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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,Quote:
Originally Posted by SimonBrown
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.
CheersQuote:
Sorry to call you 'ammonia guys'. it is a matter of speaking:D .
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.;)
No harm done. As a matter of fact, thank you for the compliment!:DQuote:
Originally Posted by lana
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.
Sorry for the incorrect terminology. I meant the liquid overfeed coil.Quote:
Originally Posted by US Iceman
Cheers:)
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.
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.
Hi Arthur,
This is one of those topics that drives everyone nuts. It did this to me to until I was forced to learn more.
If we look a a steam coil for heating purposes this coil works the same way a hot gas defrost coil does during defrost. Gas in the top, liquid out the bottom.
Using a pressure regulator simply means we have to pressurize the coil to a certain pressure. No liquid drains until the coil pressure reaches the regulator set pressure. I always thought this was sort of strange, but as everyone kept telling me "it's the way this is done."
To me, the most difficult task is to get the condensate out of the coil as fast as it forms. Doing so allows the coil to defrost much quicker.
I'm working on a project right now where we are just using inverted bucket traps from a steam system (they are SS material) to drain the condensate back into the wet suction line.
Hi Iceman, When I left School and started at work the trade that I was in and did for 14 Years was a Fitter welder on Steam boilers and systems. So you could say that I went from makeing heat to removeing it. As you say steam traps and condensate seperator's are fitted to get dry steam. I think that you should be able to use a steam trap to remove liquid from a coil. Please let me know how you get on.
Arthur.
Here is some reading material I could quickly find.
http://www.hantech.com/documents/PDF/D409.pdf
http://www.spiraxsarco.com/resources...team-traps.asp
The more you look at steam systems I think you will find a lot of similarities to an ammonia system.
A steam trap might work to remove liquid from a coil. I have used Armstrong traps to located on the outlet of condensers to feed critical charge systems.Quote:
Originally Posted by aawood1
Use caution however. The specific gravity of NH3 is different than water. If the pressure difference is too high the float may not operate.
Sound advice.;)Quote:
Originally Posted by NH3LVR
It is important to realize how a steam trap operates and the principles related to their capacity.
The specific gravity is the first item. Ammonia (depending on the actual liquid temperature) has a specific gravity somewhere between 0.5 to 0.6. The steam trap capacities are based on a specific gravity of 1.0 (for water).
Any condensate flow of ammonia has to be converted to an equivalent flow in terms of water.
The second item is the orifice size and pressure differential that will exist across the trap. More pressure differential will mean a greater flow rate capability for a given orifice diameter. Too much differential and some traps can lock up.
Also, different traps have certain limitations that need to be considered.
As with any other application of components it is important to select the component using good information with the limits of that component in mind.
what is a steam trap?
For a mechanical description it is very similar to a high side float valve.
I have interesting question. What is the pressure in bottom feed coil during defrosting? Back pressure regulator set to 70 psig.
This almost seems like a trick question Sergei.Quote:
Originally Posted by Sergei
If the regulator is set for 70 psig, the coil pressure takes a while to rise to that pressure. The gas condensing in the coil simply turns to liquid until the coil has defrosted to some equilibrium point.
At this time, the liquid blocking off the heat transfer surface begins to pressurize. This is similar I think to a flooded air cooled condenser with head pressure controls on a ***** system.
As enough liquid accumulates in the evaporator the pressure will begin to rise. As soon as the evaporator pressure reaches the 70 psig setting, the valve will pop open. A portion of the liquid inside the evaporator flashes off to the downstream pressure of the regulator. The valve closes again and more heating occurs. The pressure rises again and the process is repeated over and over until the time clock closes the hot gas defrost solenoid valve feeding hot gas.
This is one of the reasons I feel it takes so long to defrost an evaporator with hot gas. If we can remove the condensate as fast as it forms the defrost occurs much faster.
I have seen large coils defrost within 5 minutes (completely) if they are properly equipped to do so.
We have free flow of gas and condensate inside the coil, if no restriction to this flow. Orifices in liquid header are major restriction to free flow. This restriction create pressure difference inside the coil.
Example. Head pressure is 120 psig. BPR set to 70 psig. From compressor to hot gas balancing valve pressure is 120 psig. From hot gas balancing valve to orifices pressure is 100 psig. From orifices to BPR pressure is 70 psig. Actual pressure in the coil is 100 psig.
I can agree with this as it seems a reasonable and logical approach. I have never actually measured the coil pressures however.Quote:
Originally Posted by Sergei
As you pointed out, the orifices are the weak link that limits the defrost. Since the orifices are designed for refrigeration this could limit how much condensate the orifices can pass a a specific pressure differential across the orifices. Lower discharge pressures would INCREASE the defrost time, while high discharge pressures would DECREASE the defrost time.
In principle I can agree with you.
My theory is; if we can get rid of the orifices the coils actually defrost better and faster WITH lower discharge pressures.
I prefer gradual defrosting. It is better if temperature of the coil will increase gradually. Sometimes temperature shock can damage the coil. Fast defrosting creates significant parasitic load, because part of hot gas will not condense and will go through the coil on suction side. Actually orifices limit this parasitic load.
How would more hot gas escape to the suction side if a liquid drainer/steam trap/high side float valve is used? It will only pass condensate unless the drainer is stuck open.Quote:
Originally Posted by Sergei
The only way the hot gas could just flow through the coil quickly without condensing very much is if the defrost relief valve is stuck open, too big, or if the defrost cycle is too long.
Certainly, I mentioned about the coil with BPR. Every coil has several circuits. These circuits don't defrost evenly. Top circuit will defrost faster than bottom one. In the middle of defrosting top circuit will be defrosted and will pass hot gas through without significant condensation. at the same time defrosting of bottom circuit is still going on. The reason of this situation is different diameter of orifices. To equalize liquid supply in cooling mode, diameter of top orifices is greater than diameter of bottom one. However, this factor create uneven defrosting of these circuits.Quote:
Originally Posted by US Iceman
Certainly, liquid drainer is working better than BPR, but it is more expensive one. This drainer should has bypass valve for proper liquid draining.
Simon Brown originally posted this question and I haven't seen a reply from him. This is an excellent industrial refrigeration discussion. But let's take his question and consider that it is from the commercial refrigeration or supermarket application.Quote:
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 ..?
A reach-in glass door case may have a vertical evaporator in the back wall. These are piped to feed top to bottom in electric defrost applications, but are piped bottom to top for gas defrost applications. The working concept is that the defrost be gravity assisted, and that this is important enough a consideration that the evaporator piping drastically change. Either way, a flooded coil or a draining coil seems to refrigerate.
This comes up a lot when somebody wants to convert a gas defrost system to electric or vice versa. Oil trapping becomes a question, for example.
From a practical perspective, we have changed many cases from gas to electric and vice versa without changing the evaporator feed location, and have pretty much succeeded despite the legitimate questions that pop up.
Another reason why the orifice coils take longer to defrost is the liquid accumulates in the bottom of the evaporator until the defrost relief regulator pops open.
After the gas has condensed it falls to the bottom of the coil. Once the liquid collects, the defrost in that area (where the liquid is present) does not hardly occur at all due to the condensate. Cool or cold liquid does not defrost.
As the liquid begins to be purged from the bottom of the coil, the lower portion also begins to defrost.
The orifices are one issue, draining the condensate is another.
This is that I mentioned before. Top circuit will defrost faster because condensate will be removed faster due to larger size of orifice.To do hot gas defrosting efficiently we have to balance 3 parts of this defrosting: hot gas supply, hot gas condensation at certain temperature level and condensate removal.Quote:
Originally Posted by US Iceman
Absolutely.Quote:
Originally Posted by Sergei
We have had a good discussion going here, but I'm wondering what happened to the original RE member who started this thread.:(
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.
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.
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.