The density of fume hoods in biomedical laboratories has been decreasing in the last few years. Ten years ago, one fume hood per 800 to 1,000 net assignable square feet (nasf) was used. Presently, 1,600 to 2,000 nasf per hood is allowed. While equipment load for the same equipment is smaller, more electrical equipment tends to be used.

With the decrease in the density of fume hoods, along with the changes in equipment load, the calculation of minimum air change rate assumes a new meaning. For example, often for air change calculations, the entire volume of the room is used. However, this may no longer be true. The equipment in the room may occupy 15 to 20 percent of the space volume. Calculations based on total room volume may be over-estimating air change requirements by 15 to 20 percent. Therefore, the first step in calculating air changes is to use accurate information. Once this has been done, the opportunities to reduce energy for conditioning of the air can be considered.

For recovering heat from the building exhaust air, often a run-around loop heat exchange system is used. In this arrangement two liquid-to-air heat exchangers are used, one in the supply air stream and one in the exhaust. The liquid between the two heat exchangers transfers heat. This fluid is circulated by a pump. The heat is transferred as "sensible heat" (temperature rise) of the liquid. Thus for a 20 degrees F temperature difference in the fluid loop, a pound of glycol liquid will transfer 16 BTU.

In a "heat pipe" type run-around, a heat recovery loop is now being used in many projects. In this scheme, there are two heat exchangers, one in the inlet air stream and one in the exhaust. The fluid in the loop connecting the heat exchangers is an environmentally-friendly refrigerant. Thus one pound of the refrigerant will transfer 130 BTU.

Run-around refrigerant loops provide an effective method of reducing heating energy requirements for laboratory buildings, although only a modest amount of cooling energy reduction is available. Recent analysis shows that the cooling energy reductions are from the cost of energy only—the cooling equipment size cannot be reduced.

A Sample Building

A recent study of a sample biomedical building in Pittsburgh, Pa., analyzed the performance of the refrigerant type run-around loop for ten diverse climates in the United States. The building was chosen because it is a good example of large modern biomedical lab buildings.

Built in 1991, the building is 440,000 gross square feet (gsf) with 244,981 nsf of usable space. Total lab space is 130,500 nsf and the offices are 42,772 nsf. The lab support space and animal areas are 60,784 nsf and 13,000 nsf, respectively. Additional building specifications are as follows:

Refrigerant Run-Around Loop

The use of a refrigerant in a run-around loop transfers about nine times more energy per pound compared with a glycol system. The refrigerant used in the system is R-134a, which is tetrafluoroethane (CH2FCF3). This is an environmentally-friendly refrigerant and is widely available. For these systems, several parallel loops are used to provide adequate surface area in the air stream for heat transfer. Since greater amounts of heat are transferred per pound of refrigerant, larger heat transfer surfaces are needed in the air streams.

The refrigerant loop operates at a pressure of 100 psi, thus no extra special considerations are required in the piping system. The amount of fluid that needs to be circulated is a fraction of the glycol loop. Therefore, very small pumps are used and the operating cost is very low.

The potential for energy recovery is high with the use of refrigerant-based run-around loops. For a refrigerant loop, the temperature difference for recovering heat can be 2° to 5° F. In a glycol loop, the only time energy is recovered from the exhaust air is when there is a 20º F temperature difference between the exhaust air temperature and the supply air temperature. This temperature difference is due to the effectiveness of the heat exchangers used in the loop.

The Study

For calculating the performance of a refrigerant run-around loop, hourly simulations were made using TRNSYS computer code. This is a general purpose public domain computer code, and has been extensively used for solar energy systems since mid 1970s. The weather data used was TMY2 data for the selected cities of Minneapolis, Chicago, Colorado Springs, Colo., Pittsburgh, Washington, D.C., Houston, Phoenix, Atlanta, Seattle, and Los Angeles.

The following information was used to calculate savings for energy recovery in heating and cooling:

The Results

The results show that energy recovery is most effective in cold climates. The benefit is much less in warm climates. For example, in Minneapolis, the annual savings from heating energy recovery is $818,458 and from cooling, the modest savings is $3,946. The same numbers for Pittsburgh are $659,564 and $4,292, respectively.

In evaluating warmer climates, such as Houston and Phoenix, the overall annual savings are not high. In Houston, the savings during the heating season is $264,223 and in cooling season, it is $24,231. For Phoenix, the numbers are $254,847 and $73,894, respectively. In cities like Los Angeles, where the climate is mild, the savings during the heating season is $291,447 and for the cooling season, the savings is only $407.

Apart from the obvious benefits of energy saving by the use of a run-around loop, there are environmental benefits as well. By not burning gas to supply the equivalent of the recovered heating energy, a substantial amount of environmental emissions are reduced. The reductions are the greatest for Minneapolis, followed by Chicago and Pittsburgh.

Lessons Learned

By using the run-around loop for heating energy savings, the equipment size may also be reduced. Ideally, the reduction in heating equipment size should be significant in heating-dominated climates. However, the reduction in size would realistically be from 25 to 50 percent. This equipment can also be used during start-ups and guarding against sudden shutdowns of the systems.

The space required for the refrigerant type run-around loop in building mechanical systems risers is much less than the glycol loops. Using small pumps keeps operating costs low. The first cost of the refrigerant loop systems is about the same or slightly above the cost of the glycol system. The payback, of course, is much faster for the refrigerant loop systems, compared to the glycol loop system.

The refrigerant loop system must be designed as an integrated system for the buildings, and not as an added component. The design of the mechanical system also must accommodate such systems, which may require slightly more space in the ductwork, depending on the configuration of the system.

The refrigerant loop systems are usually several parallel loops, with two or more pumps, providing some degree of redundancy. This redundancy, along with smaller sized back-up heating coils, provides a comfortable operational margin for the building.

Refrigerant type run-around loops have been installed for many projects ranging from very small teaching laboratories to very large biomedical research labs. Performance data from these projects is now being collected.

This article is based upon a presentation given by S. Faruq Ahmed, PE and P. Richard Rittelmann, FAIA, of Burt Hill Kosar Rittelmann Associates at the Tradeline Research Building, 2002 conference in March 2002.

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