WAREHOUSE HV DESIGN

[Cooch42]

Hi,
I am designing the H&V for a warehouse and have a question for the experts. I'm heating and ventilating using high temp rise/ high velocity space heaters which bring in 100% outdoor air (Cambridge S-series). In the winter I wasn't planning on running any exhaust, but simply blow the space up and let it leak out through warehouse overhead door (to save energy & $$). In the summer I will run exhaust for comfort. Is there a standard ACH for warehouses for good comfort in the summer?

thanks,

 

[DrRTU]

Depending on location, building height and racking we are seeing 2 to 6 X/hr. 3 to 4 X/hr is the most common. This is for new large warehouses outside Chicago. I like to use a lower turns per hour and then move the air with HVLS fans.

 

[ChrisConley]

How does bringing in 100% O/A for heating save money? How does shutting off the exhaust save money?

For a warehouse I'd use unit heaters, or something similar for the heating and a ventilation unit for ventilating, typically a direct fired MUA interlocked with an exhaust fan.

I agree a HVLS fan would be a good addition for summer comfort.

 

[DrRTU]

In an application of an S – series Cambridge unit, the designer tries to offset the natural infiltration of the building with the fan on the Cambridge. This flow (infiltration) would have been admitted in to the space via normal leakage. The blow thru burner / fan does an excellent job of doing a high temperature rise required. The high delta T will cover the OA load and the space conduction loads.

 

[Wareagl487]

I am not a big Cambridge fan for just that reason - the 100% outside air issue. As a company policy, we do not specify them.

Let me share this - the principal of our company was invited by Cambridge to the factory for a dog and pony show to try and convince him of their worth. They wine him, they dined him, they set him up for a round of golf on an exclusive course. Then made a presentation with all the company officials present. At the end, they ask him what he thought. With a straight face he said "Let me see if I have this right. By your theory, when I get home tonight and my house is cold. I should open up all the windows and turn the heat on high? Thanks for the diner and golf." And off he went.

When ask our opinion on system types for warehouses, we recommend air turnover units with indirect gas heat (Cambridge is direct fired). For heating and ventilation, they are sized from 3 to 3-1/2 air changes per hour - as the height of the space increase, the A/C's can go as low as 2-1/2 (I did a warehouse with two of these units 60 foot tall). The plus to these is they are up to 200,000 CFM - so you can do large areas with only a few units. There is also is minimal heat stratification from floor to ceiling - 2 to 3 degrees (try that with a Cambridge). If cooling is required, no problem to add either DX or chilled water coils to the unit. It is amazing how uniform the heat (and cooling) is distributed across the entire floor area.

For ventilation in the summer, the design intent is to have a large bank of louvers on the rear of the unit that modulate open with 100% outside air. The air is then relieved through roof mounted vents. The tick is to convince the owner/operator to run the unit all night to take advantage of the cooler air.

IMO, These are the way to go on new warehouse construction.

http://ice-ww.com/ac.html

Andy W.

 

[Cooch42]

Chris,
Yes bringing in 100% OA will not save energy in comparison with unit heaters, but in comparison with indirect fired MUA's the delta T of 160 allows bringing in less air to satisfy the load, and not running the exhaust in winter keeps the heat in the space longer. I suppose the unit heaters with minimum ventilation would minimize energy usage in the warehouse, but if more ventilation is needed it could be better to use the high temp rise heating unit for heating and ventilation. Thanks for all the responses, I still am not convinced of the absolute best way to design just H&V for a warehouse, probably because there isn't one best way, but a number of determinants to lead one to the best way for that particular use.

Nick

 

[Cooch42]

DrRTU,

3-4 X/hr is a lot of CFM for large warehouses, are you only running this in the summer? thanks,

Nick

 

[mtngreen]

In my former job I did ALOT of warehouse HV designs. I used the following "rules-of-thumb"

If electric forklifts were being used:
.25 cfm/sq-ft winter OA
.5 cfm/sq-ft summer OA

If propane forklifts were being used - I doubled the OA requirements above.

Using Unit Heaters/Ceiling Fans/Air Rotation Units/MA units...depends on the climate and what the objectives are..

 

[Wareagl487]

I failed to mention in my initial response that the air change rates mentioned were for unit air flow - not outside air values. O.A. would be as outlined in ASHRAE.

I also through in 1/4 AC/Hr. infiltration in the load if there are a lot of dock doors.

Andy W.

 

[DrRTU]

Cooch42

You are correct 3 to 4 X is a lot of air for a summer ventillation design. This number can be huge with today's decks at 34' to 38'. I like to do the math with a 20% loss in volume due to space infill. I also check the number on a CFM/ft2 basis. I see 3x to 4x pop up the developer specifications. If the space is just racking, run the changes way down. I see the key to the good design is how the air is admitted into the space. Louvers / motorized dampers get expensive, bug screens on the doors are difficult to control, it gets windy below large axial supply fans and open an over head doors present a security issue. I like low sidewall louvers and roof mounted axial upblast fans. Cross ventilation is the best. A drip pan below the fan helps with moisture control. Hooded fans work nice but are more expensive and still require a drip pan. We have had dripping issues with moisture on the inside of hooded fans and butterfly dampers in the winter time. Supply fans should only be used with motorized dampers. I have one client that advises the owner to pre-cool the box in the early morning hours then keep the fans off until the building gets warmer than OA then enable the fans. You do not want the owner running 25 to 50 ten hp fans without a benefit.



Most of our new design/build warehouse go with 80/20 units.

 

[ChrisConley]

and not running the exhaust in winter keeps the heat in the space longer.

This was running in my head for a few days. It does not make sense, unless by turning off the exhaust we in actual fact have now increased the static on the supply, which reduces the O/A coming into the space. Which may or may not be acceptable.

Thanks Andy for the description of the Camberidge units. I find myself in strong agreement with the principle of your firm.