Design Chilled Water Flow Rate and Cooling Load

[nicole10g]

My friend was talking to me that the chilled water flow rate design should base on the Sensible cooling load only instead of the Total cooling load (Sensible + Latent). The control valve selection would also follow the Sensible cooling load only. His reasoning was that if there is no humidity control, the temperature sensor can only measure the sensible load and thus control the control valve based on the sensible cooling load.

He further concluded that the Sensible + Latent load only affect the air side of the HVAC equipment design and the water side of the HVAC is based on the Sensible load.

I tried to look this out in the net but cannot find any conclusive result on this subject especially on tropical climate with high latent load. Appreciated any comments on this subject. TQ.

 

[DRWeig]

You don't need a reference. How does the latent load get removed if not through the chilled water? Latent load adds heat to the chilled water just the same as sensible load does. If the latent load is high, your chilled water coil that has sensible-only chilled water flow will not be able to produce design supply air temperature.

Best to you,

Goober Dave

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[Drazen]

its' more matter of your terminal units capacity, than academic issue related to chilled water.

take a look at sensible and latent capacity of your fan coils at design flow and return temperatures of chilled water.

than you look at calculated sensible and latent load: sensible capacity of your fan coil will remove sensible heat, latent capacity will remove latent heat.

as shr ratio of fan coil almost never equals shr ratio of room served, you normally have to take design conditions.

and that's where you friend is partially right - if your fan coil controler acts on sensible temperature, your fan coil sensible capacity should match room sensible load, and latent load removal will be "you get what you have".

if you would match fan coil to latent capacity of your room, but control it by sensible temperature, fan coil would go off quicker and your would remove even less sensible load i. e. you could not utilize capacity of your fan coil.

if you would match fan coil to latent capacity of your room, but control it by humidistat, you could fully utilize latent capacity, but your room sensible temperature would be too low.

that is why in simpler application match is made by sensible load, but it is correct to say that you have partial latent load removal as well. every wet cooler has at least some latent load removal, apparently.

 

[MintJulep]

Your friend is wrong. Simple as that.

 

[nicole10g]

Thank you Mr Drazen for your reply.

This issue was came out when I was doing my chilled water pipe sizing and Balancing & Control valves selection for a Kitchen cum Dining Room. The space has a Sensible cooling load of 6.7RT and Latent load of 3.5RT (Total Cooling Load is 10.2RT, SHF is 0.66.) The space is cooled with a chilled water FCU system and controlled by a thermostat acting on the on/off 2 way control valve.

I was using the water flow rate of 24.48usgpm (based on Total Cooling Load of 10.2 x 2.4, and 10degF delta T) to do the pipe sizing and valves selection. My friend commented that because of low SHF, I should have using the water flow rate of 16.08usgpm (based on Sensible Cooling Load of 6.7 x 2.4) otherwise my system will be oversized. Since the space is controlled by the thermostat, an oversized system will keep the control valve closed most of the time.

I am now confused that

1. Should I base on the Totaling Cooling Load, or

2. Should I base on the Sensible Cooling Load, maybe at least the balancing valve and control valves selection or

3. Any value in between?

TQ.

 

[Drazen]

As mentioned in previous letter, your firstly need to select fan coil.

You can try to match sensible capacity of fc to sensible capacity calculated, than check what is latent capacity of fc at that conditions, and than size the rest based on total actual capacity of fc.

In kitchen, different to some other spaces, you should not plan to remove all sensible load by fc - most of it should be exhausted, while load of make-up air should be taken into account, at least to some extent.

 

[nicole10g]

Thanks Mr Drazen for your advise.

I will do my sizing based on the selected Fan Coil Capacity.