liquid injection triggered on low suction method

[liptow]

Hello cool friends,
Has anyone heard or wish to comment on a liquid injection system that uses a manually adjustable metering valve followed by a crankcase pressure regulator set at a very low pressure just higher than low pressure cut-out of compressor.

There would be a suction accumulator downstream of the junction and another CPR upstream of the junction set at much higher pressure. So if suction pressure climbs the main suction line CPR modulates down and if it drops way low the liquid injection CPR modulates open. Since pressure is so low a system has suction accumulator and a fine liquid metering, then liquid slugging would not happen.
Benifit? - No thermal valve in liquid line, no hot gas bypass valve/line, no short cycle of compressor, cool runnin compressor.???? These would be individual working cold labs at 4C-10C with hotgas modulation/defrost into distributor. What a great community and service. I'm proud to be involved.
Thank You All, Joe Liptow from Clev Ohio USA.

[taz24]

Hello cool friends,
Has anyone heard or wish to comment on a liquid injection system that uses a manually adjustable metering valve followed by a crankcase pressure regulator set at a very low pressure just higher than low pressure cut-out of compressor.

Thank You All, Joe Liptow from Clev Ohio USA.

I Have worked with direct expansion into the suction line when working on two stage systems.
I see no advantage other than to pre cool the discharge of another comp or cylinder.
In effect you are sugesting using a system that gives its self work so it does not have to unload.
If you size the system right you should be able to have a system running and cycleing for less energy consumption than the method you are sugesting.
You are makeing a simple system complicated in my opinion.

taz.

[liptow]

Hello Taz24,
Thanks for the come back.
Yes it is more complicated but, from what I've come to know, non-cycling condensing units are the norm for cold working labs using 1/2-2 hp compressors. The reason, rightly or wrongly, is that short cycling is bad for the compressor and the +/-.5C tolerance range in a 4C room is hard to maintain with a cycling compressor. Maybe I have blinders on. My method assumes the need for a non-cyling compressor so I'm looking for comment within that requirement. On the other hand, I'm opening the discussion to those who may feel that this is an antiquated notion given the state of the art of refrigeration technology today [ie: the more I learn the less I know]. Anyone else?
Thanks, Joe.

[NoNickName]

That is not usually referred to as "liquid injection" but "hot gas bypass". Infact it is safer to inject gas in the suction, than injecting liquid. Many suppliers of valves and the like do have a hot gas bypass valve (manually or automatically operated) in their catalogue.
For example sporlan http://www.sporlan.com/90-40.htm

[taz24]

My method assumes the need for a non-cyling compressor so I'm looking for comment within that requirement. On the other hand, I'm opening the discussion to those who may feel that this is an antiquated notion given the state of the art of refrigeration technology today Joe.

Hi Joe.

I see your point with the way the system works and if it is a tried and tested sytem then who am I to dispute.

What type of compressor does it use.
Would a scroll comperssor be able to carry out the duties required.
I ask because there are scroll comps on the market that actualy unload the duty as required.

Just a thought.

Cheers taz.

[US Iceman]

...the need for a non-cyling compressor so I'm looking for comment within that requirement.

The normal method for this type of control is a hot gas bypass only. If you use a refrigerant distributor with a side port (for hot gas bypass) and a hot gas bypass regulator that is all you need. (Well maybe an accumulator too).

When the cooling load reduces the evaporating pressure also decreases (if the compressor is constant speed/volume). As this occurs the hot gas bypass regulator simply opens to maintain a constant evaporator/suction pressure.

The benefit of doing this is a relatively constant evaporator superheat since the hot gas flows through the coil and the TXV controls the superheat.

If the compressor has capacity reduction, the hot gas bypass capacity is selected off of the minimum compressor capacity.

If the compressor has no capacity reduction and remains constant speed/volume then the hot gas bypass is sized for 100% of the compressor capacity. This way as the cooling load begins to decrease the hot gas valve begins to stoke open.

This is a very simple system if engineered properly and works extremely well for cold rooms, or other applications where extremely tight temperature control is demanded. AND, you don't need liquid injection valves with this method which is a big benefit in my opinion.

If I understand your orginal post you are using mulitple regulators in series with one another. I tend to discourage this practice as the valves tend to fight each other and never seem to reach a stable operation.

[liptow]

Thanks fellas!
US Iceman,
With the hot gas bypass, which will modulate OPEN as suction pressure lowers, won't I have to have a de-superheating TXV to inject liguid or can this be avoided.
Is it possible that the pressure increases back upstream into the evaporator and then the main evap TXV reacts by allowing liquid to flow? Would MOP TXV be something that would come into play here. Warning: I am MOP challanged and not sure what there real influence is.
Thanks for hanging with me, Joe.

[US Iceman]

With the hot gas bypass, ... won't I have to have a de-superheating TXV to inject liguid ...

Nope. The TXV does this for you.

Here is what the arrangement looks like.
http://www.sporlan.com/SD-094.pdf

There is some good information on page 20 of this:
http://www.sporlan.com/20-10.pdf

And finally, see page 1/figure 1 in this link.
http://www.sporlan.com/90-40.pdf

All this arrangement does is provide a false load to the evporator to keep the compressor suction constant.

As the evaporator load disappears, the suction pressure begins to fall, which forces the hot gas bypass regulator to open. This maintains a constant evaporator pressure and hence a constant suction pressure.

Since the hot gas flows through the distributor the liquid coming from the TXV is foced to mix with the hot gas inside of the evaporator. The mixing process delivers a relatively stable superheat that allows the TXV to control nicely.

The compressor does not care. All it sees is constant gas flow and constant superheat. Nice, neat, and simple.

If the box experiences frequent pull-downs from higher temperatures, then just use a single crankcase pressure regulator to prevent overloading the system during the pull-down.

In this case the regulators do fight each other during a pull-down condition, the hot gas bypass regulator is off anyway.

[liptow]

Hello again US Iceman,

I see your point[s]. I should have mentioned that quite
often we have long remote runs and we tend to eliminate the hot gas run to the evap which might be over 120-150 ft away. The quality of the hot gas isn't very good at those distances. So what would you suggest now? Thanks, Joe.

[US Iceman]

So what would you suggest now?

You punt!

Just kidding.

I would insulate the hot gas line very well so that the gas does not condense. Sure you may loose some of the superheat some of the time.

A lot of this will depend on how long the system is at full load (here the hot gas would be off and the line cooling down).

On the other hand, if the loads are very cyclic then the hot gas line should stay warm as the hot gas would be flowing continually or at the very least more frequently.

[Gary]

I would think an EPR would be better for regulating the cold room temp and a TEV injecting into the accumulator to regulate superheat for the compressor.