HVAC System for Warehouse/Office/Lab in NW GA

[MOHobbes]

Situation is that am looking at multiple quotes from a few vendors to heat & cool a warehouse/office section of our facility.

Details are:
- located in North-West GA
- flat-roof structure
- space is 220'x120'x20'
- has (100) 400W daybright lights
- has ~12 individuals working [well, most of the time! ]
- area functions as (a) warehouse, (b) different enclosed office spaces for shipping department, QA lab, and a engineer [me!], (c) department for finishing orders/products on the floor, and (d) department that has (6) machines that generate heat as part of processing.
- has one overhead door on east side, four on west side [loading docks here], and one into other section of complex.

Quote options are:
(a) three (3) separate heat/cool systems; featuring two (2) 50 ton AC units on each end and then one (1) 40 ton heat/cool unit in middle. Utilize "fabric ducts" to carry and deliver air to specific areas for specific needs, using engineered holes of desired sizes for each location.
(b) two (2) "air rotation" units, each to provide air such that atmosphere in the structure will not be stratified, nor experience wide differences in temperatures. System would have one (1) 130 ton condensing unit, air-cooled, and one (1) 400,000 BTU/hr indirect-fired heat exchanger.

Prices for install & units are for the most part identical.

Main concern is which route to go: (a) 3 separate units w/ fabric ducts, or (b) 2 separate units w/o ducts that distribute air in a forceful manner.

Regarding the first option, are there any concerns or negative aspects to these fabric ducts?

Regarding the second option, I am leary that such can really cool/heat this space w/o a gale-force wind, AND can such a system really keep specific areas to a comfortable level, while other areas are warmer due to their ovens?

Any other thoughts, comments, suggestions?

Thanks!

 

[ChrisConley]

One thing you could consider is not using vendor's to design your HVAC system. 140 tons of cooling is a reasonably large system, and more than likely a pretty decent capital cost.

A mechanical consulting engineer would be the best bet to design the system, with you the client in mind, rather than the sale of equipment in mind. There are a lot more options than air-turnover devices and rooftop units with fabric duct for cooling projects. While a warehouse may not be very complicated, it sounds like you have enough variable to make a good design complicated.

You may think that it isn't worth it to ask for a real design from an engineer specializing in the field….but then why did you post the question?

 

[imok2]

I like Chris's idea of hiring a professionsl design engineer. We are talking about a lot of energy use. Employing high-performance HVAC equipment in conjunction with whole building design can result in significant energy savings. Typically, a 30% reduction in annual energy cost can be achieved with a simple payback period of about three to five years. And, if the payback threshold is extended to seven years, the savings can be about 40%. These figures apply to buildings such as warehouses that offer conventional comfort (e.g., 70°F in winter, 76°F in summer).

Unless you don't have a need to maintain critical temperatures, but just regular air conditioning, a single air cooled condenser may do. However, if you have to shut it down for some repairs, you realize that in the main warehouse area will be uncomfortable in the meantime in extreme weather, there is no redundentcy.

 

[imok2]

I said there was no redundency unless the engineer stipulates a unit with about 6 compressors and seperate circuits the more circuits the better control of the loadand more reliability
Cheers

 

[jherbert]

- As Chris and imok recommend, get a consult to
design or at very least prepared the performance
requirements.

- what are your present / future operating hours
for warehouse, process and office areas?

If 24hr air conditioning for warehouse or
other spaces is required these needs may
well dictate the best system configuration,
including maintenance shutdowns
Loading dock doors can be problematic,
infiltration needs careful consideration.

- your process exhaust and or chiller heat
rejection could be utilised to reduce your
energy bill, generate domestic hot water, etc.
or use thermal wheel? you should take a look
at the economics.

- energy prices only get higher, dont
forget options for free cooling. Again
this may dictate the best system config
for your application.

- fabric duct is great for a single area,
considerations include fire risk and sound
transmission between adjacent areas.

- are the ovens in the warehouse or within
a separate enclosure / defined space?

cheers,

 

[imok2]

If you are in a high humid climate you may want the design engineer to consider a reheat system. This is a reliable and cost-effective solution to temper the supply air. It utilizes the heat-laden hot gas as it exits a compressor to reheat the air after it passes through the evaporator coil. This will results in a small increase to the operating cost of the unit but will be much less then electric reheat which will be needed in order to maintain the dew point requirement and will be a huge increase in occupant comfort.

 

[ChasBean1]

A couple of thoughts to add:

Re-roofing with a white membrane roof could reduce cooling tonnage requirements back down toward the 1 cfm/ft2 range (maybe 60-100 tons vs. the 100-150 range planned).

My last experience with fabric diffusers was that they got clogged fairly quickly, resulting in excessive pressure loss across them. The manufacturer recommended a weekly laundering of the fabric diffusers for most installations. Think about that!! If you go that route, go with a coarse mesh, not a fine mesh. And have good filters at the AHUs.

Because it sounds like there's nothing too toxic going on in there (at least not on a large scale), I like a heat recovery wheel for this application...

 

[MOHobbes]

Just a quick note to say THANKS! to all that replied to my query - interesting and helpful replies all.

At the time I posted this message on this forum, I had just started working w/ a consultant, and was looking at all possible avenues for ideas and advice - that is the reason behind the original message.

Most likely, I will be going w/ air return unit(s) instead of the fabric duct units - unless something drastically changes in the next few days.

Again, thanks!
Rob