Chiller vs Direct Expansion system question

[coldkeeper1]

Dear fellows:

I´m facing my very first design job of a large A/C system. Most of my previous designs involved residential and light commercial DX systems, but this one is of industrial size. I remember from a seminar that there´s a particular system size range where the benefits coming from potential short-term energy savings begin to offset the increased inital costs of chilled water systems compared to DX systems. I cannot recall what system size range was it. Could someone help me out with this? Thank you.

 

[KiwiMace]

This is the kind of question we love to hate.

There is no one number for everyone. Operating costs, equipment costs, hours of operation all vary.

BUT! you are in luck. There is a simple little calculation where you spend an hour or two figuring out the ballpark costs of the equipment in each option and estimate the energy costs. From this you get a rough payback. Then you go to your client and say, here are your options and here are the costs associated with each, and here is the payback for the more expensive option, and here are the maintenance requirements. Which one do you want to pay us to design?

 

[TXiceman]

There is not a right answer that fits every case. You need to review the operating cost, longivity of equipment, how it is operated, installation cost, first cost, maintenance cost and available space to install.



Ken
KE5DFR

 

[coldkeeper1]

Thank you both of you for your kind responses. I understand that there´s no magic number of tons when deciding when to install chilled water or DX, but more or less as a rule, for NEW well designed single owner commercial buildings (no retrofits), without any special space limitation, nowdays I don´t see many DX systems beyond the 80 to 100 ton range, particularly in locations with mild winters. Most of them switch to some kind of water loop for cooling.

 

[KiwiMace]

I just went through a couple of ASHRAE Journal articles on Walmart from Sept/Oct 07.

The guts of it was an energy comparison with different types of experimental systems some DX and some water based etc and the results did not appear overtly conclusive. Now admittedly this did not include maintenance, which is a big aspect of the decision, but is clearly relevant to what you are trying to determine.

The average supercenter is about 200,000ft2, which by anyones rule of thumb (irony?) exceeds the numbers you suggested.

This american love of packaged DX systems at all costs is a tad mysterious, but low cost installation, low repair cost and redundancy must count for something.

 

[HVAC68]

It's not only the size of the building that matters. The application is important. Part load profile plays an important role too. There's no magic answer to this. It's project-specific and you have to carry out a life-time cost analysis for various options - if you were to do full justice to the comparison - before arriving at the final decision.

Good luck.

HVAC68