Critical charges

[VTP99]

Ever wonder how some critically charged systems can work and hold temp till there all most empty on gas ?

[76olds]

Based upon my experience, they don't. The customer does not check temperature until the unit warm enough to get their attention. Then after they made sure the problem wasn't caused by not closing the door, dirty conderser coil, and so on they will place a service call and say the unit just quit.

[VTP99]

But what about that cooler you have to fill once a year that holds 9oz's or so and it would cost your customer way more to find a small leak versus just refilling it ? The leak is at a steady rate given a constant ambient & its losing its critical charge as soon as you walk away. The temp is not off for a whole year.

[dangpgt97]

I would say by extended run times. For example on a freezer thats critical charge you may find a situation where it was low on charge but still maintaining temperature due to the steady cold product load. But if that freezer goes down for an unrelated reason you find that it won't have the capacity to pull down again because it's low on charge. I've seen this and pretty sure you get the idea.

[76olds]

Small cooler, small charge, and a small leak. At first it would be ok. Then the compressor will run longer and longer to try to maintain temp . Eventually the compressor will be running non stop. The customer might notice the temperature is warmer than normal during heavy use, but at the proper temp in the morning. In my experience, the unit would be ignored until it would no longer maintain temperature. We had a old cooler in our shop that was a leaker. The beer would be cold but we would hear it running longer and longer. Put a gauge on it, had a 3-5 lb. suction pressure on it.

[surenuff]

Low charge will start making run cycles longer and longer until it either ices up the coil on a cooler or just can't ever reach set temp on freezer unless like other poster mentioned, an un-related issue brings the problem to the fore front.

[VTP99]

I don't see how the compressor running longer is going to get the evaporator any colder.

[surenuff]

Who said it makes the evap any colder??

[VTP99]

In your post #6 you stated that low charge will make run cycles longer and longer.I took that as if by running longer the evaporator would reach its target temp of around 25*. Maybe not colder but would reach that temp.Would not the coil get warmer & warmer as it loses its charge? How would that coil at say now 30* ever get you to a off cycle at a 34*set temp?

[surenuff]

I don't really know where to start on that. I can only say what I say. I can't help how you take it or what you read into it. As a unit looses charge(assuming it is not all at once, don't want you to read into that), The capacity of the unit gets less and less. THe fact that we are low on charge does not mean that we can not reach target temp of box and shut the t-stat down and cycle the unit off. But as we get lower on charge, the time it takes for that freon that is left to do the work at hand takes longer and longer. The unit is depending on off cycle to defrost,(I am talking about a cooler OK), so the fact that we get to where we run all the time trying to satisfy t-stat causes us to ice up. As we lose charge, our suction pressure drops making the evaporating temperature even colder than it was. So instead of a full coil at 25* as you put it, we have 3/4 of a coil at 20*....1/2 coil 15*.....1/4 of a coil at 5* and so on. It is a progressinve thing that gets worse as time goes by. But just because we have lost capacity by losing some of the charge does not mean that we can't ever cool the box down and shut off. The problem is, that most of the time once our charge gets very low, the unit will be icing up which won't go un-noticed.

[cboe]

with the compressor runing constant , evap wormer, not getting cool suction gas to the windings will couse the comp. to go out on internal thermostat. keeping the comp. off for hours. then the customer calls for help. thats when you get the water hose out and let the water trikkle over the comp. to cool it down. find the leak and fix it.

[surenuff]

It can take a very long time for a slow leak on a unit to even be noticed. especially when they keep cool product in the box and are not in and out all day long

[VTP99]

Don't take it personal i am just picking your brain. The more detailed your response the less i have to assume.Did you not answer your own post #8 ?

[surenuff]

nothing personal! Some just think that losing freon actually makes it warmer as adding freon will make it colder. I'm just tired and grumpy! Get tired of not being able to sleep much.

[VTP99]

Just so were on the same page I'm talking about a cap tube system here. As they are considered critically charged. Could a cap tube specialist please chime in ? Not trying to say you didn't no this.

[surenuff]

I was talking cap tube or other fixed orifice critical charge unit.

[Airmechanical]

leaks come in ALL different shapes and sizes

to put it into perspective

after a certain percentage of refrigerant loss, temperature is impossible to maintain

some leaks are so small the cooler can maintain temp. for a year

some leaks are so big, they might hold temp for a few hours, or not at all

that is why a system that holds 9 ounces with a super slow leak can cool for a year



.

[uniservice]

And after you have lost the first couple of ounces of the critical charge, you have also lost compressor subcooling. I've been at this a few years, and the most critical thing about small cap tube systems, are people not monitoring their equipment. A unit can lose refrigerant for a long time and get very low, before the manager or owner or the cook notices it. Like it has been said, then "It just happened yesterday."