Dehumidification with a pre-heat and then a cooling coil???

[BronYrAur]

I'm second-guessing myself

I know enough about psychrometrics to know better, but I have had 2 different "seasoned professionals" tell me on different occasions that you can dehumidify by pre-heating air and then cooling it down. The purpose of pre-heating is so that you can run the cooling coil at 100% and not over cool the space.

I understand psychrometrics and know that you must cool the air below dew point to remove the moisture. Then typically you reheat it, but in this case there is no reheat coil. I was quick to dismiss the claim, but I have been pondering it for a while now. Without a reheat coil, you basically have 2 ways to attempt to dehumidify.

1. Modulate the cooling coil chilled water valve to maintain space (or discharge) temp. By doing so, the water flow is reduced and the amount of cold coil surface area is probably also reduced. So the average coil temperature is higher. This gives you a certain amount of dehumidification along with the cooling.

2. You preheat the air just enough so that the cooling coil operating at full water flow gives you the desired space (or discharge) temperature. I could see this scenario giving you more cold coil surface area and a lower average coil temperature. Would this allow more moisture to condense even though the entering air is hotter???

Not sure anymore???? Anyone have experience with this?

 

[Compositepro]

By preheating the air you then have to remove this heat before any dehumidification (condensation) can take place. So no, preheating makes no sense at all.

 

[SAK9]

When the room load is less than design,coil operates well below capacity reducing the extent of dehumidification.Preheating air is equivalent to adding to room sensible heat so this will help operate the coil at near full capacity.

 

[BronYrAur]

but will that ultimately lead to more dehumidification? Or is that full capacity coil just removing the sensible heat that the preheat coil added?

 

[urgross]

The situation you are talking about depends on the application. As an example, for an area served having a large internal sensible load which is always on, such as a server or equipment room, or an electronics room, where minimal sensible heating load is of no consequence, using dew point control works fine. Sucks when the load is turned off. Then the heat load has to be picked up at the expense of humidity control and mold becomes an issue.

For operatories, preheat reset for dew point control is common practice where reheat is available, like most operating tooms. That only works well when lower sensible temperatures are desired, which is frequent in a gown or tyvek environment. For certified environment, I have typically seen two sets of coils, one for dew point (humidity knockdown) and the downstream set for sesnible temperature control. All the animal labs and operatories I've worked in use dual coil, all the human operatories I've seen typcailly run only dewpoint control with reheat sensible temperature control,plus RH control from separate, dedicated humidistat.

 

[Compositepro]

"Preheating air is equivalent to adding to room sensible heat so this will help operate the coil at near full capacity."

SAK9, that is not correct. Reheat is the equivalent. Preheat energy never gets to the room to be sensed by the thermostat.

 

[ChasBean1]

The seasoned professionals you reference might want to think of a different line of work. Cool to dehumidify; reheat to try to make the space comfortable. If you preheat during warm/humid conditions, you are adding to the global warming energy crisis

 

[BronYrAur]

Well, I convinced at least one of those "seasoned professionals" that the concept won't work. I did find, however, a couple of cases where I could achieve more latent cooling from a coil by first preheating the air. The problem is that the discharge air conditions are still unacceptable.

I ran a scenario where the entering air was 75 deg and 50%. In one case, I throttled the chilled water valve until I reached a discharge temp of 55 deg. In the other case, I let the chilled water valve run wide open and then turned on the preheat coil to match the same 55 deg discharge temp. I actually got more latent heat removal from the coil when I preheated the air first and let the chilled water valve run at 100%. The problem, however, is that both options offered little dehumidification. If I sensibly heated both discharge air conditions back up to 75 deg, the RH% were at 45% and 47%. A little drier from the unit with the preheat coil running, however, a lot of wasted energy for just a small improvement. Plus, 55 deg air may be too cold now with no way to heat it up. Remember, this was a constant volume unit in a humid area from internal sources; so, it will be humid even when the sensible load is small.

So, I decided to raise the discharge temp to 60 deg since I have no other means of heat on this constant volume system. Again, when I turned on the preheat valve and ran the cooling coil at full flow, the coil did remove more moisture than just simply throttling the chilled water valve. However, both conditions left the discharge air at 60 degrees and nearly saturated, which translates to about 57% RH at room temperatures. This is not exactly comfortable. The discharge temperature must be colder to get the moisture out, but then the space will probably get too cold.

So, I guess it is not completely incorrect to claim that preheating the air will allow for better dehumidification; however, the discharge air conditions are probably never going to be acceptable. I suggested that they switch the coils in the unit to turn the preheat into a reheat.

 

[urgross]

If you are trying to maintain dew point control on an OR, dry bulb is about 60* F, RH above 80%, that is about the time when latent load will exceeed sensible load. Any time that the sensible load divided by total load exceeds the SHR ratio for the coil and sensible load is already met, the only way to increase latent capacity is to increase total load (which means increasing sensible load).

A "seasoned" chiller plant operator or a mechanical design engineer that does not understnad that would not bbe a "seasoned" engineer in the health care field fo long.

 

[ChasBean1]

As a general rule, cooling coils do very well with sensible heat. Pre-heating increases sensible but does not have any effect on latent. If you fully heat with the preheat coil at 100% capacity then re-cool, the CHW coil will usually do the job. The only thing is, you’re a greenhouse “gas-hole.” You just added much unnecessary energy to an environment that is staged to fix all this.

By you: “In the other case, I let the chilled water valve run wide open and then turned on the preheat coil to match the same 55 deg discharge temp. I actually got more latent heat removal from the coil when I preheated the air first and let the chilled water valve run at 100%.”

That’s great. And I averaged 55 on the freeway with the brake fully pressed because I overcame the issue with the accelerator.

In the summer, when it's warm and stuffy, cool to a temperature above which you do NOT want your dew point, then reheat so the people don't think it's cool and clammy in the space.

If you want to do this in reverse order, see me after class -)

 

[BronYrAur]

trust me, I am NOT AT ALL advocating that preheating the air prior to a cooling coil will help with dehumidification. I was trying to demonstrate the exact opposite to someone else. All I was saying in my last post is that I did find a couple of cases where a cooling coil gave me slightly better latent heat removal if I preheated the air first. That's because I was able to run a colder coil then I otherwise would by throttling the chilled water valve. it is still not a successful strategy for dehumidification and I wasn't trying to make that claim.

 

[cry22]

An enthalpy wheel in your fresh air will solve your problem of course. If not, you need to sub-cool to dehumidify and then reheat - just look at the psych chart, it speaks for itself.
I do not know about your climate but reversing the preheat into reheat as you suggest - you end up freezing your cooling coil in winter.
If this is an office type application, ASHRAE 90.1 does not allow simultaneous cooling and heating - allowed only for special process.

 

[urgross]

Most OR's do not allow enthalpy wheels, not even the 3 angstrom. Same for A/BSL-3.

Not saying this is an energy efficienct procedure, but when an OR is active, efficeincy will always lose out to safety.