The air at Applebee’s
A test is done at three of the restaurants to come up with solutions to ASHRAE Standard 62, a mandate that’s boosted outside air requirements.
Buildings across the country are feeling the impact of ASHRAE Standard 62 (1989) which increases outside air requirements significantly. While the adoption, interpretation and enforcement of this standard vary according to the location and application, fresh air ventilation requirements for dining areas have been on the upswing.
This has increased latent loads and indoor relative humidity (rh), retarded HVAC system performance and negatively impacted comfort levels and building maintenance. As a result, facility and design engineers have been considering alternative treatment strategies.
“Code officials are enforcing ASHRAE Standard 62 in restaurants,” says Stephen Yborra, president of Energystics, Mount Airy, Md. “Until recently, most of the outside air in restaurants was introduced and exhausted at the kitchen hood. Now it has to be brought to the dining area. This is creating an added load on the restaurant HVAC systems, which weren’t designed for 43 percent outside air”.
A lengthy and detailed study was recently completed on three nearly identical 5,000-square-foot, 198-seat Applebee’s restaurants in the Philadelphia and Southern New Jersey area: Vineland, N.J.; Lansdale, Pa.; and Quakertown, Pa. Underwritten by the Gas Research Institute, Chicago, and the Oak Ridge National Laboratory, Oak Ridge, Tenn., the study was co-sponsored by Applebee’s International, which is headquartered in Overland Park, Kan., and PECO Energy, Philadelphia, and managed by Energystics.
“We got involved with the study to evaluate the benefits of enthalpy heat wheels vs. desiccant wheels,” explained Ray Bond, director of facilities management for Applebee’s.
Bond explained that Applebee’s had enthalpy heat wheels in some 30 locations, but no quantitative data on their effectiveness. “This seemed like an excellent opportunity to have a professional evaluation done where we could weigh all the factors including utility and maintenance costs7
ENTHALPY HEAT WHEELS VS. DESICCANT WHEELS
Because of the similarities between enthalpy heat wheels and desiccant wheels, the two technologies are often confused. There are differences though, both in the level and consistency of their moisture removal.
Like desiccant wheels, enthalpy heat wheels have a desiccant-coated honeycomb and are placed between two air streams. Enthalpy wheels, however, contain very little desiccant, turn 60 times faster than desiccant wheels and use dry air drawn from the building’s exhaust system instead of heated air to reactivate the desiccant.
In operation, enthalpy heat wheels remove moisture from incoming outside air and transfer it to the exhaust air However, they can only do so when the exhaust air is drier than the outside air. Also, the amount of moisture removed is typically 70 to 90 percent of the difference between the two air streams. A desiccant wheel will remove about twice as much moisture, and will do so regardless of the humidity of the outdoor air and the dryness of the exhaust air.
It would seem then that desiccant wheels do a better job. And in many applications they do. However, enthalpy heat wheels do have a number of advantages.
Since the dehumidification process is passive, no extra heat has to be added. This lowers operating costs.
Enthalpy heat wheels can be used to recover heat from exhaust air during colder months and recover cooling from exhaust air during the warmer months. Desiccant wheels cannot be used for this purpose.
However, enthalpy heat wheels have disadvantages as well:
1) The exhaust air must be free of oil and grease and brought to the same location as the incoming fresh air.
2) No dehumidification can be achieved unless the exhaust air is drier than outside air.
3) No dehumidification can be achieved when enthalpy heat wheels rotate at low speeds, which may be necessary to avoid overheating under certain conditions. Therefore, their application for dehumidification is limited.
FACILITIES AND EQUIPMENT
Despite the similarities between the three restaurants tested, the HVAC systems in each were decidedly different because of local code requirements.
The Vineland restaurant. This location had the standard Applebee’s package. That consisted of three rooftop units (RTU) of 35-tons combined capacity, supplemented by a makeup air (MUA) unit feeding the kitchen exhaust hood. The local code allowed the exhaust hood make-up air to be counted towards the ventilation calculations, so each patron received about 8.1 cfm of fresh air and Applebee’s standard package figured to be capable of handling the load.
The Lansdale restaurant. Here, local codes were more stringent so the exhaust hood make-up air could not be counted toward the ventilation calculations and 15 cfm of outside air per patron was required. To meet this increased load, an enthalpy heat wheel was added to each of the three RTUs (35-tons combined capacity).
The Quakertown restaurant. Here, 15 cfm of outside air per patron was also required. The combined capacity of the three RTUs was reduced to 30-tons though, since a desiccant system was added to pre-treat the outside air, which was fed to the return duct of each dining room RTU. Since exhaust air was not required for desiccant wheel regeneration, it was available for the kitchen to supplement MUA supplies for the hood exhaust system. This allowed MUA volumes to be reduced significantly.
For 11 months, temperature and humidity levels, energy use, and other relevant data were monitored at each of the three using a site-mounted data logger connected to a phone modem.
Perhaps the most significant fact unearthed from the study was the ability of each system to maintain relative humidity levels at or below 60 percent during occupied periods. Dining room relative humidity at Vineland (standard HVAC package) exceeded 60 percent more than 40 percent of the open hours. At Quakertown (enthalpy site) that number was reduced to 22 percent and at Lansdale (desiccant site) it was a mere 6 percent.
The study noted that mold and mildew growth increases significantly when rh levels exceed 70 percent, which is important since the presence of mold and mildew and the associated odors is “detrimental to the appearance and perception of the restaurant environment and therefore, its profitability”. The enthalpy and desiccant systems both performed much better than the standard Applebee’s configuration at the 70 percent level.
The overall comfort at the desiccant site was also found to be much better than at the other two, particularly during low-sensible/high-latent load periods when the other two sites’ indoor relative humidity was frequently above 60 percent. This allowed the dining area temperature set point to be raised several degrees while remaining within the ASHRAE Comfort Zone and reduced energy costs even further.
As always, system costs were considered extremely relevant. The desiccant-integrated HVAC system cost more than the other two even though part of the added cost was offset through downsizing the dining room RTU package to 30-tons. However, the study pointed out that cost comparisons were unrealistic since the standard Applebee’s package did not work under increased ventilation conditions while the enthalpy system did, but not nearly as well as the desiccant system.
“From a profitability standpoint, it is extremely important to restaurants like Applebee’s that the patrons be made as comfortable as possible,” says Yborra. “If they are, they tend to stay longer and revenues tend to increase. If only 15 checks per day are increased by $4, which is the average price of one additional drink appetizer or desert, the increased revenue will pay for a desiccant dehumidification unit in less than a year.”
While that may indeed be true, Bond said it would still very difficult to sell that concept to upper management. “Like most major restaurant chains, we’re constantly looking to lower first cost. Adding dehumidification technologies go against that philosophy. There’s no doubt that they lower relative humidity. It’s just not all that clear-cut what the relationship is between that and increased sales and profits.”
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