How to avoid a common chiller design mistake

We were recently called out to a job site visit because the commons area and hallways of a multi-million dollar condominium building had humidity issues. 

These areas were being served by a chilled water make up air system; an air handler with a chilled water coil connected to a 20 ton split chiller (a 20 ton chiller barrel and a 20 ton DX condensing unit).

We noticed a couple of potential issues right away. First, there was no reheat. This isn’t necessarily a problem with a chilled water system though. If there is a large enough storage tank and the pump never shuts off, the discharge air shouldn’t fluctuate too much. It could however lead to overcooling the space in the summer time. 

We continued to investigate and noticed that the discharge air temperature wasn’t as low as we expected it to be. Do we have a capacity problem? A control problem? A refrigerant leak?

The investigation continues… 

We find out that the system was originally designed with a 20 ton DX condensing unit (the one that is still there) piped to a 20 ton DX cooling coil. 

Assuming there are no leaks and controls are working properly, we just figured out the issue: we don’t have enough capacity. 

But if 20 tons was enough capacity before, why not now?

Here’s why: a 20 ton standard A/C condensing unit only provides around 15 tons of cooling as a chiller. 

Why is that you might wonder? When operating as an air conditioner, suction temperatures are typically around 45-50 degF, but as a chiller, suction temperatures need to be around 36-38 degF to provide 44 degree water. And cold suction temperatures DERATE the condensing unit by 10-20%.

So now our 20 ton condensing unit is a 16-ish ton condensing unit. 

The moral of the story: Anytime you are sizing a split system chiller based on nominal capacity, make sure to check the condensing unit capacity charts at colder suction temperatures to make sure you have enough capacity. 

A good rule of thumb is to oversize your condensing unit by 20% for a chiller application. 

Have you ran into this issue before?

Brad Telker
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