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# Test #1

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Member Posts: 1,380
Close, but determining that this system is undercharged is a common mistake.

Hint: The subcooling is zero, not low.

• Member Posts: 1,380
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Test #1

To keep things interesting, I have decided to, on occasion, post system scenarios and give you guys the opportunity to evaluate the system and post possible causes for failure and repair otions.

Here is the first.

You have an air conditioning system that is operating with R-22 as its refrigerant and is equipped with a fixed bore metering device. The operating conditions are as follows:

Low side pressure: 26 psig

High side pressure: 156 psig

Outside ambient temperature: 85 degrees F

Evaporator outlet temperature: 54 degrees F

Condenser outlet temperature: 85 degrees F

Compressor discharge temperature: 250 degrees F

Return air temperature: 82 degrees F

Supply air temperature: 77 degrees F

You make the call.
• Member Posts: 27
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I'll

go with not enough freezone..I mean freon.

I'm seeing high superheat and low subcool temps,or in your
case no subcool temps.

But I can not be sure without checking for proper air across the coil.

• Member Posts: 1,380
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One question

What will happen if there is insufficient airflow through the evaporator coil? Will the temperature split go up or down? Hmmmmmmmmmm
• Member Posts: 27
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Depends

On rather is was iced over or partly ice over.However in this case I would say up.

• Member Posts: 1,380
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Evaporator Temperature Split

Correct.

If the airflow through an evaporator is decreased, the delta-t across the coil will increase and the evaporator saturation temperature and pressure will decrease. In this case, the evaporator saturation pressure and temperature are lower than normal and the temperature split across the evaporator is also low.

Let's clarify that the airflow through both the evaporator and condenser is correct.

• Member Posts: 327
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With the air flow

across the evaporator and the condenser coils being correct, a superheat of 52 and a subcooling of zero I would say the system is undercharged.

Locate & repair leak.

Replace filter driers.

Pressure test with dry nitrogen

Evacuate system below 500 microns.

Weigh in charge accoring to rating plate.

Check superheat and delta t across the coil.

How'd I do?

• Member Posts: 15
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Overfeeding

metering device,couple with a unit that overcharge?Liquid
stacking in the condenser.
• Member Posts: 1,380
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Good Try, Don

Let's take a look.

If the metering device was overfeeding, the suction pressure would be high and the superheat swould be low.

We have low suction pressure and high superheat, so an overfeeding metering device can't be the problem.

Try again, I think you'll get it now.
• Member Posts: 327
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If a fixed bore

Metering device is over feeding the coil would have too much liquid ref in it and the the superheat would be low not high.

But then again I though I had it the first time :-(

Hope we resolve this tonight

I won't be able to sleep.

• Member Posts: 109
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Residential ? metering device restricted
• Member Posts: 15
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Ok

its a restrictive liquid line drier.Wait before you answer I've got the help going to the truck to get my temperature
guage.
• Member Posts: 4
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If the condensor

outlet temp (85) is being measured after the liquid line drier then I would say the metering device is restricted. If it were a blocked filter drier there would be a temp drop across it.
• Member Posts: 1,380
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Ding! Ding! Ding!

Either a blocked/restricted liquid line filter drier or a clogged/blocked capillary tube will result in the conditions provided.

Since the refrigerant cannot flow into or through the metering device, the refrigerant backs up in the condenser and reaches a temperature equal to the outside ambient temperature, giving us zero subcooling.

In addition, the amount of refrigerant being introduced to the evaporator is greatly reduced, so it boils very fast in the coil. This results in excessively high superheat.

Since the superheat is very high, a great deal of the coil is filled with vapor refrigerant, not liquid. Since more heat is transferred during a latent heat process (boiling or condensing), the amount ofheat absorbrd by the evaporator is reduced since the small amount of liquid refrigerant boils quickly at the inlet of the coil. The remainder of the coil facilitates a sensible heat transfer, as the temperature of the vapr refrigerant is increased. This reults in a lower delta-t across the evaporator coil.

Thanks for playing, guys. You can all be correct.

And yes, Tom, a restricted liquid line drie will have a temperature (and pressure) drop across it.

See you tomorrow with Test #2.
• Member Posts: 15
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No

Thank you professor...I will say that liquid line driers
have always kick my butt when it comes to trouble shooting.

Even with the three degree drop across it and no subcooling
I still find myself looking some where else.

Cap tube are no problem I just disconnect the fan and see which one not freezing.

Bring it on...this is fun.
• Member Posts: 4
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Thanks Professor!

> Thank you professor...I will say that liquid line

> driers have always kick my butt when it comes to

> trouble shooting.

>

> Even with the three degree

> drop across it and no subcooling I still find

> myself looking some where else.

>

> Cap tube are

> no problem I just disconnect the fan and see

> which one not freezing.

>

> Bring it on...this is

> fun.

• Member Posts: 4
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Thanks Professor!

Great way to sharpen trouble shooting skills! Thanks for taking the time.

• Member Posts: 794
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Wouldn't......

....your high side pressure reading be greater than what is listed if your liquid line filter or metering device was clogged?

Starch
• Member Posts: 1,380
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Not in this case...

If the metering device or liquid line drier was completely blocked, the answer would be yes.

We know that there is not a complete blockage since there is still substantial pressure on the low side. A complete blockage would result in a low side pressure in the vacuum region.

I was waiting for someone to bring that issue to the table and Starch, you win the prize.

Thanks for playing.
This discussion has been closed.