1. Professional Member
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Jan 2004
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Thanks for the post, eventhough I did not learn anything from it. In fact you got me confused. Are you saying to measure Subcoling at the Evap. coil ?

both usually taken at o.d.u. service valves

Tell me, on Salad cooler (at 40 F)running on R12, what would be the Superheat (yes, it is using TXV) ?

it should be controlled by txv and at manafacturers spec
a txv is usually pre set dont mess with it unless you are "real good"

Also, when switching from R12 to R409 - how do you adjust that "gizmo" as you call the TXV ?

probably with different refrigerant you would change valves unless the properties of the refrigerants are nearly the same and i doubt they are

Is the Superheat meaningfull on a fixed metering device, if so , then what is the proper Superheat on a domestic fridge ?

most are designed to operate in room temps of about 70 degrees with captubes and superheat will vary with box temp(load)

hope i cleared up some of your confusion

[Edited by resolutetech on 04-07-2005 at 07:00 AM]

2. Banned
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May 2004
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Originally posted by umd
Ok, I'll take the bait...The dry bulb delta T across the evap coil will be a function of the amount of sensible heat removal from the airstream and the amount of air going across the coil. Now if you were only doing sensible coiling (not removing any moisture) the formula would look like this: BTU/HR(sensible)=CFM X1.08 X DELTA T. Unfortunately in the real world you are usually removing some latent heat which is not measurable by dry bulb temperature. To accurately predict the delta T, you would need to know the wet bulb temperature of the entering and leaving air. You would also need to know the airflow across the coil. Using a psychrometric chart you could predict what your delta T would be by calculating the actual enthalpy per pound of heat in your entering and leaving air and use the formula: BTU/HR (total)= CFM X 4.5 X DELTA H (actual heat content of air).
The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T.

Sorry if you were offended.

THAT is how a professional answers a vague question!
It depends.... and then.... the bottom line...BAM!
Nicely done UMD

3. Regular Guest
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Originally posted by umd
The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T.
If the unit is properly charged and the air flow is correct for the conditions, delta T should always be close to 20 °F once indoor humidity has stabilized. If it is more than that, efficiency is being compromised. The higher the coil temperature the more efficient the system.

In high and dry Denver, Colorado area just about every HVAC professional believes the correct air flow is 400 CFM per ton. But when the relative humidity is only 15% on a hot day, 400 CFM per ton can result in a delta T close to 30 °F and a very inefficient system prone to freezing in the evening. Increasing the air flow to 600 to 700 CFM per ton and an 20 °F delta T will solve both of these problems. Unfortunately, the ducts can seldom accommodate any additional air flow.

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