Grocery stores must keep two things at just the right temperature: products and people. Unfortunately, those two needs can sometimes be at odds. But with the right planning and management, they can complement instead of conflict.
Supermarkets, like all retail businesses, have to maintain a proper indoor temperature for the comfort of customers and staff. But they face a unique challenge in doing so: the presence of coolers, refrigerated display cases, freezers and other units that chill or freeze food. These can work against the heating/ventilation/air conditioning (HVAC) system.
Ron Chapek, director of product management for the ProAct Enterprise Software Services unit of Emerson Electric, St. Louis, gives the example of a major retailer that greatly expanded its grocery offerings—and suddenly found that its HVAC and its refrigeration were at odds.
“The refrigeration [suppliers] really didn’t care about the HVAC, and the HVAC didn’t care about the refrigeration,” Chapek says. “It has taken some pain and time for [the retailer] to get the vendors to work together so that both can be optimized.”
This conflict manifests itself in several ways. Coolers and freezers discharge cool, dry air into the store, especially when they’re open, no-door units. This affects the HVAC load, not only increasing it in winter but also creating cold spots that can be uncomfortable even in summer. Retrofitting open units with doors, or replacing them altogether, is an obvious energy-saving solution, but it’s no panacea. Doing so can alter the HVAC load in a way that the system wasn’t designed to handle.
Doors can create another challenge: fogging up. Condensation forms when the door’s surface temperature goes below dewpoint (a function of the temperature and humidity of the ambient air). Not only is this unattractive, but it also forces shoppers to open the doors to see what’s inside, defeating the whole purpose of having a door.
Door fogging is a symptom of a larger problem: keeping humidity at the proper level. This is especially tricky for supermarkets. When humidity is too high, it fogs up the doors of coolers and freezers and also can frost their cooling coils, forcing them to work harder. If high enough, humidity can also lead to condensation on floors and even dry-goods packaging. If it’s too low, it can cut the shelf life cause of fresh produce, and even cause it to wilt.
Mechanical cooling of any kind inevitably generates heat, when the refrigerant is compressed in preparation for expansion in the evaporator (cooling) coils. This heat is usually vented outside the building—that is, wasted. It represents a potentially rich source of free heat, but to take full advantage, HVAC and refrigeration systems have to work in harmony.
Not working in harmony is the source of many of the biggest problems grocers face with HVAC and refrigeration, says Ani Jayanth, director of product marketing for food retail for Emerson.
“At the end of the day, there’s a divide between where refrigeration’s focus and concerns are and where the HVAC air handling focus and concerns are,” Jayanth says. “The two have to cohabitate, and they’re slightly not doing that at times.”
Fog on cooler and freezer glass doors is perhaps the most common, and most obvious, manifestation of an imbalance between HVAC and refrigeration. There are several ways to deal with it. The simplest is to warm the doors above dewpoint with what are sometimes called “anti-sweat heaters.” Muskegon, Mich.-based Structural Concepts, a manufacturer of refrigerated display cases, offers heat tape that warms doors directly and fans that blow warm air over the glass, says marketing manager Christopher Latta.
The problem is that heating the doors this way uses energy, both directly and because it increases the unit’s refrigeration load. According to an estimate from Amesbury, Mass.-based Munters, door heaters can consume up to 40 kilowatts per hour.
“That is an obvious issue that is existing today, that instead of anybody looking at the root cause and effect, they just go to fix the problem as it comes up,” Jayanth says.
The root cause of fogging is an improper level of relative humidity (RH). Grocers should aim for an RH of 40% to 45%, compared with the 50% to 55% found in most other retail businesses. (RH expresses the moisture level in air as a percentage of the moisture in completely saturated air at the same temperature and pressure.) The source of humidity inside a building is almost always air from outside, and it’s up to the building’s HVAC system to lower it.
There are several ways to do so. The simplest is just to cool it more, because as air is cooled, its humidity drops. If a low level of humidity is required, a system can keep cooling the air until it gets there.
But this wastes energy in two ways: It increases the initial refrigeration load, and it may very well cool the air past the point of comfort. This requires it to be reheated once it enters the store, which uses more energy (unless it’s done with waste heat from the condenser coils).
Another option is to use a desiccant system as part of the HVAC. This forces ambient air through devices that remove its moisture, either mechanically or by spraying a chemical into the air stream. Desiccant systems are effective and reliable, but they require a lot of energy, especially for large spaces like supermarkets, either to blow the air through a mechanical moisture remover or to heat and reconstitute the chemical desiccant.
On the Right Path
Properly structuring airflow both within and outside the building can greatly increase efficiency while maintaining optimal humidity levels.
A “single-path” system uses overcooling to dehumidify, but it does this judiciously, to a reduced volume of outside air. This is then mixed with air that bypasses the cooling coil, resulting in the right temperature/RH mix entering the store.
A “dual path” system uses two cooling coils, one each for air entering and leaving the building. Because the latter is cooler and drier than the outside air, it can be conditioned with less energy for mixing with outside air to reach optimal temperature and RH. Dual path systems can be enhanced by grilles and ducts underneath refrigerated cases that draw in the cool air they expel, reducing the load of the coil that treats the exit air.
The refrigeration process invariably produces heat, removed from the coolant—whether chemical refrigerant or water—before it gets chilled for the next cycle. This heat is usually vented outside, but some advanced systems harvest it for various purposes. It can be used to reheat air that has been supercooled to remove moisture; some advanced systems use it, via heat exchangers, to warm the air that’s vented into a supermarket during cold weather. Danfoss, a Danish manufacturer of refrigeration compressors, says it has helped supermarkets near its headquarters reduce their carbon dioxide emissions by one-third through heat harvesting.
Another ecological initiative has to do with choice of refrigerants. The most commonly used chemical refrigerants, hydrofluorocarbons (HFCs), are being phased out due to their potential for atmospheric harm. Carbon dioxide is a popular “natural” alternative. It can be used in two basic ways: retrofitted into existing HFC systems as a secondary refrigerant that is cooled by the HFC and in turn cools the refrigerated/frozen cases; or installed as the primary refrigerant in a new system. Weis Markets recently put in a carbon-dioxide-based refrigerant system as part of the renovation of a former A&P market in Randolph, N.J.
Refrigeration and HVAC costs are among the biggest operational expenses a supermarket faces. Making them work together is a way to bring comfort to both customers and the bottom line.