Difference between residential and data center split system AC units? 1

[Jasper_1]

I'm reviewing a design for the replacement of split system AC units in a server room (with outdoor condenser). The existing system is residential AC units, which are unable to meet demand during hot days. The proposed design is to replace the existing residential units with Data Aire mini-plus split system units of the same capacity.

I'm not sure how replacing a unit with one of similar capacity is going to fix the cooling issue. Could someone explain the difference between data aire or other server room specific units, and a typical residential split system unit? Are there features that will allow it to provide more cooling under certain conditions, even though the rated capacities are equal?

 

[Compositepro]

Obviously if the capacities truly are the same there will be no improvement. However, specifications for residential equipment are often exaggerated by using tricks in the test methods. One may be capacity under best case conditions and the other may be capacity under worst case conditions.

 

[MintJulep]

Sensible heat ratio.

 

[itsmoked]

Residential are world renown for cutting every last penny off the cost even to the point of partially crippling the resulting equipment. Data center equipment is NOT going to do this. They will include many extra features to improve reliability and will typically provide for day after day of full heat transport. You will pay a premium for this but it will likely prove worth it.

That said, having a system even 'just' an A/C system that's not optimized optimized can often get you considerably more capacity. A quality refrigeration guy and I do mean refrigeration and not 'air conditioning' could possibly look at what you have and with only minor modification solve your lack of capacity problem. A lot of A/C people are what are termed as "board changers", "part swappers", or "bottle jockies" as they don't have the training or knowledge to do anything other than to try things.
Don't call A/C companies to do commercial or industrial refrigeration.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lukaiENG]

I would look at the condensing ambient temperature. I would hazard a guess that your residential units are designed to reject XX tons/BTU/kW of heat at an ambient temperature of 90 or 95 degF. The "data center" rated unit would be designed to reject the same amount of heat at 105 or 110 degF. If the capacity is rated at a lower ambient temperature then it will have a smaller capacity (apples for apples).

That being said, you should be specifying the ambient design temperature so double check what the ASHRAE/client requirements are for your location.
Also, you should also check with Data Aire what minimum temperature their units operate down to and compare this to your site design requirements. Some systems do not operate well in a cooling mode at low ambient conditions. in the smaller range, Mitsubishi have a 'low ambient kit' which they apply to their condensers which allows them to operate down to ~-20 degF. The kit is basically some dampers which close to stifle the fans, trapping the heat.

 

[EnergyProfessional]

You should ask the designer if he or she considered the current low performance and calculated load correctly. The above explanations make sense (rated for different ambient etc.).

Commercial units are also built to run 24/7 and at low ambient temperature, which a residential isn't rated to.

It is possible the old unit wasn't charged properly. Often they only get set by measuring DAT. the actual line pressures and temperatures should be measured and charged to appropriate superheat or subcooling.

 

[Jasper_1]

Thanks for all the responses. I visited the site last week and was able to talk with the maintenance staff for awhile. It turns out the existing units were meeting demand just fine for about a year after installation, but could not keep up after that. I also spot checked the designers sizing, it appeared fine. So I'm thinking the replacement with equal capacity DataAire units makes sense.

 

[FreddyNurk]

Bear in mind that if the equipment worked for a year and then couldn't keep up, then there's a good chance that either the equipment may not be maintained correctly and requires cleaning and servicing, or more likely, additional equipment has been added to the server room, also increasing the heat load.

Thus, even if you're swapping to commercial equipment, it'd be a good idea to allow for extra capacity for further heat load increases if possible.

EDMS Australia

 

[itsmoked]

Unless more waste heat HAS been added it's probably just a service issue. A system that runs frequently for a year (unlike a typical domestic A/C) very likely needs to have all the evaporator and condenser heat exchangers cleaned. Also all air-in-motion filters should replaced and the liquid site-glass scrutinized.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

If they worked fine for a year then "could not keep up", you really need to find out why.

If it is lack of maintenance / not cleaning filters etc then the commercial units will suffer the same fate, ditto if in fact something has changed in either the heat load or the air flow or whatever.

A deep clean / renewal of operating fluids would seem to be valid after a years probably continuous operation.

If on the other hand it is because some part has worn out, failed, started malfunctioning or tripping then the domestic units might just have failed prematurely.

Just replacing like with like I think you'll be buying another new one in a year or so's time. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mech110]

The explanation regarding ambient temperature consideration sounds logical. Proposed equipment's catalogs should be referred to for this information. On the other hand, I would like to add that CRAC units (Computer Room Air Conditioners) or "Precision Air Conditioners" are used for Data centers as they provide a very precise cooling up to the required temperature. As in sensitive data centers, we cannot afford temperature variations. Furthermore, there are differences (between residential and CRAC) of air distribution and sensible/latent capacities too.