas defrost). But having said that, I am also basing this on the use of a liquid distributor to evenly distribute the liquid into each coil circuit (much as we do DX coils).

With this, the hot gas or condensate flows past the distributor tubes and freely drains. That is exactly one of the problems of hot gas coils. The need to drain the condensate as it forms during defrost.

Arranged like this the coils are free draining for both cooling and heating (re defrosting). Another interesting aspect is how we drain the condensate.

Look at how a steam coil works. You put steam into the top of the coil and condensate runs out of the bottom. Steam guys use steam traps to drain condensate.

For some reason refrigeration people think the draining should be done by a pressure regulating valve, which does not control condensate volume flow. A pressure regulating valve only controls pressure.

If a steam trap works for draining condensate from a steam coil why aren't we using the same thing. We can of course is the answer. The devices are called high-side float valves or liquid drainers, which is the same thing as a float type steam trap.

[US Iceman]

But I am sure ammonia guys can give you the best explanation.

No harm done. As a matter of fact, thank you for the compliment!

The top-feed/bottom-feed argument only seems to come up in industrial refrigeration circles and only applies to liquid overfeed systems.

A flooded coil is ALWAYS bottom-fed, otherwise it would not work.

[lana]

A flooded coil is ALWAYS bottom-fed, otherwise it would not work.

Sorry for the incorrect terminology. I meant the liquid overfeed coil.
Cheers

[US Iceman]

No need to apologize lana. They are just words and terms. That's the best part of this forum. By exchanging ideas and concepts everyone benefits.

[aawood1]

Hi when I started with Ammonia we where tould that Hot Gas should be piped into the top of the Evaporator/ Coil. On freezer Evaporator's with there bigger volume it's better to use a mix of a Flot Valve with a back pressure regulating valve set about +10 oC. and on small coils just use and Back pressure valve.
Arthur.

[Terry Collins]

I guess I'll chuck my two cents worth in too lol. Most of the coils I worked on did not use a distributor feed system. They used hand expansion valves and headers on the evaporators. With a bottom feed coil you feed the hot-gas into the suction line. This forces the liquid out of the coil and back up the liquid line. The liquid/hotgas is then vented into the suction line via a Pressure Regulating valve and cross-over lin at approx 5-600kpa. This requires a check valve in the liquid line downstream of the solenoid and valve train. With top feed coils the hotgas is fed into the liquid line downstream of the solenoid and vents into the suction line via an auxilliary Pressure Regulator either around or an integral part of the Suction Solenoid. The bottom feed set-up works equally well for Liquid Recirculation/Overfeed as it does for Flooded Evaporators.

[Terry Collins]

I guess I'll chuck my two cents worth in too lol. Most of the coils I worked on did not use a distributor feed system. They used hand expansion valves and headers on the evaporators. With a bottom feed coil you feed the hot-gas into the suction line. This forces the liquid out of the coil and back up the liquid line. The liquid/hotgas is then vented into the suction line via a Pressure Regulating valve and cross-over line at approx 5-600kpa. This requires a check valve in the liquid line downstream of the solenoid and valve train. With top feed coils the hotgas is fed into the liquid line downstream of the solenoid and vents into the suction line via an auxilliary Pressure Regulator either around or an integral part of the Suction Solenoid. The bottom feed set-up works equally well for Liquid Recirculation/Overfeed as it does for Flooded Evaporators.

[Terry Collins]

Hot gas defrosting has it's limitations. The lower the room temperature the less effective it becomes. Where possible we fitted a manual water defrost system. This was to give the low temperature coils a water defrost/clean-up when required.