De-humidification

[walden]

I have, up to now, always designed de-humidification systems by forcing on the cooling (usually by opening the liquid solenoid valve) and then allowing the heating to be switched on as the temperature falls.

Just lately, I have been told that it would be better to do the reverse! That is, to switch on the heating to reduce the RH and then allow the cooling to be switched on to counteract the rise in temperature.

What are your thoughts on this new suggestion?

I have problems on one site where the system oscillates between heating (de-hum) and cooling without ever reaching a stable state.

[NoNickName]

Heating does not reduce humidity, it just increases temperature. By doing that, the RH reduces. Infact the absolute humidity remains intact.

On the other hand, cooling causes condensation and so water vapor is removed from the treated air.

Therefore, priority must be given to cooling, and heating only if the temperature falls below the temperature setpoint by 1°K or 2°K.

You are right, but only if heating capacity is the same or more than cooling capacity; if not, the forced cooling will cause too low temperature in the treated room.

The suggested operation is not convenient, because the dewpoint must be reached. Eg: 20°C 50% has the same dewpoint of 25°C 37%, which is 9.6°C approx.
It is useless to heat up to 25°C, since in order to dehumidify you will be forced to cool the air down to 9.6°C.

The correct cycle in this case will be 20°C 50%, down to 9.6°C 95% with sensible cooling, down to 5°C 100% with sensible plus latent cooling, and then reheat up to 20°C for a 40% humidity (10% net humidity reduction at 20°C corresponds to 1.5g/Kg water removal)

[Abby Normal]

If you were trying to dry out a space that had been flooded for example, you would want heat added to the space, similar to the discharge from a self contained dehumidifier, that removes moisture from the air, then reheats the air with the heat of rejection.

The warm temperatures, encourage mositure to evaporate from the wet surfaces. Air movement helps as well.

As far as the equipment is concerned, the more saturated the air is, the more moisture it would remove. You want reheat on what leaves the coil, not preheat on what enters.

[Brian_UK]

Go along with NoNickName but would add that if you are getting temperature overshoot then consider improving the control of the electric heaters.

What sort of control do you have presently?

[winfred.dela]

NoNickName's cooling and then re heating is the best way. However, in reheating the cold air, i usually use the hot gas heat from the compressor before sending the refrigerant back to the condenser. With hot gas reheat, the problem you mentioned on the cooling/heating oscillation will be minimized. A good humidity controller will also help.
To handle a very low dehumidification load when door will not be opened for very long period of time, it would be practical to add a small dehumidifier. ;)

[Abby Normal]

winfred

York will sell packaged units with two cooling circuits. It allows the heat of rejection of one ciruit to reheat the cool air produced by two circuits. Results in dry air roughly the same dry bulb as the entering air being supplied.

Other manufactures offer a solenoid that will allow a small reheat coil to be in parallel with the regular condenser, and sometimes on a dehumidifcation call, you end up again with dry supply air roughly equal to the entering dry bulb.

I have also used small dehumidification circuits in series with a deep row coil. An evaporator and condenser coil connected to a smaller compressor in series with the air flow leaving a large 8 row coil.

This reheat and the fan motor heat usually results in a discharge of 18C with dewpoint down arounf 10 or 11 C.

Have you seen anything that modulates the amount of hot gas to maintain a discharge air set point?

[winfred.dela]

Hi Abby,
Thanks for the info, if I could take hold of units like York, that would have been great.
But you know. . . we are in the Phils., down South it's just not possible to have these equipment fast (as we always find customers who need it yesterday or if possible NOW. . .).
So, we always do the Mcgyver thing: Find the available Split Aircon Unit, check the compressor duty, add bare pipe condenser (for hot gas reheat) install it next to evaporator, add the minimum controls (solenoids, check valve, temp/humidity sensor, etc.) and. . . you have the cheapest dehumidifier.
Re hot gas modulation for reheat based on the temperature, i have not done it yet. I just have to close the hot gas solenoid valve based on the room humidity.
Again, thanks.

[Abby Normal]

Most of the packaged unit OEM's are coming out with hot gas or liquid line subcooling reheat, to get around using electric reheat.

Trane is selling a curb with a dessicant wheel in it even.

Bard was offering a hot gas reheat coil in their wall mounted units. Be nice to see a partial hot gas reheat available in spit system air handlers. Something that does not blow 120F dry air into the space.

[walden]

Be gentle with me! I’m only an electrician trying to understand his psychrometric chart!

I get involved with two types of installation. One installation is an AHU with separate heating and cooling coils. With the AHU, heating to reduce the RH and then cooling seems to work.

The second installation is a conventional evaporator using the defrost heaters as air heaters. In the second type of installation heating before cooling is generally a disaster with hunting between heating for de-hum and cooling to regain control of the space temperature.

Your comments appreciated.