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The major laboratories are being constructed by private companies, government agencies and universities. Within these groups there are medical, pharmaceutical, electronics, materials and environmental laboratories.

Establishing Lab Requirements
The purpose of a laboratory is to create an environment where experiments, analyses and observations can take place under controlled conditions. In order to achieve this, it is necessary to be familiar with the needs of the users.

Often a lab consultant or programmer is retained to assist in the development of space configurations and equipment needs early in the project.

In addition to laboratories, many R&D facilities also include pilot plant setups to test product manufacturing ideas for full scale production. Pilot plants require processes which are unique to a particular product. Process engineering can usually be handled by the design engineer working with the users; however, it is the user's ideas which have to be implemented.

It is important to understand the organization of the user, particularly in the case of corporate clients. The design engineer is required to coordinate the needs of corporate engineering, facilities engineering and the user groups. On occasion, this can make for a very exciting design process.

The Exhaust Hood
The major impact on the laboratory mechanical system is the exhaust hood. The primary function of this device is to protect the technician from potential hazards. A secondary but equally important function is the control of the local environment to avoid contamination of the work or surrounding areas.

Figures 1A and 1B show diagrammatically the basic types of specialized hoods available.

The amount of air exhausted by the hoods in R&D facilities is usually the basis for calculating conditioned supply air quantities. The quantity is normally more than adequate to compensate for internal gains from lab equipment and the building heat gains.

Hood Velocity
The general standard for face velocity through hoods is 100 feet per minute (fpm), although some facilities permit velocities down to 70 fpm and other facilities require up to 150 fpm.

To reduce energy consumption, most hoods operate with the sashes partially closed. When the sashes are fully opened for set up, the hoods are permitted to operate at a reduced velocity.

Virtually all labs exhaust 100% of the supply air through a combination of hoods or general exhaust, which requires large investments in capital plant and operating expense to heat and cool 100% outside air as opposed to recirculation systems. As a result, methods to reduce and recapture energy are usually implemented.

Energy Reduction
Systems have been developed to reduce energy usage. Some facilities use make up hoods to reduce energy (see fig. 2). In this type of system up to 60% of the air exhausted by the hood is supplied from a separate system which is only tempered (i.e., limited cooling and heating) and discharged at the hood face. This type of system is not often used since it can cause operating problems such as condensation within the laboratory and hood and can be annoying to the user. Further, this system requires a separate duct system to be installed above the ceiling. Although the total air quantity is no more than the conventional system, the necessity of having two independent duct systems can lead to serious space and coordination problems.

Today most facilities use bypass hoods, and to reduce system air flow, the hoods are designed to operate at 100 fpm with a reduced opening, usually about 50% of the fully open position. During set up of the hood, the face velocity is permitted to be less than design. This, in effect, reduces the exhaust flow of a system by up to 50%.

Another method to reduce energy is to provide variable air volume (VAV) hoods. With this system, the volume of air exhausted through the hood is proportional to the opening. To avoid potential condensation and settling in the duct, the minimum exhaust is limited to between 40% and 60% of the rated hood capacity. The controls for this type of system are relatively complex, although with the development of modern DDC, consistent control is easily attained.

Energy Recovery
Another method to reduce consumption and reduce the installed plant is the air-to-air heat exchanger, which is an energy recovery device. This device is installed between the exhaust and outside air intake ducts.

There are four basic system configurations to recover heat: the air-to-air plate heat exchanger (fig.3), run-around coil (fig.4), thermal wheel (fig.5), and the chemical spray unit which in operation is similar to the total heat thermal wheel.

The method most often used is the air-to-air plate heat exchanger. It is very efficient and while primarily a sensible heat only transfer device, can under certain conditions provide some limited dehumidification of the outside air when the exhaust air is provided with an evaporative spray section. It also separates the supply and exhaust air eliminating potential contamination problems.

The run-around coil utilizes two coils containing a common fluid (usually a glycol solution). One coil is located in the exhaust and the other in the supply intake plenum. A pump circulates the fluid between the two coils; if not designed carefully the system will consume more energy than it saves, due to the resistance of the coils in the supply and exhaust systems.

The thermal wheel, which uses a chemically impregnated wheel, is available for sensible only and total heat exchange. The wheels are housed in a sheet metal housing and outside air and exhaust air are passed over the wheel. This type of heat recovery device has only limited application in R&D facilities since the potential exists for contamination of the two air streams. A major advantage of the total heat wheel is its ability to allow sensible and latent heat exchange to take place. A new device which uses a molecular sieve can avoid contamination problems.

The chemical spray, similar in results to the thermal wheel, allows for both sensible and latent heat exchange to take place, using a solution of lithium bromide. Normally two units are used, one for the exhaust air and the second for the outside air. This system is complex and expensive. Care must be exercised to avoid chemical contamination of the occupied spaces.

All of these devices can reduce installed primary refrigeration and heating capacity by up to 20% and reduce annual operating costs by up to 30%.

Separate Exhaust System vs Common Header Exhaust System
The exhaust distribution from hoods can be either individual, connected to a common header, or a combination of the two. The greatest heat recovery potential is achieved when hoods are connected to a common header.

The decision on whether or not to provide a common exhaust header depends on the materials being exhausted. If there is a possibility of reaction between products from different hoods and the reaction is either potentially explosive or produces toxic material they should be exhausted separately. Similarly, if there is a possibility of cross contamination (either biological or chemical) the hoods should be exhausted separately. It is also possible that particulate matter may settle or condense requiring very high velocities for a specific hood. Under these conditions separate exhaust may be necessary for this type of hood.

Whenever possible exhaust hoods should be connected to a common header. This configuration is usually less expensive, allows economical multiple exhaust fan backup, and most importantly, it allows for the inclusion of a single heat recovery device to be installed such as air-toair plate heat exchangers.