View Full Version : HVAC: DX System vs. Air Cooled Chiller
What are the dis/advantages to the 2 systems if you were prescribing one to a 5 level (20,000SF/Floor) Building in Florida. Building Dimensions are 350' X 62' and a 75' Height. Mixed use ranging from medical office space to thermal spa on top level.
I am trying desparately to convince this owner to go ground mounted air cooled chiller system vs. this DX system which would operate on 6-7 zones per floor.
Can anyone give me a great argument and not the obvious one, why the hell wouldn't you. The pricing for the air cooled chiller system is to be in the range of 2.0 to 2.2 million design build. I can not put a number figure on this DX system which would be broken into multiple 10 ton units throughout.
thanks in advance to all the knowledgable souls on these page....
07-12-2006, 08:10 PM
How about heat pumps?
W/ a cooling tower and what not
The split systems leak
and have oil return problems
heat pumps will work better for what he wants!!
I work w/several in albany NY
07-12-2006, 08:51 PM
Need more info dude, is this an existing building? If so, what is there now? If your starting from scratch, i'd say air cooled chiller is the only way to go, unless noise would be an issue. Water source heat pumps with closed loop tower, and a small boiler for heating will probobly be the system of choice, due to the energy savings. There again there might be a noise issue, due to the units being above ceiling grid and compressors running. Of course the package unit route would negate the noise and water piping problems, and have the side benefit of not all being off when one goes down, unlike a chiller, or wshp tower that when down takes out the equipment on high pressure. But if the ductwork is not in place it could take a lot of work to provide air to the bottom floors.
it is a new construction project, here is a brief:
1st Level - Diagnostic Imaging Center( 1 Siemens 64 CT Scan) / 152occupany Auditorium
2nd Level - Technical Office Spaces
3rd Level - Lasik Eye Surgeon / Cosmetic Dentistry / Cosmetic Plastic Surgeon(still up in the air)
4th Level - Alternative Office Space - Chiropractor / Physical Therapist / Etc.
5ht Level - Thermal Spa and Gym Area
approx. 20,000SF per level ~ 100,000SF
Post Tension Flooring - Precast Concrete Panel Exterior
thanks for the responses so far.
07-12-2006, 09:41 PM
why not chiller with cooling tower ? or air cooled condensor with a chiller and pumps orrrrr
All your mechancial equipment in one box, cust down on your labor and the owner gets to pic there equipment win win for both :D
07-12-2006, 09:48 PM
I AGREE WITH "CROONED". NEW ORLEANS, LA HAS MANY MEDICAL FACILITIES USING WSHP, A COOLING TOWER, AND ELECTRIC BOILERS TO CONTROL COMFORT COOLING.WATER TREATMENT IS A MUST ON CLOSE- LOOP AND TOWER LOOP. EASY TO ZONE INDIVIDUAL OFFICES AND SEPARATE INTERIOR OFFICE FROM PERIMETER ZONES.
07-12-2006, 10:00 PM
I have to disagree with the heatpumps down here, especially in Louisiana with the "limited" winter season we have the energy savings isnt really there, especially if they are going to be 10 ton units. Chill water with modulating valves or VAV is the way to go.
07-12-2006, 10:08 PM
THERE IS ALWAYS MORE THAN ONE WAY TO SKIN A CAT !!!!
I BELIEVE WE BOTH AGREE 10 TON SPLIT SYSTEMS IS NOT THE BEST DIRECTION.
07-12-2006, 10:26 PM
is it possible to go with one rtu per floor with vav's? the only other way i would go is with water source hp's. with what you describe as future tennants you would want to have something there that would not effect the entire building when the cooling fails. thats why i would suggest the rtu's.
if you have wshps, if the building went down, it wouldnt take much to get it back online,ex:loop pump failure or cooling tower.
i know that the air cooled chiller option would only take up a 20' X 30' space on the ground. Operating (2) units. Our goal is to utilize the roof as a sun deck as much as possible and a bunch of DX boxes really doesn't do it justice. We're looking at approx. 500 tons.
It will take several(6-7) zones per floor to utilize a DX system and the thought of having 10 ton mechanical spaces all around the floorplan is not really appealing. The initial cost factor involved is coming to almost 2wice the cost when the two systems are compared(DX vs. air-cooled). This number is the obvious problem here.
Has anyone developed and designed a 5 level building using a DX system as described? and if so what are the downfalls. thanks for the repies...
07-12-2006, 10:46 PM
TO MUCH REFRIGERANT. A LEAK AND OIL RETURN NIGHTMARE.
DO IT RIGHT THE FIRST TIME AND BY THE WAY THE MECHANICAL SYSTEMS NEED ADEQUATE SPACE FOR SERVICING. VERTICIAL WSHP FIT NICELY IN MECHANICAL ROOMS AND ARE EASY TO ACCESS
07-12-2006, 10:49 PM
This is my opinion not seeing the job
If you go DX 10 ton you would need to get a unit with 2 compressors to try to carry the load properly and then you are stuck being at either 0,50, or 100%.
If available(like I said without seeing the job), multiple fan coil units with chill water valves you can modulate anywheres from 0 to 100% and control tighter(if not a low bid job I would look into chiller redundancy too), also with the fan coil option you can add a controls package(actually on anyone) and show a payback value to the customer.
07-12-2006, 11:09 PM
BEING A MECHANIC I WILL WORK ON ANY OF THE SYSTEMS TALKED ABOUT TONIGHT.THE MOST IMPORTANT THING IS YOU SELL THE JOB, TURN A PROFIT, AND ARE PROUD OF YOUR COMPANY'S EFFORTS.
07-12-2006, 11:20 PM
air cooled chiller and piped into each floor off a riser supply 3 way valves.pumps in the basement,built up AHUs in MEC behind elevators.VAV application for all the medical offices with the chilled water and fan powered VAV for all the perimeter areas as required.fresh air duct dropped down the middle of the building for required air changes and some OA free cooling.if the building is built tight the heat from the offices will hold during the fall and winter and when occuppied you will need cooling.advantages no roof penetrations,no duct risers all on the floor they supply,no work within the spaces to maintain...keep the service guy out of the spaces with filters/belts/ceilings condensate lines NOISE!!!! away from the tennets...quiet cooling.water cooled with chiller in the basement +++ $$$$$ to install, cheaper to run but more maintanance.
07-13-2006, 03:37 AM
chilled water vav system somewhere along where maxster said , or self contained watercooled packages placed in centrally w zoning and vfd for static, ive seen this in many 10 stories w great success, wshp are short cut install to me and in longrun customer pays #1 alot in water treatmeant and nuisance related trips , #2 alot more in service just due to the fact of the sheer amount of pieces of equipment and them things(wshp) ARE NOT FUN to work on ;)
07-13-2006, 08:52 AM
Please no more CAPS your hurting my ears :D
07-15-2006, 11:33 PM
This is more of a "tenant" perspective..
WSHP is particularly good for moving heat around a building, though, such as mixed-use where you have retail space needing year-round cooling and residential space calling for heat, or the south-facing half of the building wanting cooling and the north-facing half wanting heat on a winter day.
Come to think of it, in a climate with almost no heating demand, *is* there a good reason to use WSHPs? I can see how they make sense due to lower first cost and some operating efficiencies in certain buildings. I can certainly see how providing four-pipe type service to tenants with a two-pipe system is attractive.
It does feel like a shortcut overall vs. four-pipe. But it's sure nice to have as a tenant.. the office building I work in has McQuay/AAF WSHPs with Mark IV boards and no one can stop me from running the AC in January. :D (And most of the heat gets rejected into the office next door that's trying to reach 80 degrees anyway.)
07-16-2006, 12:02 PM
Here's my take as one who's worked on both DX AHU's tied to VAV's and chilled water AHU's tied to VAV's. For this application, chilled water IMO wins hands down. DX strikes me as far clunkier in this context in regard to capacity control, at least how I've seen it done.
Capacity control is a big issue with VFD driven AHU's feeding VAV boxes. Another poster here referred to oil return problems with DX. Such is true, especially if no hot gas bypass is present or is not commissioned properly (if at all). I had oil return problems on a 40 ton DX split system serving VAV's until I figured out how to set the hot gas bypass parameters correctly. Nobody from the installing company had ever bothered to set it up right from the start. The system chewed through at least two compressors until this, among other problems, were discovered and remedied.
With a chilled water system, a three way valve, or at the very least a chilled water differential bypass in the plant with a two way valve at the AHU coil, is the way to go for good modulation as load demand changes. The whole idea behind VAV systems is that they vary in accordance to demand, so the refrigeration system attached to them must be able to vary as well. At a minimum, a centrifugal chiller's capacity can be varied with guide vanes; more elite configs vary the speed of the chiller's compressor itself in response to demand. I remember years ago seeing a chiller compressor coupled to a natural gas fueled six cylinder engine where the throttle was controlled by system demand.
Drawbacks are if you only spring for one chiller and tower and either one takes a dump, the whole building is down. Additional equipment means more expense but ensures cooling, particularly in a medical facility, will not be lost if one component craters.
RTU's on a roof have noise transmission issues and will require ductwork penetrating multiple floors to reach their intended zones. Air cooled chillers can be noisy if that is a concern, although they are less maintenance. As someone else mentioned, water source heat pumps are an option but put compressor noise over the T-bar and absolutely require the cooling tower never go offline to avoid every unit tripping out on high pressure.Or that there be more than one cooling tower so at least one can be idled for maintenance without taking down the whole building.
It's also no joy for the tech that has to squirm his way above the T-bar to change a WSHP compressor. I once had to do this in a plastic surgeon's suite lobby...bad enough trying to wrestle a 5 ton compressor up a stepladder without being distracted by the clientele in the lobby waiting for an appointment! :D
thanks for all the repsonse and suggestions. please keeo them coming.
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