first estimate of system charge

[cool-hvacer]

Hi all....

I have a pretty basic question that I am getting mixed answers on.... I have researched alot of previous posts, and am still struggling a bit.

I have a air cooled glycol chiller with r134 and a TXV (approx 9 ton capacity). There is a receiver in the system as well. (Documantation on the system MIA, and we cannot seem to get any support from the original manufacturer - and so I have been trying to figure most stuff out, first principles)

OK- the question is what would be a good estimate of proper refrigerant charge? I am getting two different answers looking at past posts.

1) all of Liquid line volume + receiver volume + 30% condenser volume + 70% evaporator volume

OR

2) about 80% of the receiver volume (thus making the receiver big enough to hold 125% of the charge)


A few extra bits of info from the system......

- measured our receiver and it is roughly 1.31 cu ft or big enough to hold approx 105 lbs of r134

- I found an old "startup" sheet and it has "scribbled" on it that 160 lbs r134 used for startup. I don't think I trust this / but it could be right....

- we have been trying to charge the system by subcool/superheat method after replacing the TXV. We are unable to get a clear sight glass all the way up to 160lbs charge, and then compressor starts slugging.

- our system does have head press control via condenser flooding, but I have it all forced through the condenser right now while we try to get the proper charge (I am keeping the pressure up with fan cycling).

........ Any thoughts?!?!

[Brian_UK]

Are you near to design water/glycol temperature.

How much was taken out of system?

[cool-hvacer]

We took all the refrigerant out to replace the valve, and to be able to "weight in" a known amount....

The system is fully loaded, but unable to bring the temp down to setpoint. We reduced the load to partial load (but still well short of full specified capacity) and the temp came down a bit further, but still not close to what it should be.

We added r134 back into the system all the way back up to 160lbs total, when we started to have "slugging" problems. We removed back down to 100lbs total, and the system now runs, but we have never gotten the sight glass cleared.

We are suspicious that the TXV is not modulating properly (just staying fully open)..... But I am not sure if subcool/superheat measurements are worth anything until I get solid liquid to the TXV. Or we could still be short of r134?!?!

Which brings us back to the original question?!!? How much r134....

The service techs we have had helping have all shaken their heads and said that they think 160lbs is way to much for a system this size....

[nike123]

We took all the refrigerant out to replace the valve, and to be able to "weight in" a known amount....

The system is fully loaded, but unable to bring the temp down to setpoint. We reduced the load to partial load (but still well short of full specified capacity) and the temp came down a bit further, but still not close to what it should be.

We added r134 back into the system all the way back up to 160lbs total, when we started to have "slugging" problems. We removed back down to 100lbs total, and the system now runs, but we have never gotten the sight glass cleared.

We are suspicious that the TXV is not modulating properly (just staying fully open)..... But I am not sure if subcool/superheat measurements are worth anything until I get solid liquid to the TXV. Or we could still be short of r134?!?!

Which brings us back to the original question?!!? How much r134....

The service techs we have had helping have all shaken their heads and said that they think 160lbs is way to much for a system this size....

You could check proper function of TXV by dipping his bulb in water (or glycol if low temperature is needed) with ice. It should close and superheat should rise.
Give us current figures:
superheat subcooling air temperatures pressures etc...

What is TXV make and model #? Why did you changed valve?
What is original valve model #?

[Peter_1]

...
.. air cooled glycol chiller with r134 and a TXV (approx 9 ton capacity). ...receiver in the system...
1) all of Liquid line volume + receiver volume + 30% condenser volume + 70% evaporator volume
OR
2) about 80% of the receiver volume (thus making the receiver big enough to hold 125% of the charge)


I should go for answer 1 but +/-20 % of the receiver. If you calculate it full, then you never can pump down the system.


- measured our receiver and it is roughly 1.31 cu ft or big enough to hold approx 105 lbs of r134
- "startup" sheet ...160 lbs r134 used for startup.
- trying to charge the system by subcool/superheat method after replacing the TXV. We are unable to get a clear sight glass all the way up to 160lbs charge, and then compressor starts slugging.

Under- over overcharging has nothing to do with compressor slugging but with a TEV which isn't working properly for whatever reasons.
If this happens, then measuring SC and SH isn't possible.
You must first besure that you have the correct TEV, correct orifice, correct SH setting, preferable beginning with factory setting,...
Having no clear sight glass can be because you have flash gas.


our system does have head press control via condenser flooding, forced through the condenser to get the proper charge (I am keeping the pressure up with fan cycling)

What are your High pressures with and without condenser pressure control?
There's something wrong with your new TEV.
What was the old one and what is the new one?

[Lowrider]

Peter_1 is right on the money!

How ever I have some questions as well! You have changed the expansion valve, so did you change it completly or just the orifice and powerhead? Did you do any brazing on it? Did you evacuate the system? to what pressure?

And then: What's the make and type of chiller? Electronic or mechanical expansion valve?

[Gary]

- we have been trying to charge the system by subcool/superheat method after replacing the TXV. We are unable to get a clear sight glass all the way up to 160lbs charge, and then compressor starts slugging.


Describe the step by step subcooling/superheat method you are using. I am not aware of any subcooling/superheat method which calls for charging until the sightglass is clear.

If the TXV is wide open clearing the sightglass will give you a gross overcharge... and the TXV is going to be wide open until the glycol is close to design temp.

If the compressor valves are broken, the sightglass is not going to clear no matter how much refrigerant you put in there.

Charging to a clear sightglass is NOT the best way to charge a system.

If you are slugging the compressor, you obviously have low superheat. What does your subcooling/superheat method tell you about low superheat?

[cool-hvacer]

Hey - you take the weekend off, and it's like Chrismas all over again with all the great answers. Thanks everyone - I'll respond to each.....

NIKE123:

I was thinking about trying to dip the bulb in... something. Problem is that chiller is set to produce glycol at 10F. Right now we are getting 20F, but that means I would need to dip into something pretty cold. I am not sure if ice cubes stirred in glycol would be low enough. I may be able to drum up some LN2 on site and make the glycol cold enough..... definitely worth a try!

I have no superheat and subcool #s as the sight glass has never cleared. (doesn't this make those measurements invalid if not solid liquid to TXV)?

The valve is a sporlan O-JE-12-CP60. Same as the original. It is rated a touch higher than 9tons, but I called the sporlon people with the details, and they said it should be just fine in this case.

We did the replacement because we noticed oil drops on the floor below the valve.

PETER_1:

I am going to reply to the first part of your reply in my next post (because it gets to the just of my original question).

I thought that a symptom of "undercharge" is bubbles in the sight glass? I am able to get the high side pressure all the way up to 200psi and down to 90 psi by playing with the fan cycling. I have been trying to hold it at around 150psi.

The new valve is the same as the old one.

LOWRIDER:

Actually, I replaced all of the "guts" of the valve as well as the powerhead. I left the body in place to avoid torch work. We have since taken the valve apart again to confirm that it was reassembled correctly.

The service techs pulled a vacuume overnight before we charged again (pretty small section). It is a mechanical valve (with gas bulb and equilization port).

The chiller is a "one off" / built up system.

GARY:

I thought that you needed to charge to clear sight glass (means you are close and TXV will be working properly). Then use Superheat/subcool measurements to nail it down (fine tune).

I could take the measurements, but do they mean anything without solid liquid at sight glass??

BTW - the service techs took the compressor valve covers off and visually inspected - all looked OK.....

Thanks for all the replies guys - hope this helps. I'll add another post for first part of PETER_1's question

[cool-hvacer]

PETER_1:

Ok let me try to understand what you are saying...

Volume of charge should = Liquid Line + 20% of receiver volume + 30% condenser + 70% evaporator.

Your point about pumping down the system is valid, but my question them becomes

If you have subcooled liquid flowing to a reciever, and subcooled liquid flowing out of a reciever, can this "gas" volume you are speking of actually exist for long in the reciever.

i.e. As the process runs, "gas" (saturated r134) will eventually cool and that volume will turn to liquid, thus pulling more subcooled liquid from the liquid line / condenser into the reciever. Thus you will not have the 30% volume of liquid in the condenser?!?!?

I think this is kind of the heart of my original charging question....

:confused:

[nike123]

The service techs pulled a vacuume overnight before we charged again (pretty small section). It is a mechanical valve (with gas bulb and equilization port).


Is it correct to say that pulled vacuum is not measured?

[Lowrider]

Is it correct to say that pulled vacuum is not measured?

Was my first thought as well!

Even if the system is charged correctly one can have bubbles in the sightglass!

So the only way to know for sure you have the correct charge is to have the chiller running full load as near as possible to it's setpoint/ design leaving water temperature. You'll have to know what it's designed for, otherwise all logic is out the door!

[cool-hvacer]

Ummm..... Yes - it is correct to say that the pulled vacuume was not measured/recorded (I would have to ask the service tech. he may actually know, as this was a detail I was not on top of).

But I am not sure that I understand what you are getting at.... Are you suggesting that the charge might be contaminated, and that this would prevent the sight glass from clearing?!!?

[Gary]

GARY:

I thought that you needed to charge to clear sight glass (means you are close and TXV will be working properly). Then use Superheat/subcool measurements to nail it down (fine tune).

I could take the measurements, but do they mean anything without solid liquid at sight glass??



On the contrary, the measurements tell us what we need to know. Without them we are just guessing.

We need to know the entering and leaving glycol temps, the condenser entering and leaving air temps, the high and low pressures, the receiver outlet liquid line temp and the suction line temp at the TXV bulb.

[Peter_1]

PETER_1:

Volume of charge should = Liquid Line + 20% of receiver volume + 30% condenser + 70% evaporator.

....If you have subcooled liquid flowing to a reciever, and subcooled liquid flowing out of a reciever, can this "gas" volume you are speking of actually exist for long in the reciever.

i.e. As the process runs, "gas" (saturated r134) will eventually cool and that volume will turn to liquid, thus pulling more subcooled liquid from the liquid line / condenser into the reciever. Thus you will not have the 30% volume of liquid in the condenser?!?!?
....

If you have gas and liquid in your receiver then you don't have subcooled liquid but saturated. When a substance exists in gas and liquid together, then you have a saturated condition.

The subcooled liquid will not pull the liquid out of the condenser. The liquid flushes out of the condenser into the receiver by gravity together with some gas if there's no subcool section incorporated in the coil .

Why should the gas cool in the receiver? Pressure in receiver and condenser is +/- the same. If your receiver is colder then the ambient, then this is possible but you only will then subcool it a very little bit after liquid from the condenser flew into the receiver.

[Gary]

The old axiom that you cannot have subcooling in the presence of vapor nor superheat in the presence of liquid is just simply not true.

Imagine a half full container of refrigerant sitting on a block of ice with a heating pad on top. You now have superheated vapor on the top, subcooled liquid on the bottom and saturation at the liquid/vapor interface. All in the same container.

Is it possible to have vapor above subcooled liquid in the receiver? Yes, and in fact this is the norm.

[Peter_1]

I see it this way because this explanation is for me more hypothetical.

There's an exaggeration of the situation to a no real-life situation and even then, nobody ever tried this or has proved this.
And event then, it all depends on the volume of gas and liquid in the container, the material of the container and the amount of heat and cold you 'add' to the system.

If you heat it on the top, then the relative colder liquid will cool the gas down to a new equilibrium temperature and some of the gas will condense.
The colder liquid at the bottom will be heated immediately by the relative warmer liquid.

But this is something that doesn't happens in a real-life liquid receiver. There's a continuous flow of liquid coming in at a relative stable situation and going out at the same temperature.
I measured this a hundred times.

If you have steam above water, then it will condense immediately and if you continue to add steam, you will get some sort of circulation in the gas.

That's how I see it.

[cool-hvacer]

NOW WE ARE TALKIN!! These are the several concepts in these last three posts that I am trying to get my head around....

If we first discuss the case of a "properly operating" system:

1) Superheated gas before the condenser (and in the first few loops)...
2) Saturated liquid and gas in the middle loops of the condenser...
3) Subcooled liquid in the last few loops of the condenser, and on the way to the reciever...
4) Perfectly insulated receiver (in our case, it is very well insulated, but the receiver temp is little above the room temp, so there may even be a bit more cooling)

I do understand that superheated, saturated, and subcooled can exist together, but when you start to consider our "steady state" case:

We have subcooled liquid coming in to the receiver (from the condenser), and we have saturated OR subcooled liquid leaving the receiver (we don't know which, but from receiver geometry, it is definitely a liquid). Over a period of time, won't the gas in the receiver lose heat to the subcooled liquid all around it, and liquify.

It is exactly like your example Gary... except switch that heat pad you had on top to another block of ice.... Or an even better "parallel example" - remove the heat pad, and insulate everything but the ice interface.... It make take a while, but pretty soon it will all be liquid....

This is what I meant when I said that the reduction of gas volume in the receiver will draw liquid out of the condenser. I guess what I really meant was that as the volume of gas in the reciever changes to liquid, your liquid level in the condenser will simply go down..... I THINK?!?!?

And wouldn't this be the "steady state" of operation for a properly operating system?!?! i.e. no gas in the receiver?!?!?..... HELP?!?!?!

[Gary]

Static gas laws go right out the window as soon as energy is applied. Then the system is not static, it is dynamic. The "steady state" is a "steady state of change".

If you measure temp and pressure at the liquid line sightglass as you charge a system, you will find that you are measuring subcooling long before the sightglass is clear. You have a mixture of saturated vapor and subcooled liquid, with the subcooled liquid in the majority. It takes a substantial amount of subcooling to clear the sightglass.

Similarly, the suction line will contain liquid droplets until you have picked up a substantial amount of superheat. A suction line with 1 degree of superheat at the compressor inlet will flood the compressor with liquid.

[Gary]

It is exactly like your example Gary... except switch that heat pad you had on top to another block of ice.... Or an even better "parallel example" - remove the heat pad, and insulate everything but the ice interface.... It make take a while, but pretty soon it will all be liquid....


No, the liquid is flowing out of the receiver at the same rate it is flowing into the receiver. In fact, in a stable series loop system the mass refrigerant flow is equal at all points in the system.

During normal operation, the receiver contains a relatively small amount of liquid and mostly vapor.

[Peter_1]

If flow in = flow out of the receiver, why we then need a receiver?

Why we need a vented pipe on top of the receiver to the inlet of the condenser?

Then what with a pack/rack where several evaporators are shutting down? Compressor is pumping for some time a lot more to the condenser then there's leaving the receiver.

[Gary]

While flow is equal at all points in a stable system, that flow can be in liquid or vapor form (or a mixture). The receiver ensures that the leaving refrigerant is in liquid form and it stores surplus refrigerant in liquid form when flow is reduced.

The purpose of the equalizer line on remote condensers is to make sure the flow from the condenser to the receiver is in liquid form, so as to not back up liquid in the condenser and also to equalize pressures if the condenser pressure is lower than the receiver pressure due to cold ambients, which can restrict flow.

In pack/rack systems, as well as other systems, there are times when the flow is de-stabilized, such as a pumpdown, during which time the flow is obviously not equal at all points in the system until the flow is once again stable.

Reduced flow at any point in the system eventually becomes reduced flow at every point in the system.

And I'm thinking we are getting far more complex than is needed for this discussion.

[nike123]

Ummm..... Yes - it is correct to say that the pulled vacuume was not measured/recorded (I would have to ask the service tech. he may actually know, as this was a detail I was not on top of).

But I am not sure that I understand what you are getting at.... Are you suggesting that the charge might be contaminated, and that this would prevent the sight glass from clearing?!!?


If vacuum is not measured, then it could be that charge is contaminated. Then, air would end up in condenser and reduce his effective surface (trapped air cannot condense), therefore it is possible to have low or no subcooling (and elevated condensation pressure and bubbles of evaporated refrigerant) because of that. Without measuring we cannot tell what is happening.

[cubecool]

Hi, just a question:

Did the system ever work fine?
Isn't there a restriction before the expansionvalve?

Cubecool

[cool-hvacer]

Hi all... Sorry got pulled off onto something else, and have neglected this a bit.

It looks like this thread has somewhat drifted away from the original question, but it is still very interesting. So I will continue replying to this one, but I will repost the original question, along with a new one we began discussing (about the receiver) - because I think they are both good questions to have on record.

Well, ok, I think our system has a TXV problem. At least that is the conclusion I draw from these numbers I took:

P@compressor = 105psia
T@compressor = 88 deg F
(approx 5 deg super)

P@condenser inlet = 103psia
T@condenser inlet = 86 deg F

T@condenser outlet = 81 deg F

P@receiver entrance = 88 psia
T@receiver entrance = 80 deg F
(approx 7 deg super <------?!?!?!?!)hmmm

T@sightglass / TXV entrance = 60 deg F

T@TXV exit = 23 deg F

P@compressor Suction = 30psia
T@compressor suction = 15 deg F
(approx 0 superheat)

Looks to me like TXV is not even close to operating properly.

A few observations......

The 7 degrees superheat at the receiver entrance is weird. There is also about a 20 deg F temperature drop from the receiver to the txv/sightglass. There is a filter in this path - looks like it needs to be checked!

I am going to visit the facility again later this week, so if you have any suggestions as to what to try, let me know. For now, I am going to

1) try to get the txv to close
2) try to get the high side pressure up
3) look at that filter for blockage
4) add more glycol to our mix - (its at 40% now, but I am curious about the pressure drop between the txv exit and the compressor suction)

:D

[nike123]

Duplicated post, ignore!
Why there is no delete button, grrrr!

[nike123]

1) try to get the txv to close
2) try to get the high side pressure up
3) look at that filter for blockage
4) add more glycol to our mix - (its at 40% now, but I am curious about the pressure drop between the txv exit and the compressor suction)

:D


You have no evaporator superheat and very low compressor superheat. Also you have big pressure drop between condenser outlet and TXV inlet. Low high side pressure is result of low heat absorption at evaporator.



do not touch TXV
check evaporator for proper flow (by measuring flow is preferable). Cooling capacity, 5K deltaT, and glycol mixture determine required flow.
change filter drier and clean TXV screen
Check for other obstructions from condenser to TXV
Evacuate and measure achieved end vacuum and check for rise in pressure for moisture
Leak test
Charge initially for amount same or lower as volume of liquid receiver
Evaluate charge by subcooling while condenser is covered with cardboard or something else (manually switching fans) and high side pressure is maintained to about 150 psia
Wait until glycol mixture is near design temperature and then check superheat
Fine tune charge by subcooling (7-8K)
Adjust superheat at TXV if necessary (4-5K)
Check and adjust charge again

Than take all measurements again and this time include chiller water in and out temperatures and condenser air in and out temperatures.

P.s. Remove cardboard.:D