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Improving Supermarket Operations Through Desiccant Dehumidification

December 20, 2010
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A supermarket environment is one of the most unique in the commercial arena. The dynamics of temperature and humidity have a strong impact at several levels and offer HVAC designers a major challenge.

Indoor air that’s too warm, too dry, too wet, or too cold causes numerous problems for building owners and customers. If conditions are too hot, products spoil and smell; too dry, products shrink or get hard; too wet, products mold, freezers frost, glass fogs up; too cold, customers and employees complain.

Conventional attempts to control humidity can provide an uncomfortable environment for customers. Uncontrolled humidity can wreak havoc on refrigeration systems, causing energy inefficiencies.

Desiccant dehumidification is a cost-effective and energy efficient option to reduce a supermarkets’ moisture load to lower energy costs, ensure food safety and customer comfort, and provide the proper amount of fresh outdoor air at the precise temperature and humidity necessary for the space.

THE TYPICAL SUPERMARKET

Supermarkets feature a varied mix of applications under one roof - each with specific HVAC requirements to heat and cool the space and provide the required amount of ventilation.

In one part of the store, refrigerated cases provide a steady stream of cool air into the aisles, while on the other side produce is misted and sprayed to keep products fresh. Cooking equipment that ejects heat provides the convenience of prepared foods, but that heat must be exhausted.

In addition, the number of people in the store causes cooling loads to fluctuate continuously during store operating hours, requiring precise calculations during HVAC system design.

With so many indoor environment variances, it’s vital for mechanical designers to find an effective solution to efficiently control the space and reduce energy usage.

The photos above and below are a before-and-after representation of product in a supermarket display case. Without desiccant dehumidification (above), there is frost build-up on the product. With desiccant dehumidification (below), the frost is eliminated. (Click on each image for an enlarged view.)

REDUCING HUMIDITY KEY

One section of the supermarket that requires full attention is the refrigeration display case area. Because these cases provide the majority of the store’s sensible cooling, the goal is to reduce their latent load.

Display cases are designed to operate in a wet bulb condition of 64°F or less. A typical issue is that humidity resulting from ventilation air and internal loads increases the refrigeration load and causes frost formation on the evaporator coils. This makes defrosting of the coils necessary to continue providing refrigeration, but will increase energy consumption and compressor run time.

The anti-sweat heaters that help prevent fogging on case doors can consume between 25-40 kW per hour. The amount of “on” time needed by these heaters is directly related to the store humidity levels. By lowering the humidity and keeping the store dry, those heaters can be cycled off, saving electrical load with no risk of condensation or fogged display doors.

By reducing the latent load on the refrigerated display cases, the mechanical designer can reduce the amount of defrost cycles and anti-sweat heater run times. In addition, product display, product shelf life, and energy costs will benefit from having reduced humidity within the supermarket.

Conventional DX systems are not an efficient option to manage latent loads as they must first overcool the air and then reheat it to maintain store comfort. Overcooling and then reheating air to maintain the stores’ humidity levels will substantially increase energy and operating costs.

With a desiccant approach, humidity is controlled independently of temperature. Also, warm, dry air from a desiccant dehumidifier can be targeted toward the coldest parts of the store so there is no need to use reclaim heat to warm up the aisles. That saves the cost of extra refrigerant piping and, most importantly, frees up heat reclaim capacity for producing the hot water needed by the deli and bakery departments.

Munters, for example, offers a variety of cooling, dehumidification, and energy recovery systems in various sizes and configurations to meet building needs. Munters systems cool and dehumidify 100 percent outdoor air using one-third the energy of conventional DX systems.

Utilizing systems to cool and dehumidify, an HVAC designer can hold a store at 40 to 45 percent rh that results in 10 to 15 percent savings in the refrigerated cases operation, while maintaining superior product display.

Shown is a rooftop installation of a Munters DryCool™ unit. The system combines desiccant dehumidification technology with packaged refrigeration. (Click on the image for an enlarged view.)

DESIGNING THE HVAC SYSTEM

Once it has been determined to take a desiccant approach, several design practices should be observed to minimize the possibility of humidity issues.

First, the mechanical designer should separate sensible and latent loads. Sensible and latent loads for a supermarket are different from most retail applications. The effect of the refrigerated display cases negatively impacts both sensible and latent categories, and the designer should account for this to avoid oversizing the HVAC system.

It’s also recommended to treat the outdoor air before it comes into the store. Ventilation air in most cases is the major source of moisture. The HVAC designer needs to account for exhaust hood activity and introduce the proper amount of outdoor air to offset exhaust.

Vestibules and door openings are another source of moisture so a positive air pressure is recommended. In terms of energy, it is less costly to treat outdoor air with the HVAC system than the refrigerated cases.

It’s also important to take advantage of a lower humidity to raise temperature for occupant comfort. Without humidity control, maintaining the store at 75° for customer comfort means employees are too hot, which has a negative effect on productivity. Employees feel comfortable even at 75° if humidity levels are maintained at 40 percent.

Finally, maintain 40 percent rh to reduce defrost cycles. Most major refrigerated case manufacturers recommend that indoor humidity levels be held at 55 percent rh or less to reduce defrost cycles. As the rh level is reduced, refrigerated case operation improves.

APPLYING DEHUMIDIFICATION

There are three basic approaches to applying desiccant dehumidification in supermarkets. Each has its advantages and disadvantages and must be evaluated to best meet the needs of the client.

The first option is a single central system. This application can meet all the temperature and humidity control needs of a typical supermarket. Dry air is distributed throughout the store, improving the comfort conditions for both customers and employees. Central systems do the work of multiple conventional units in larger stores, and save on wiring, ductwork, rigging, and roof work, in addition to reducing operational costs.

Another option is a targeted approach to dehumidification. Some stores have a high concentration of refrigerated cases and benefit from the dry air directed in that zone. The majority of the stores’ ventilation air is processed through this unit, with multiple rooftop packages serving the general merchandise area.

A third method is a dedicated outdoor air unit ducted either directly into the space or into a return air duct of a central system or multiple rooftop package. This option treats outdoor air separately to effectively dehumidify the establishment.

In addition to the benefit of in-store comfort, store operators who utilize one of these approaches will achieve a more comfortable environment, lower operating costs, and better indoor air quality.

Publication date: 12/20/2010

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