Condenser Liquid Seal

[Chseybold]

Have any of you ever had trouble maintaining one in a shell and tube condenser?
How do you know if you have a good liquid seal?
We have a system that has constant flash gas in the liquid line that we can't get to clear up. The system is 404a and it's supposed to be at 105condensing but even at 350psi (which is way overcharged) we can't supply clear liquid.
The condenser water side operation seems ok and have checked everything else.
http://www.refrigeration-engineer.co...ad.php?t=19004

This is a Standard Refrigeration condenser and is rated at 500k btuh at the operating conditions, the system rejects aprox 300k btuh max.

The liquid line is really hot which indicates the hot gas is going through the seal, any of you have experience with this?

[Grizzly]

Hi Chesybold
Slightly off track here.
But is there a liquid line filter before the sight glass.
If there is what is the temperature difference across it?
Grizzly

[Chseybold]

No temp differance across liquid line filter. Sightglass before filter and another by txv valve

[al]

What temp is water in/out, what temp liquid after condenser, is it piped correctly?

al

[US Iceman]

Where is the liquid line? And, how long is it?

Does the liquid line flow up or down?

[Gary]

the system rejects aprox 300k btuh max.

A detail you neglected to mention in the other thread.

BTU = GPM x dT x 500

300k = 40 x 15 x 500

So, your dT would be 15 at max output.

Your incoming water is 78F, so the water out would be 93F.

Your approach is 32F.

Add that to the water out temp and the SCT would be about 125F. The high side pressure would be about 331psi at max output.

[Gary]

The approach is excessive. It should be no more than 20F. If it were 20F, the SCT would be about 113F, which would give you a high side pressure of about 280psi.

So if the approach were reduced, the pressure would be 280psi.

With a 60 GPM pump, the dT would be reduced by 5F, as would the SCT. The SCT would then be 108F with a pressure of 265psi.

So, if all were well and the pump size were increased, you would have 265psi when the system was at max.

[Gary]

Seems my math was way off last night. I have corrected the mistakes in the above posts... that's what I get for posting at 2AM.

[US Iceman]

Any water-cooled condenser I have seen operates with a condensing temperature no higher than 105°F, if it was designed properly.

If the discharge pressure is too high and the system appears under-charged you most likely have a fouled condenser. That would give you high discharge pressure... and if the system appears under-charged you would have hot gas flowing down the liquid line.

You need to provide some data on the condenser....

Water flow
Water temperatures in & out
Condenser refrigerant outlet temperature

[joe magee]

I'm curious. How do you know you have flash gas? If the refrigerant flashes at the sight glass that doesn't mean that it's flashing before the valve. Did you torch the receiver? I started a five ton walk in freezer with 404 a and could not clear the sight glass even though the receiver was at fifty percent. When I asked our engineering department why it was flashing I was told that with the condensing unit on top of the box with 404a and a vertical liquid line once the refrigerant enters the s/g it will have a tendency to flash. I have been servicing and starting up supermarkets for years and this is the only unit I have seen do this on every occasion. Sometimes I don't try to figure everything just makes sure it works.

[US Iceman]

...I asked our engineering department why it was flashing I was told that with the condensing unit on top of the box with 404a and a vertical liquid line...

Hi Joe.

Vertical liquid lines can cause flashing. The reason is due to static pressure. When the liquid flows up the static pressure decreases, which causes the liquid to flash due to the lower pressure. You need subcooling to overcome this.

If the liquid flows down, the static pressure increases. Therefore the liquid stays in liquid form only.

[joe magee]

The liquid line coming from the receiver went down to the txv. I had 10 to 12 degrees of subcooling so I knew there was liquid. If you looked at the sight glass you could see the refrigerant rushing in. The s/g was halfway filled with refrigerant if that makes sense. My thinking was I should have a seal from the txv back to receiver. One of our engineers told me because the s/g was a larger diameter then the liquid line it would flash. Now I know that isn't always true, but in this case it was.

[US Iceman]

You can't always trust sight glasses for charging systems. I've seen them clear full, partially full, and with bubbles or froth. You have to keep this in context with what the system is doing also.

If you are starting up a hot system, nothing is where it will be after the temperatures have stabilized. Sometimes systems have a mind of their own too...

[Billy Ray]

try checking pressure drop between compressor discharge & TXV.

Exessessive pressure drop would possibly show these symptons i.e undersized liquid line!