Reply to Thread

First off, fan cycling is a very poor method to control head pressure.

The wide pressure swings cause problems with the TEV operation.

The TEV not only needs a solid column of liquid (which fan cycling is often hard put to provide) but also wants a steady quality of liquid entering it. If that varies a lot, you'll get poor superheat control.

Best option on smaller equipment is a headmaster valve. Next option is a combination of ORI/ORD valves.

As these valves flood the condenser as ambients decrease to maintain proper head pressure, it is vital to have a proper charge in the system so that there is enough refrigerant in the system to fill the condenser at the lowest ambient AND continue to provide liquid to the TEV.

The easiest way to do this that I've found is by computing the charge required using Sporlan's 90-30-1 chart.


http://www.sporlanonline.com/90-30-1.htm

Clear the sight glass, if it has a pump down make sure when it pumps down your not over charged, that's it!

This time of the year without a headmaster is hard to get a fan cycling control to level out and get a true solid column of liquid without blocking off the condenser.

In my opinion, especially on TXV systems, you do not charge by superheat. Only on cap tube systems as a verification after weighing in the charge. I do not see the reasoning that you dont clear a sight glass if the system is R22? If the system is designed properly you should always clear the sight glass unless you exceed 300 pounds of head pressure on R22. If you overcharge the system you may have a pumpdown capacity issue and may go off on high headpressure. Superheat is obtsined by a metering device such as a cap tube or TXV etc. Saying not to clear the sightglass or charging to 20 degrees of superheat is like saying fill your gas tank of your car and stop filling it when you reach 40mpg.......,,kind of........,

Originally Posted by bunny

Your statements here raise a few questions:

Did you charge to a full sight glass, or charge to superheat as your boss requires?

In either case, why was the system operating with 20 deg superheat?

What does your boss want the SH to be?

In additon are you talking evap SH or pump SH?

Originally Posted by marter

Got the box to 42° with a 20° superheat and then box was down to 38° when I left.

Ive always been told to keep a full glass to ensure all liquid to txv. Manager says to charge to the correct superheat and not worry how full the glass is and coworker says only with R22 you should charge to a full glass

Your statements here raise a few questions:

Did you charge to a full sight glass, or charge to superheat as your boss requires?

In either case, why was the system operating with 20 deg superheat?

What does your boss want the SH to be?

Originally Posted by Snapperhead

I never see a sight glass on cap tubes , i usually have to bring out the crystal ball

Crystal ball? What's wrong, don't you have x-ray vision?

Originally Posted by Snapperhead

I never see a sight glass on cap tubes , i usually have to bring out the crystal ball

Smart man

I never see a sight glass on cap tubes , i usually have to bring out the crystal ball

This one never goes away.......it depends on what type of system you are working on. The answer to this question is always yes. However, it depends on what type of system you are working on. The debate rages on that if you have a full column of liquid in the liquid line you should have a clear sightglass. On cap tube systems, due to may reasons, the sightglass may always flash and never clear. There is a better reason to use a sightglass on systems. And that is for moisture reasons. I have my guys install sightglasses on all refrigeration systems for this very reason. On top of that, they are a great way to visually check the refrigerant charge on any system along with any moisture issues. Although charging by superheat on cap tubes in the best way, second to weighing in the charge, I don't think charging a system by superheat, let alone sub-cooling is the correct way either. What is the correct way? By using the recommended refrigerant charge on the piece of equipment. If no tag is available, then I usually just let the system equalize after a recovery, pressurization, deep vacuum and let the refrigerant fly in under pressure until equalization occurs. You will be pretty close to a full charge. As far as sightglasses on ice machine, I recommend installing them for moisture indication purposes and a quick visual check of a refrigerant charge. MOST of the time, a clear glass is a full or overcharged system.

The only time you cant use a sight glass is when you have a headmaster , because even after the sight is cleared you still need to add a little more , and there is a procedure for measuring that amount.

All ways an interesting one.
As stated above a sight glass is a tool and used as an aid to charging, generally speaking your pure refrigerants, are normally better than mixtures using a sightglass method. (with mixtures very small pressure drops, valves, driers etc. can cause bubbles) At times of very high load, it sometimes difficult to get a full sightglass when the valve is 100% open especially the TXV is slightly on the larger size of the required duty.
Superheat is also a good method, but normally only when your room or product (one pass chillers) is close to set point.
So it more down to understand at what stage your system is at and what condensing control you have. (also stated above)

Originally Posted by marter

I understand this but he signs the paycheck.

and when the call comes back in you were the tech who worked on it last...

Try blocking off the condenser like Capz suggested

If fan cycling is throttling my pressure up & down, first I make sure the fan control is set up as accurately as possible for the refrigerant being used. Then I try and maintain a head pressure of 180 (R22) maybe using a piece of cardboard to partially cover the condenser and then can watch the sight glass and clear it at this point. If I still see bubbles at a lower head say 160psi, I might add small amount more but being careful not to over charge. I don't even call the boss. I am the boss...

There's a way to respectfully argue your point to the boss who you work for. Have your facts straight (which you are trying to do by being here) and just state your disagreement and back it up with why. Explain you will make him more money by doing it the right way the first time.

Originally Posted by markettech

You don't have to prove a flashing sight glass via a picture, I would hope. Why can't you properly charge the unit (clear the sight glass) AND keep the boss happy by simply saying you charged the unit to XX superheat?

Bingo!

Do the right thing and tell the bossman what he wants to hear.

Originally Posted by marter

I understand this but he signs the paycheck.

You don't have to prove a flashing sight glass via a picture, I would hope. Why can't you properly charge the unit (clear the sight glass) AND keep the boss happy by simply saying you charged the unit to XX superheat?

Originally Posted by marter

A field installed unt no weigh in charge, you would have to have some type of head pressure control to handle low ambient tempreture though.

Headmaster would be the way to go.

A field installed unt no weigh in charge, you would have to have some type of head pressure control to handle low ambient tempreture though.

I understand this but he signs the paycheck.

Originally Posted by marter

Manager says to charge to the correct superheat and not worry how full the glass is and coworker says only with R22 you should charge to a full glass

If the TXV does not have a solid column of liquid to work with, how is it supposed to maintain a 'correct superheat'?