Hi Guys, Presently, I am designing

[Buildtech2]

Hi Guys,
Presently, I am designing a chilled water system for residential blocks which can accomodate 400 workers, the chilled water system will also serve to re-creation center and a factory.

I have proposed two air cooled chillers of 300 tons each with a constant primary pump for each chiller and a 3 nos. duty / standby variable distributed pumps on the roof of three buildings. The design delta T for this system is 16 F.

There are 400 fan coil units, one for each worker room.

Now main worries are as follows.

1. Due to 16 F delta T, the chilled water flow in each fan coil unit is very low (1-1.5 gpm) and the pipe size is limited to 3/4" and chilled water velocities in the pipe is less than ASHRAE recommended. Do you think it will work..?

2. The variable distributed pumps are located on the roof of buildings (8m high) away from chilled water plant. Do we have to consider static head..?

3. Will there be any specific operating porblem in the system..Please advise.

Thanks

 

[Drazen]

what recommendation for minimum pipe velocity do you have?

in quick calc, velocity should be about 27 fpm, which is far above acceptable minimum according to my references.

moreover, is there some reason why you should stick at 3/4" as minimum diameter?

 

[jkauwale]

At 600 tons of cooling, it is typically recommended to use a water cooled plant.

Justin K, P.E.

http://www.engproguides.com

 

[KiwiMace]

1. The fan coils should be selected for a 16degC dt, and therefore not have low flow.
2. You can pipe to the fan coil in any size you like.
3. Both the primary and secondary loops are closed, so you don't have to add the static head. What goes up must come down.
4. Supplying lots of small coils with low velocity flow - I would recommend adding filtration to the system.

 

[Buildtech2]

Thanks for your valuable comments.

The recommended chilled water velocity in the pipe as per ASHRAE is 1.5 fps and I think that 3/4" pipe size is the minimum pipe size as per industry standard. According to Drazen, the minimum pipe velocity is is 27 fpm which is 0.45 fps.I do not know the reference of this velocity. Due to lower velocity in the piping, there is a possibility of laminar flow and it can also accumulate air in the system.

If we can use 1/2 " pipesize, the velocity can be increased but I do not know how for it is acceptable practically.

Due to maintenance issues the water cooled system is avoided., so we have to use air cooled system.

The expansion tank pressrization may have to be increased to maintain positive suction pressure in the pumps which are located on the buildings.


KiwiMace has advised for adding filtration to the system. Can you please specify.

Regards,

 

[KiwiMace]

You can pipe main runs close to units and keep the branches short in smaller pipe.

Filter sediment and corrosion product out of your water using a sand or centrifugal filter, this will collect in your small coils.