How to design the orifice of the evaporator

[ntfreezer]

Hi,everybody
In ammonia pump system,I have a doubt how to design the orifice of the evaporator,every tube shoud has a office in evapotator.

[lana]

Hi ntfreezer,

What do you mean by oriffice?
In ammonia system when the liquid is pumped into the evaporator then you have liquid circulation.
Maybe ammonia experts can clarify that.
Cheers

[nh3simman]

Maybe ntfreezer refers to the distributor?

[Dan]

I think he is speaking of the orifices that enter each tube of the evaporator from the liquid header. regrettably my book is at the shop. If I recall correctly, these are to placed to assure sufficient pressure drop to feed each tube equivalently.

[US Iceman]

In ammonia pump system, I have a doubt how to design the orifice of the evaporator, every tube should has a orifice in evaporator.

You actually have two options if you are designing the evaporator circuits, which is what I am assuming you are describing.

You can use either for liquid overfeed evaporators:

1. A refrigerant distributor as used in direct expansion systems. Each distributor tube would feed a separate evaporator circuit. The circuit tubes from the distribution have to be sized for the mass flow and differential pressure. Remember this is low pressure liquid, not high pressure so the available liquid feed pressure is dependent on the refrigerant pump head.
2. A separate orifice plate for each evaporator circuit. The orifice plates are just a metering device from one pressure to another pressures for a specific mass flow. If you know the evaporator pressure you need to know what the inlet pressure to the orifice is. The inlet pressure consists of the available liquid feed pressure plus the static head in the liquid header.

This establishes your pressure differential across the orifice plate. Next you size the orifice for the required mass flow for each circuit at the differential pressure already established.

[lana]

A separate orifice plate for each evaporator circuit. The orifice plates are just a metering device from one pressure to another pressures for a specific mass flow. If you know the evaporator pressure you need to know what the inlet pressure to the orifice is. The inlet pressure consists of the available liquid feed pressure plus the static head in the liquid header.

Where is this orifice plate installed? I apologies if this is a very basic question.

Cheers

[US Iceman]

Hi lana,

No need to apologize. Evaporator circuit design is another of those "secret arts" that is not well documented. Otherwise they would not be secret.

The orifice is installed at the entrance of the circuit tube. It is intended to meter the refrigerant into that circuit tube only. The liquid header connects all of the circuit tubes and supplies liquid to the orifice.

It is common to find the orifices with different openings also. The static head in the liquid header affects the differential pressure across the orifice. The total pressure should be used, not just the liquid feed pressure.

[lana]

Thanks a lot USIceman,

I am used to DX coils with distributors. That's why I am confused . I have to see one to get the idea.
Thanks again.
Cheers

[US Iceman]

Using a refrigerant distributor offers a better method of liquid distribution I think for overfeed coils also. They are a slightly more expensive because of the distributor, but personally, I think they work better.

Especially if you have to hot gas the coil for defrost.

[750 Valve]

Its amazing how people forget the past and what they've learned to this point in time... As a few of you know I'm tooling around with some r744 systems of late, pumped liquid. Engineering wise, some people think they've gotta reinvent the wheel to use this stuff. The MT coolroom evaps are a step back in time... inlet header, outlet header - no orifice or distributor. When you try to explain uneven circuit flow , starving top circuits and the ice that forms on the bottom circuits due to it requiring longer defrosts to boil off the dense r744 - they stare blankly at you, when you ask if a distributor may be required you get told that will cost an extra $23.50 per evaporator and the projects already over budget.
Why do I persist?!?!

[US Iceman]

I have seen the same argument used. The distributors cost money! Don't mind the fact that this eliminates some of the problems with defrosting and even circuit loading and probably several other problems.

The logic (if you want to call it that!) continues to be: that's the way we have always done it!