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04-07-2010, 10:46 PM #1
Wet Bulb probe (working out target superheat)
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
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05-07-2010, 12:51 AM #2
Re: Wet Bulb probe (working out target superheat)
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.
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05-07-2010, 06:24 AM #3
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05-07-2010, 07:38 PM #4
Re: Wet Bulb probe (working out target superheat)
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05-07-2010, 09:52 PM #5
Re: Wet Bulb probe (working out target superheat)
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06-07-2010, 02:15 AM #6
Re: Wet Bulb probe (working out target superheat)
Hi RF Chiller.
Do a search on superheat settings, there is a lot of discussion on the subject on this site.
Magoo
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06-07-2010, 03:24 AM #7
Re: Wet Bulb probe (working out target superheat)
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.Last edited by hvacrmedic; 06-07-2010 at 12:49 PM.
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06-07-2010, 07:10 PM #8
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06-07-2010, 11:27 PM #9
Re: Wet Bulb probe (working out target superheat)
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
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06-07-2010, 11:49 PM #10
Re: Wet Bulb probe (working out target superheat)
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?Quidvis Recte Factum Quamvis Humile Praeclarum.
Latine dictum, sit altum videtur.
Si hoc comprehendere potes, gratias age magistro Latinae.
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06-07-2010, 11:54 PM #11
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08-07-2010, 04:23 AM #12
Re: Wet Bulb probe (working out target superheat)
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.Last edited by hvacrmedic; 08-07-2010 at 04:39 AM.
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08-07-2010, 08:16 PM #13
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08-07-2010, 08:18 PM #14
Re: Wet Bulb probe (working out target superheat)
Although can anyone answer the question on what they use for the wet bulb reading?
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20-08-2010, 12:43 AM #15
Re: Wet Bulb probe (working out target superheat)
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.
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