Coil Leaving Air Temperature 1

[HVACHawaii]

I have a condition where I require 52 degree coil leaving air temperature. We only have 47 degree chilled water supply temp and can not get it any colder. We are actually getting 55 degree coil leaving temperature. Will increasing flow to the coil, hence reducing delta across the coil, cool down the coil leaving air temperature? Any help with a solution is appreciated.

 

[imok2]

The only way to get the air colder is to slow down the velocity of air flow across the coil.

 

[HVACHawaii]

Air flow cannot be slowed down due to ACH requirements for hospital. Coil is oversized and not currently operating to capacity. I figure by increasing flow we will decrease average coil temp, hence reducing supply air temp. This may be faulty logic, just wanted another opinion

 

[imok2]

If the coil in oversized then it's worth a try and you may improve the situation however I would expect the delta T to get somewhat smaller

 

[trashcanman]

Add more rows of cooling coils.

 

[Yorkman]

I figure it's worth mentioning, if the coil is not piped in a counterflow configuration it can increase the approach of the coil. Chilled water supply should enter on the leaving air side of the coil,to get the most efficiency out of the coil.

It would'nt be the first time this type of piping error has been made, and not discovered until several years later.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[SAK9]

Try to increase the chilled water flow.If the coil is oversized and there is not sufficient water in the coil,laminar flow conditons will resul reducing the heat transfer coefficient

 

[AbbyNormal]

Increasing water flow will make a difference but I do not think it will be much.

If you cannot make the water colder or slow down the air flow, then you are going to need a deeper row coil, with more fins per inch. While not impossible I think 47F is going to still be problematic. Be a fan penalty

If the 52 is wanted for a low dewpoint, perhaps add a dehumidifier

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[gepman]

Increasing the water flow will reduce the air temperature leaving the coil. In order to calculate by how much, you need to know the coil water inlet temperature (assumed to be 47), the coil water outlet temperature, the coil air entering temperature, and the coil air leaving temperature (assumed to be 55). You also need to know the coil configuration (how the water is piped in the coils and the airflow) so you can calculate the log mean temperature difference (LMTD). Then with this information you should be able to calculate the difference obtained by changing the water flow.

The suggestion by Yorkman would increase the LMTD if the coil was piped concurrent instead of countercurrent. As imok2 says if the coil is operating near its design point increasing the water flow will not help much since the LMTD will not change much.

The formula for the heat transfer is Q=UAdeltaT where deltaT is the LMTD. You can either increase A or LMTD. Once your get fairly high Reynolds numbers (on both the water and the air side), as touched upon by SAK9 you will not improve U much (overall heat transfer coefficient).

 

[RossABQ]

It's sure worth a try; I assume you have a circuit setter that is limiting the flow now? And you are going to crank it open?

The details of the coil circuiting will determine if you are successful.

 

[SAK9]

you have mentioned in your second post that is an hospital application and ACH needs to be maintained constant.How is the chilled water control valve controlled? If the room temperature is controlling it,there could an issue here.

 

[HVACHawaii]

Cooling coil is controlled by humidistat. The reheat coil is controlled by room temperature. Our room temperature is controlled nicely at setpoint. However, because the physicians like it nice and cold they crank the thermistat down to 67, which I'm sure you know will result in high RH. This is below design criteria and the specified leaving air temp will result in high RH. Therefore we are trying to accomdate them by reducing supply air to acheive an acceptable RH.

Thank you all for your input it is greatly appreciated, especially gepman for you very detailed response.

 

[Yorkman]

HVACHawaii,

I teach apprenticeship/journeymen classes in advanced psychrometrics, and found your postings a typical problem with systems trying to control dehumidification and reheat. Could I ask you to post a few more details regarding the room/space conditions, so that I might include it as an example problem in my classroom.

Sensible Heat load BTU/HR?
Latent Heat load BTU/HR?
Outside air reqirements %min. or CFM?
Typical osa temperature if applicable (summer)
Typical osa % relative humidity if applicable (summer)
Is this a constant volume fan system?
% Relative Humidity setpoint for space (design)?
Room temperature Setpoint degrees F (design)?
Is the reheat; electric, hot water, or steam?

I understand if what I'm looking for would take too much of your time, be straight up about that if it is I'll understand. Thanks ahead of time for any data I could get.

I'll even give your site credit by name if that's all right!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[HVACHawaii]

Yorkman,

I will provide you this information on Monday

 

[ChasBean1]

HVACHawaii,

An ‘acceptable RH’ should be one that prevents condensation on room surfaces. Sources like AIA might give an allowable band, say 30-60%, but an allowable RH has to correlate with a certain temperature, otherwise it’s a conceptual error. Condensation on surfaces is more a function of total moisture in the air than the amount of moisture in the air relative to the temperature. In other words, if they like the thermostats low, the RH will increase but it doesn’t change the amount of moisture in the air.

I’d estimate that your 55°F leaving temperature would have about a 53°F dew point. This is just a guess. It would depend on the coil bypass factor, but it’s probably pretty close to typical.

So if your OR is happy at 72°F with a 53°F dew point, the RH looks great at 51%. Now if someone lowers the thermostat to 67°F, the dew point is still 53°F, but now the RH looks bad at 61%. The fact that any surface in the room will have to be 53°F or cooler to cause condensation hasn’t changed.

So? I’d say don’t let your controls react to the room RH. Have your controls technician create a building automation point for each room’s dew point, calculated from its temperature and relative humidity. Control dehumidification from that and not the room RH. If the calculated room dew point exceeds a certain value (say 53°F), have the dew point control the chilled water valve, until it becomes fully open at some higher dew point (say 56°F). The reheat can continue to maintain room temperature, as you mention.

Sorry this went on too long. I hope it helps. -CB