Sensible and Latent Heat Cooling Coil

[John_187]

There is a project with a server/electrical room. As such, the sensible heat ratio should be very high - very close to 1 - because there are no internal sources of humidity or people in the room.

Also, there is no real source of outdoor airflow, and this shouldn't change the sensible heat ratio, as outdoor air is a source external to the room, correct?

Due to no real presence of outdoor air and a very high sensible heat ratio, I think it is reasonable that the majority (95 %) of the cooling load on the coil will be sensible. This is determined based on the psychometric chart and the logic that the coil will dry out the constantly recirculating room air to a humidity ratio equal to that of the cooling supply air at 55 F.

The reason I'm asking, is that often manufacturer's publish data about cooling coils - saying for example - 50,000 Btu/hr sensible, 70,000 Btu/hr total. However, that doesn't mean that the coil isn't CAPABLE of having the vast majority (95 %) of the total load be sensible right? There is no limiting factor on what percent of a coil load is sensible or latent, it is strictly determined by the conditions that the coil is subjected to?

Please comment on the above assumptions/questions, thanks for your time

 

[GT-EGR]

The manufacturer's data is the limit on the unit's sensible capacity. Think of that latent value as more of an add on, that while cooling that much sensible heat (drop in actual air temperature), it can also extract a certain amount of moisture along the way.

Get the manufacturer to do a selection on your actual operating conditions, give the entering air conditions, and they'll let you know how much sensible heat they can remove from the air. That will give you the real answer.

 

[greenen]

Hi John_187, I came to a similar conclusion when working on one of my projects. I found that a unit's sensible heat ratio is completely dependent on air inlet conditions. Several of the big manufacturer's of RTU's such as Carrier and Trane publish charts of the unit's sensible and latent capacity at a range of DB and WB temperatures. From what I saw, although these types of "comfort" units are designed with considerable de-humidification capacity, even they can have an SHR of 1.0 if the entering air DB is high enough (80-85 deg. F.) Check your individual unit's capacity table to confirm, but the principles should be the same.

Make sure you have some form of ventilation to meet code

 

[willard3]

Most coils are designed for human comfort conditions, ie: fin spacing, fin type, number of rows and etc. A 100% sensible coil will differ from these and you should contact the manufacturer for coil choices.

 

[lilliput1]

So long as the temperature of the evaporator coil is below the dewpoint temperature of the space you will get latent heat cooling capacity. So you will get 100% sensible cooling when the space dewpoint matches the temperature of the evaporator coil.

 

[MintJulep]

The coil bypass factor exists to help you understand why you can't add rated sensible + latent capacity together and achieve the sum as 100% sensible capacity.

https://www.engproguides.com/coil.html

Simply put:

Some air passes through a coil without being cooled at all because it is insulated by the air around it.
When water condenses on a coil the film thickness causes the bypass factor to decrease. Because the space that air can pass through gets smaller.
When a coil operates dry the bypass factor increases. Because the space that air can pass through gets bigger.


100% sensible cooling coils need to have their fins closer together than wet coils.