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Krzysztof
24-04-2008, 09:41 PM
Hi :), it's me again,

I'd like to ask You, how large is the circulating rate in a gravity flooded ammonia plants. As I know in overfeeded systems with NH3 pumps this rate is about 3-4 up to 7. So, I think that circulating rate in gravity flooded systems is less than in forced-flow systems and is about 1.5
So I'll be grateful if You could tell me if my thinking is correct...

Thanks for the answer...

My best regards...

US Iceman
24-04-2008, 11:20 PM
Hi Krzysztof,

Actually the overfeed rate in a gravity flooded evaporator (like an air-cooling coil) is determined by the heat load applied to the coil surface.

If the coil is operating at 100% capacity the circulating rate will be much higher than if the coil is frosted and absorbing very little heat. This is due to the greater volume of gas created by the boiling at 100%.

Depending on the pipe sizes and installation I suspect the circulating could be higher than 10. The principle that limits how high the circulating rate goes is the pressure loss of the piping.

NH3LVR
25-04-2008, 03:05 AM
The principle that limits how high the circulating rate goes is the pressure loss of the piping.
Lets get into that a bit. Sounds interesting.:)

US Iceman
25-04-2008, 04:15 AM
OK.

As the heat load on the evaporator increases more liquid is boiled off into vapor. The vapor volume increases which has to flow through the same pipe size as it does when the heat load is minimal. As the vapor volume increases more liquid refrigerant is moved through the evaporator and piping.

Since the volume of vapor is increasing with higher heat loads the pressure losses (in the piping and coil) start to increase also. At some point the static head available pushing liquid into the evaporator has to balance out with the pressure losses in the pipe and coil.

This is when the highest circulation rate is achieved.

These are called gravity flooded but operate the same as a thermosiphon oil cooling system on a screw compressor. There is no difference other than the operating temperatures and what you are trying to cool.;)

Krzysztof
26-04-2008, 10:56 AM
Thanks for advice. So as we see I was in mistake :)

Can we say in conclusion, that circulation rates in gravity flooded evaporators are similar to the forced-flow evaporators with NH3 pumps?

Best regards...

US Iceman
26-04-2008, 04:37 PM
Can we say in conclusion, that circulation rates in gravity flooded evaporators are similar to the forced-flow evaporators with NH3 pumps?


I don't think I would go so far as to say it that way. A pumped overfeed system will be supplying the same amount of liquid to the evaporators all of the time.

However, I think there is a similarity in the evaporators. As the heat load increases or decreases, the liquid & vapor fractions (percentages of each) of the refrigerant would be changing.

John Hunter
03-05-2008, 12:24 AM
Gravity feed systems for thermo syphon operations , as US Iceman says are excellent for oil cooling on screws etc. provided correct design is use at the installation concept. In my early years in the Industry there were many gravity systems simply working off the operating head pressures. There were no pumps in the set-up. The evaporator units were larger than the dry coil units that prevail today as each had an accumulator vessel in to which fed the sub cooled HP liquid and from which the coil circuits were fed. The evaporated gas end of the coil returned to the accumulator above the operating liquid level. The top of the accumulator was returned to the LP suction vessel and hence to the compressors. This in essence was a 1-1 circulating system although sometimes when load fluctuations were quite large some un-evaporated liquid could be entrained with the returning gas so there was always the need for a suction flash vessel. This vessel in most plants was used as a sub-cooler of the HP liquid feed to the evaporators in that it had an internal heat exchanging coil.
These systems have all but disappeared here as they would be costly to manufacture and install nowadays but they were excellent heat exchangers in large meat stores that used to prevail.
One problem I remember was the times when oil would build up in the evaporator accumulators. All had oil drains for the porpose but it was never a pleasant job.
Hope I am not boring you with my memories , maybe I am getting too old , but this thread brought things back to my mind.