water chiller

[dwallan]

I am trying to establish the benefits, if any, of using a chilled water coil in a ahu as opposed to a dx coil.

There are obvious benefits if the coil is some distance from the chiller, i.e. no refrigerant in the building, but I am trying to understand why someone would install a chiller with remote condenser, with the ahu approx 2 metres away. From our first inspection it seemed like a lot of extra work with the recirc pumps, tanks etc, when they could have installed a straight dx coil in the ahu.

It appears to be a fairly old installation ( open drive dunham bush compressor), was this how things were done way back when.

Am I missing the obvious explanation?

David

[Brian_UK]

I'm not sure about the age thing but I suppose it could have been someone trying to use up some existing, or old stock, equipment

[BritCit_Juve]

Dont knock chilled water for an airconditioning system. At the end of the day chilled water is a very efficient secondary refrigerant, especially if a variable speed pump or modulating valve is fitted. Although DX is easy to install it is harder to correctly size a retrofit evaporator.
Chilled water systems were at one time a principle method of cooling and is primarily used in large projects be they newbuild or renovations. I think just lately it has been growing in popularity again, thanks to environmental concerns.
It has been doing what VRV/VRF systems try to do for years and has the advantage that if it springs a leak within the building then the leak is soon found.
It may be possible that this coil is operating on a full fresh air system where the load changes so a modulating valve will be fitted to adjust the cooling to maintain an air off temperature.

[Mike Hopkins]

My thoughts would be along the line of what BritCit_Juve has said.
Is this some type of process that requires close control of discharge air temp? Chilled water with a modulating valve would fit that bill well, whereas DX would require extra controls and use of hot gas or elaborate staging, unloading abilities and real good design engineer with closely matched equipment. Design engineer
can be a little sloppy with chilled water design as long as they don't underestimate load or maybe pulldown factor. Also sometimes they do that if say they are thinking to add more Ahu's
to chilled water loop and so size for anticipated increase in load.
Hard to do with DX without adding another whole system. Redundancy can be another factor. If the system is critical, a mobile chiller could be brought in and hooked up in a hurry as
opposed to DX where repairs could take too long waiting for parts, etc. Sounds like they weren't thinking like that from the age of the system though.

Mike Hopkins