Ammonia Circulation rate - by Frick/ Vitler

We are working on a project where in the capacity of total plant is 100TR at -40 Deg C. We are having Four compressors & Two Independent systems. i.e. TWo Hp / TWO Lp Vessles. to have an 100% stand by plus to offset the non peak conditions.

The flow rate for the pumps ( Again total 4 pumps- 2 S.B.) is coming out to @ 5.3 Cmt/ Hr. I remember that Frick has given some specific figure for 4:1 Circulation rate. I do not have that chart now. Bit I believe that our calculations are almost 40% above the figures given by Frick? What could be the reason. has any one those Frick/ Vitler charts?

If your calculations are off by 40% there might be two probelms:

1. The calculations were not performed correctly, or
2. The mass flows rates you calculated include the flash gas. To find the actual pump flow you use the latent heat only of the refrigerant at the evaporating temperature. If you used the liquid enthalpy at condensing pressure then this is not correct. You want to use the liquid enthalpy at the evaporating temperature.

Would it be a simlar outcome if you used low side total mass flow times 4 to select a pump for low side and same calculation for high stage comps ?(from a laymans point of view).

I think We could get what was missing in earlier calculations.

I think you need 2 100litre per minute capacity liquid pump for your requirement,,,,,,,

If you use the total evaporator capacity and divide it by the latent heat of ammonia at the desired evaporating temperature this will give you your total mass flow rate for evaporation. Then, take this value and multiply by 4. Then convert to volume and you have the total volume flow rate needed to size the pump.

Personally, I am beginning to no longer use the 4:1 ratio anymore. The last project I designed used a ratio of 1.25:1 and it works just fine. I did use some different evaporator liquid feed designs of course!

You can even use 3:1 or less but you have to inform the evaporator manufacturer so that they can properly design the coils.

4:1 is not a magic number that you have to have to make these system work.

My thoughts to add are you have to have some formula for sizing a liquid pump. 4:1 is standard on most evaporators,as if you size pump to small your life can be a misery to get it to work .
Circulation rate can easily be adjusted by manual expansion valve to achieve any circulation rate required.
Liquid pump if oversize should not matter if Qmin or bypass line of discharge of pump is sized correctly.
To me there are only certain size liquid pumps to pick from (except belt drive plus VFD applications for minor adjustments to pump output/pressure).
How many times do plants have something added to it .In this event pump can usually handle it .
Unless price of pump is to high its better to go for something that will do it compfortably or be a bit conservative.

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Originally Posted by Ranger1

...if you size pump to small your life can be a misery to get it to work.

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That's very true, however, it is important to distinguish between pump head and volume flow. They are two separate functions the pump must be able to provide. If you do not provide enough head then of course the pump will not supply ample refrigerant to the evaporators. On the other hand, if you do not size the pump to provide a sufficient volume of liquid you can have a similar issue.

The other problem we have with these systems is the use of universal terminology, which can be misleading. There are two terms it is important to remember. The circulation rate (4:1, or whatever) is the value used to size the pump volume flow, while the overfeed rate is the mass fraction of liquid to vapor coming out of the evaporator coil.

The pump supplier needs to know the GPM or liters/second which is based off of the circulation rate. The evaporator manufacturer needs to know the overfeed rate to ensure he designs the coil circuiting properly.

As a word of caution, it is not just necessary to oversize the pump for additional pumping volume if you do not increase the size (diameter and storage volume) of the low-pressure receiver also. A larger pump volume for additional liquid also means you need more separation capacity for the additional evaporator load.;)

Most of the plants we work on we seem to be able to plug on a fair few extra evaps without any problems. The design side is always checked of course.Maybe they were not to mean when sizing vessels in the first place.
If you have competition and want to win a job i quess it can give you an edge.

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Originally Posted by Ranger1

Most of the plants we work on we seem to be able to plug on a fair few extra evaps without any problems.

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Yep, I've seen the same thing. The fact that the circulation rate is so high provides a lot of safety factor in a suitable volume of liquid flow to the coils. And, coupled with the fact that the separator design is not an exact science also allows some margin for additional loads to be applied with no discernible effects.

As a side note I have seen some of these systems where a significant additional load was connected to the pumps and it also worked OK (up to a point).

Dear US ICeman,

We too have started designing systems with 2:1 circulation ratios & they are working fine. Even !.25 : 1 is good enough. The point is that the liquid sould boil giving way to Latent heat & not only the sensible heat. That makes systems very efficient.
Many thanks for the feedback.

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Originally Posted by smpsmp45

We too have started designing systems with 2:1 circulation ratios & they are working fine. Even !.25 : 1 is good enough.

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That's great! DX systems are used in a majority of applications and they work on a 1:1 rate, so by definition any circulation rate greater than 1 is a liquid overfeed system.

The fact that we have the internal coil surface wet with liquid refrigerant provides the highest heat transfer. Also, by minimizing the liquid volume in the pipes (especially the wet suction line) we also reduce the inherent problems with risers and liquid hammer and potential liquid trapping in the suction line itself.

What type of coil circuiting are you using on the evaporators?

Ofcourse the majority of the times we are using this system for Plate freezers or Trolley freezers. For cold stores we stick to Monoblock units on R 404A itself. Though now few jobs are in the pipeline wherein the client is going in for Ammonia evaporators even for Cold stores!