AHU offcoil temperature

[wankidal]

Hi all, wondering is there a formula to calculate desired offcoil temperature if we want a room temperature of 24 degrees celsius.

[Peter_1]

Sorry, there isn't, even not a rule of thumb. I should say, the warmer the better.
Q= m.c.DT

[frank]

Hi all, wondering is there a formula to calculate desired offcoil temperature if we want a room temperature of 24 degrees celsius.

You need to ensure that the air off temperature is low enough to ensure good enthalpy exchange in the room. I would suggest at least 6K below the room required condition

[TXiceman]

You need to go back to basic psychometrics and plot the process on a psych chart.

[Peter_1]

Another one...again,...,coming here once with a question reading some answers and then disappears, even without a simple 'Thanks'

[james10]

Another one...again,...,coming here once with a question reading some answers and then disappears, even without a simple 'Thanks'
part of the job spend the day on a roof in the pissing down rain then off you go without a kiss my arse or a thank you:(:D

[Gary]

The coil air off temp needs to be 11K lower than the room temp in order to achieve 50% RH.

[wankidal]

ok thanks a lot peeps. sorry was busy and left this site unattended. well i guess should put more time into this forum. hope that we can discuss more topics here

[Gary]

If we plot our target room conditions (24C@50%RH) on a psychrometric chart, this gives us a target dewpoint of about 13C (11K below room temp).

Keep in mind that moisture can only be removed if the coil surface temp is below the air dewpoint temp.

As the air moves through the coil, moisture drops out, reducing the dewpoint of that air, until it's dewpoint matches the coil surface temp. At this point, no more moisture will be removed, because the surface temp is no longer below dewpoint.

In point of fact, due primarily to coil bypass factor, the coil air off temp is somewhat higher than the coil surface temp, but this difference generally compensates for normal latent loads. Going by air off temps puts us in the ballpark for most applications... and air off temp 11K below room temp is a viable rule of thumb.

After a few days of operation, we may want to check the actual room conditions and adjust accordingly. If the actual dewpoint is running 1 degree above target, we might want to lower the air off temp 1 degree. If the actual dewpoint is running 1 degree below target, we might want to raise the air off temp 1 degree... etc.

[wankidal]

I am still trying to understand your post. Kinda lagging in certain technical terms. Its been ages since i read my acmv textbook. thanks a lot anyway

[taz24]

.

Gary says 11K and I have no problem with that.

I was always told to work to a rule of 10.

Air off is 10 degs lower than air on.
Evap temp is 10 degs lower than air off.

This is a rule of thumb that works for me, but it is not the only answer and
the application will alter the correct amounts.

A rule of 10 is a good start point and then take it from there.

taz

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