Sensible Load Only, with SHR = 1

[nevinjohn]

I have been asked to size a coil, with no latent loads.
The air conditioning unit is serving a switch gear room, in re circulation mode with no people. So, no latent loads. The on coil conditions are 23.8/7.7 and off coil at 13.7/2.5
The RH is close to 0! Is this practical?

On plotting the SHR with the above conditions, the coil ADP shows sub 0 values. Is this process achievable, or should I doubt their heat load calculations?
15782

[frank]

Surely the room will recieve some amount of latent load from infiltration....unless the room is airtight?

[nevinjohn]

Yeah, practically some infiltration should occur its but not taken into account for heat load estimates.
I am now focusing on the coil ADP.
For the above conditions, upon plotting the SHR I get this! What should be the ADP?
The black line is the SHR, its probably -10?
15783

[frank]

Sensible Heat Ratio is defined as Sensible load divided by Sensible + Latent, which would normally give an answer below 1. As you have no Latent load then your SHR will be 1.

As you can see from your results, the leaving air temp 13.7 is before Dew Point 12.8, due to a lack of moisture.

Can't say that I've ever come across this senario of 0%RH, but have many experiences of machine rooms with no fresh air input or humidification, where the cooling coil starts to freeze as RH drops below 40%. Not mentioning the rise in static charges at low RH conditions.

[nevinjohn]

Can't say that I've ever come across this senario of 0%RH, but have many experiences of machine rooms with no fresh air input or humidification, where the cooling coil starts to freeze as RH drops below 40%. Not mentioning the rise in static charges at low RH conditions.
_
How does it freeze?
The coil should still be at 7 or 10 (ADP), or were you referring to winter conditions when ambient temp drops?

[frank]

Coils on A/C fan coils are designed to handle Sensible+Latent heat. If they are used in machine rooms with no fresh air make up, i.e. just recirculation, and without steam humidification, then, as humidity levels in the room drop in line with the cooling effect, there is insufficient total heat available to be absorbed to achieve saturation.

On lower end makes of A/C where freeze protection isn't built in, the coils tend to freeze.

I suppose that this shouldn't occur when the coil is designed for sensible heat only, but I have no experience of these.