TimR wrote:

Stormy Weather wrote:

If your intention is to control the humidity, then yes run your
de-humidifiers. The trade off is more money. The heat added by the
de-humidifiers will cause the a/c to run longer...

The heat is bad news, in summertime.

...Just let the AC filter get dirty.

To reduce the airflow over the cold side...

Well, it is wrong, but filters isn't the answer. (Although they may
help with the constant volume problem.)

I disagree. (What's the "constant volume problem"?)

I don't think his intention is to control humidity, I think it is to
save energy.

We might use night air for cooling, with AC mainly for dehumidification.

...Normal household AC does not control humidity. It controls temperature,
and reduces humidity by happy thermodynamic accident.

It's no accident. If C cfm of 70 F air at 50% RH with Pa = 0.374" Hg enters
a box with a 20 ft^2 40 F coil and mixes well before leaving, we might have
something like this (viewed in a fixed font):

1/C 1/40
70 ---www-------www--- 40
|
T = exit air temp = 40+30C/(C+40).

cfm T RH@70F = 100e^(17.863-9621/(T+460))/(2x0.374)

100 61.4 50%
50 56.7 50%
20 50.0 49% |
10 46.0 42% | dirty filter
5 43.3 38% |

One way to get better control is to use a humidistat and add reheat.

Reheat is bad news, in summertime.

A dehumidifier could sort of function that way. I think you would want
to change your basic AC parameters if you wanted to do that though.

We might add a freezestat...

Nick

wrote in message ...
TimR wrote:

Stormy Weather wrote:

If your intention is to control the humidity, then yes run your
de-humidifiers. The trade off is more money. The heat added by the
de-humidifiers will cause the a/c to run longer...

The heat is bad news, in summertime.


You don't bother to state your assumptions. If you are below your
design conditions, you have capacity to handle the heat. If you are
at your design condition you are getting good humidity reduction and
your dehumidiers don't run.



...Just let the AC filter get dirty.

To reduce the airflow over the cold side...

Well, it is wrong, but filters isn't the answer. (Although they may
help with the constant volume problem.)

I disagree. (What's the "constant volume problem"?)

Simply that a constant volume airconditioner can never adequately
reduce humidity at part load. It's basic thermodynamics. See the
paper on the Trane page, it's pretty clear. How often are you at part
load? Well, if you design for a 2.5% day, then about 97.5% of the
time. If I did the math correctly. This is the problem VAV systems
solve.


I don't think his intention is to control humidity, I think it is to
save energy.

We might use night air for cooling, with AC mainly for dehumidification.

...Normal household AC does not control humidity. It controls temperature,
and reduces humidity by happy thermodynamic accident.

It's no accident. If C cfm of 70 F air at 50% RH with Pa = 0.374" Hg enters
a box with a 20 ft^2 40 F coil and mixes well before leaving, we might have
something like this (viewed in a fixed font):

1/C 1/40
70 ---www-------www--- 40
|
T = exit air temp = 40+30C/(C+40).

cfm T RH@70F = 100e^(17.863-9621/(T+460))/(2x0.374)

100 61.4 50%
50 56.7 50%
20 50.0 49% |
10 46.0 42% | dirty filter
5 43.3 38% |


Your system is designed to have variable air volume by adding
resistance to the filter section. Bad idea for two reasons: The way
you want to vary the CFM is by a variable speed fan. Also, you can't
turn airflow down to 5% on a DX coil, you'll freeze it solid.

One way to get better control is to use a humidistat and add reheat.

Reheat is bad news, in summertime.


No, there's nothing wrong with reheat, IF you have capacity. And you
always do, at part load. At full load, your reheat doesn't run. It
doesn't need to, you have (usually) sufficient humidity control.


A dehumidifier could sort of function that way. I think you would want
to change your basic AC parameters if you wanted to do that though.

We might add a freezestat...

Nick

Tim

On 20 Jul 2004 03:27:11 -0700, (TimR) wrote:

Also, you can't
turn airflow down to 5% on a DX coil, you'll freeze it solid.

Yeah, with any cheap commercial, cap-metered unit with no ORI (EPR) valve.
Unless you can cycle the pump and keep it from short-cycling.

Ane evap cpil will ice up, if the compressor runs too long, regardless of
airflow, UNLESS there is a good EPR downstream in the suction line.


--
-john
wide-open at throttle dot info

Hi ~^Johnny^~, hope you are having a nice day

On 21-Aug-04 At About 14:36:13, ~^Johnny^~ wrote to All
Subject: running dehumidifier with central a/c, yes or no?

~ From: ~^Johnny^~


~ Ane evap cpil will ice up, if the compressor runs too long,
~ regardless of airflow, UNLESS there is a good EPR downstream in the
~ suction line.

Just so eveyone is clear this is not true. if the air flow and temp are high
enough you could run the compressor as long as you want and it would not
freeze. but if you let the air flow rate, or temp, drop too low it will freeze
in no time flat.

-= HvacTech2 =-


... *IT IS* documented, look under "For Internal Use Only."

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