Wet Bulb probe (working out target superheat)

[RFChiller]

When taking you're indoor temp from of the Air inlet to Evaporator, do you use an actual wet bulb thermometer or do you just use a dry bulb(& improvise)?



Thanks :)

[Magoo]

The dry bulb temp change will give you sensible heat capacity, the wet bulb change will give you total heat capacity. Deduct the sensible capacity from the total and you will get the latent load.

[hvacrmedic]

When taking you're indoor temp from of the Air inlet to Evaporator, do you use an actual wet bulb thermometer or do you just use a dry bulb(& improvise)?



Thanks :)

I use a digital psychrometer.

[RFChiller]

I use a digital psychrometer. It doesn't take a bit more time to measure wb temp than it does to measure db temp.

If you're going to improvise, then why measure anything? It won't be charged right when you're done, so you might as well skip the SH chart too, and just go for a beer-can-cold suction line. FWIW, try this: For the next several calls on fixed orfice systems, guesstimate the indoor wb and then make an actual measurement of it. Then check the SH chart to see how much your guesstimate caused you to be off. I think you'll be surprised to find how far off your assumed target SH was on most of those calls. That's actually one of the reasons that I gave up on guessing, because after double checking it, turns out I'm not very good at guessing the wb temp. I even got carried away with it and reduced the SH chart to a formula so that I'd always have the means to calculate the target SH on hand.hmmmm..i wonder if you the wrong end of the stick with my question.

I was merely asking what you use to take the measurement, as i have read of people using the normal dry bulb thermometer and applying a little wet tissue.

[cool runings]

hmmmm..i wonder if you the wrong end of the stick with my question.

I was merely asking what you use to take the measurement, as i have read of people using the normal dry bulb thermometer and applying a little wet tissue.


Why do you need to know?

Because you ask about air on temps but mention superheat in your title!!

To measure temp use a dry bulb.
To measure humidity use a wet bulb and a dry bulb.
To measure superheat you need gauges and thermomiters.

All the best

Coolrunnings

.

[Magoo]

Hi RF Chiller.
Do a search on superheat settings, there is a lot of discussion on the subject on this site.
Magoo

[hvacrmedic]

hmmmm..i wonder if you the wrong end of the stick with my question.

I was merely asking what you use to take the measurement, as i have read of people using the normal dry bulb thermometer and applying a little wet tissue.

My bad. Well, in that case the answer is no, I don't improvise.
You can use a wetted thermometer sensor or bulb, but it'll typically read a little high. If accuracy isn't that important, then go for it. For adjusting or checking SH on a fixed orifice A/C unit, it'll probably be close enough (within a degree or two) as long as you hold it in the return air stream. But with no air moving across it, it'll be off a couple or maybe even several degrees.

[RFChiller]

Why do you need to know?

Because you ask about air on temps but mention superheat in your title!!

To measure temp use a dry bulb.
To measure humidity use a wet bulb and a dry bulb.
To measure superheat you need gauges and thermomiters.

All the best

Coolrunnings

.....To measure target Superheat you need wet bulb & dry bulb readings.

[cool runings]

....To measure target Superheat you need wet bulb & dry bulb readings.


Is your definition of superheat the same as mine?

Superheat is the amount of heat added to a vapour above boiling or saturation temperature is it not?

So to work out superheat you need to know the evaporating temperature and the actual temp of the vapour exiting the evap.

To know the saturation (evaporating) temp of the refrigerant you need to fit your gauges.

Why do you need a wet bulb reading. Wet bulbs work out humidity. Humidity in air means the air is wet and wet air can't be superheated.

coolrunnings

[TRASH101]

Don't worry cool runnings

The way I see it is, RF chillerman is refering to working out the combined latent and sensible load that will act on an evaporator (with a known duty)during a pull down causing an increased superheat to what is the design superheat. This of course must have a maximum value so as not to take the compressor out of its operational envelope.

Is that right RF?

[cool runings]

Don't worry cool runnings

The way I see it is, RF chillerman is refering to working out the combined latent and sensible load that will act on an evaporator (with a known duty)during a pull down causing an increased superheat to what is the design superheat. This of course must have a maximum value so as not to take the compressor out of its operational envelope.

Is that right RF?


Thats why I love this site.

You should never stop learning and I learned somthing today.

Cheers Trash..

Coolrunnings

.

[hvacrmedic]

Thats why I love this site.

You should never stop learning and I learned somthing today.

Cheers Trash..

Coolrunnings

.

The "target superheat" is provided by a chart or table. The "actual superheat" is the measured SH that the system is currently running. For fixed orifice A/C systems the charge and/or airflow must be adjusted so that the actual superheat equals the target superheat. This is for charging by the superheat method.

I've attached a Target SH table. This table (which is the most common type) requests entering air wet bulb temp and ambient temp. Not all fixed orifice charging charts ask for wet bulb temp or ambient temp. Some use system pressures, and some use pressure and temps. But the attached chart should work for almost any fixed orifice A/C system regardless of the charging chart provided by the manufacturer.

It has nothing to do with pull down, despite Trash's best efforts. :) HTH.

[RFChiller]

The "target superheat" is provided by a chart or table. The "actual superheat" is the measured SH that the system is currently running. For fixed orifice A/C systems the charge and/or airflow must be adjusted so that the actual superheat equals the target superheat. This is for charging by the superheat method.

I've attached a Target SH table. This table (which is the most common type) requests entering air wet bulb temp and ambient temp. Not all fixed orifice charging charts ask for wet bulb temp or ambient temp. Some use system pressures, and some use pressure and temps. But the attached chart should work for almost any fixed orifice A/C system regardless of the charging chart provided by the manufacturer.

It has nothing to do with pull down, despite Trash's best efforts. :) HTH.At last :D

[RFChiller]

Although can anyone answer the question on what they use for the wet bulb reading?

[BeBee]

I use a wet piece of gauze held on my temp probe with a rubber band... Stick it in the flow of return air and wait for it to cool down to it's lowest temp.