Maintaining comfort in the supermarket
Just how does the a/c system keep all these areas in harmony?
The important issues in supermarket systems are “humidity, temperature, people, and outside air,” says Bipin Patel, P.E., hvac department head for Clive Samuels and Associates, Princeton, N.J., with humidity being a primary consideration.
Ron Woodcock, senior product manager for package rooftop systems, Carrier Corp., Syracuse, N.Y., agrees. “The most important thing in a supermarket application is maintaining store humidity at reasonable levels.”
If you don’t maintain the proper humidity level in the store, you’ll have condensation problems on the refrigerated cases, notes Patel.
“Then you can start frosting up and icing up on your display cases,” remarked Woodcock. “And that turns shoppers off. That’s why it’s so critical to maintain the humidity at low levels.”
The relative humidity in a supermarket should typically be 45% to 50% rh, Patel says. Wetbulb temperatures will typically be around 49Â° to 50Â°F, states Woodcock.
One method of controlling humidity would be to use a humidistat to fire up a cooling coil, Woodcock says, then use some type of reheat, such as hot gas reclaim from the refrigerated cases.
Airy yet dryBut while you’re controlling store humidity, you also have to deal with ventilation air requirements.
“You’ve got to bring in a lot of ventilation air and yet maintain a very dry store,” Woodcock says. “So there’s the challenge for the equipment.”
He relates that ventilation is important “especially today with these superstore-type supermarkets, where you have bakeries, etc. When you have these bakeries, you have more exhaust air, so you need ventilation air to make up for that exhaust.
“Also, you want to maintain a healthy IAQ in the building, so you want adequate exhaust air for the people.”
Woodcock continues, “So you have to say, what dictates the amount of ventilation air? Is it the people, or is it the amount of exhaust air I’ve got going through the kitchens, etc.? With that, the ventilation loads become very critical.”
If a store’s exhaust air requirements aren’t that high, says Woodcock, “What they may want to look at is CO2 sensing as a way of maintaining lower levels of ventilation and adequate indoor air quality.” He notes that Carrier recommends demand ventilation using CO2 sensors as an energy-saving alternative, bringing in ventilation air only when needed.
In the summer, when outside air is more humid, Woodcock adds, it’s important to wring all the water out of the ventilation air, and also the moisture from the return air due to infiltration and higher internal loads, and while doing that, not to overcool the space and make it uncomfortable for shoppers.
Know your flowAirflow patterns around refrigerated cases are also important so that drafts across open cases don’t raise case temperatures and cause refrigeration problems.
Patel and Woodcock concur that ducts and diffusers need to be laid out properly. Be sure to supply treated air in refrigerated case areas, points out Woodcock, “to make sure you don’t put a big load on the display cases.”
Because of the different departments with different requirements within a store, zoning may be implemented to properly condition each area.
You may “have a dedicated rooftop for everything that has its zone profile,” Woodcock says. “Your bakery is definitely going to have a different loading pattern than a frozen food section. Trying to feed them with the same zone may not be wise. So you can zone by putting multiple rooftops on the store.”
For more information, you can reach Bipin Patel, of Clive Samuels and Associates, at 609-520-1600. Ron Woodcock, of Carrier, can be reached at 315-432-6330.