Basic System Types

There are three basic categories of HVAC system:

1. once-through 2. recirculated

3. exhaust/extract systems 8.2.3.1 Once-Through HVAC Systems

Once-through HVAC systems supply treated outside air to satisfy the design conditions for a space. This air is then extracted from the space and exhausted to the atmosphere.

Figure 8.2: Once-Through HVAC

Advantages of this system:

• This system provides an abundance of oxygen rich fresh air to dilute contaminants and assure the health of personnel.

• The system can handle hazardous materials without recirculation into supply air; however, the extracted air may need treatment before it is discharged to the atmosphere.

• Lower risk of cross contamination of products from another room, via HVAC system ducting • Exhaust fans may be located remote from the AHU making exhaust duct routing simpler.

• As there are fewer concerns about noise in the extract ductwork, it can usually be sized for a high velocity and smaller diameter, making it easier to route. Higher velocity also may be necessary to convey powder materials to an air cleaner device before discharge to the atmosphere; however, higher velocity requires disproportionately more energy to achieve.

Disadvantages of this system:

• More expensive to operate than an equivalent recirculating system, particularly when cooling and heating.

Energy recovery often is justifiable.

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• Air Filter loading will be very high leading to frequent replacement.

• Potential need for air treatment (e.g., scrubbers, dust collectors, filters) for exhausted air contaminated by the process.

• Room conditions may be more difficult to control as the system needs to be sized to handle extreme outdoor air conditions, but may operate most of the time under much less load.

8.2.3.2 Recirculating Systems

This system type is widespread; the room supply air is made up of a portion of treated outside air mixed with some of the air returned from the space. An equivalent portion of the air supplied to the room is either discarded (e.g., exhaust from a containment isolator) or lost through leakage to adjacent areas, due to local area pressurization. The amount of outside air is driven by:

• IAQ requirements (see ASHRAE 62) (Reference 22, Appendix 12), about 20 CFM (35cuM/hr) per occupant • the need to offset exhaust from the area

• the need to provide excess air to pressurize the area Figure 8.3: Recirculated HVAC

Advantages of this system:

• A smaller range of challenge to HVAC systems may result in better control of parameters (e.g., temperature or RH), as heating/cooling equipment may be smaller and may not need to handle as large a heat range as a once-through system.

• Usually, less AHU air filter loading is required; therefore, less filter maintenance is required, and there is an opportunity for higher grade air filtration at a lower replacement cost.

• Usually, lower heating/cooling energy cost than once-through air systems.

• A single once-through AHU may pre-treat outdoor air for numerous individual recirculated systems, concentrating pre-heating and humidity control in one unit, with potential energy savings.

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Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

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Disadvantages of this system:

• Return air ductwork routing back to the AHU may complicate congestion above ceiling and make duct chases larger.

• Potential for cross contamination, via the HVAC system. Requires adequate supply air filtration (and sometimes return air filtration to prevent contamination of the AHU).

• Potential for recirculation of odors and vapors. Poor design may result in inadequate fresh air supply for the health of personnel and room pressurization.

8.2.3.3 Exhaust (Extract) system

Exhaust systems may be stand-alone systems that remove airborne contaminants, either solid particles or gasses/

vapors, from a work space. They may be interlinked to a once-through or recirculated air supply system. Used alone, the extract/exhaust system will create a negative DP in a room or space, drawing in air from the surroundings.

Figure 8.4: Exhaust System

Advantages of this system:

• Simple to operate. Makeup air for the fan is pulled from surrounding spaces.

• It can be used to draw fresh air into an unventilated building, such as a warehouse, that does not require heating or cooling.

Disadvantages of this system:

• If used to capture large quantities of contaminants, such as from open processes, high energy costs will be associated with conditioned air being discarded (see once-through system above). Exhaust system energy is greatly reduced if emissions from processes are contained within process enclosures.

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Mr. Gerardo Gutierrez, Sr.

Mexico, DF, ID number: 299643

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• If exhaust air is not adequately cleaned, stack height and velocity should be adequate to prevent re-entrainment in HVAC systems and to prevent personnel hazards. See ASHRAE Fundamentals Handbook. (Reference 22, Appendix 12).

• Temperature and humidity are influenced by the surrounding area.

Detail about design of operation of exhaust and dust collection systems is provided in Chapter 5 of this Guide.

8.2.3.4 Use of Air Handling Units in Parallel or Series

AHUs may be placed in series, e.g., if a higher air pressure is required to offset the pressure drop through HEPA service. These fans commonly are alternated or run at reduced capacity.

In a reduced redundancy system, the multiple fans are capable of less than full capacity with one fan out of service.

As duct pressure drops with a reduction in flow, each fan in a reduced redundancy system will deliver more airflow with one fan out of service than with all fans running.

Example

A system with 2 fans each capable of delivering 50% of flow at design pressure may deliver as much as 70%

of design capacity with one fan out of service because of a reduction in static pressure. When configuring units in parallel serving one duct system, care should be taken to assure that the fans can be isolated and started

independently and that air does not flow backward through the idle unit. Automatic isolation dampers and variable fan drives assist in managing these factors.

8.2.3.5 Series Configuration for Outside Air Pretreatment

In recirculating air systems, a separate, series air handler may be used to pre-treat incoming outside air rather than overcooling or chemical dehumidification of an entire recirculated air stream. This is a common configuration, particularly where the outside air condition represent all or most of the latent load addressed by a system (e.g., in the tropics). In its most economical application, a central pre-conditioning unit may serve multiple recirculation systems.

This configuration has limitations and it may not be suitable for all applications. For further information, see Chapter 2 of this Guide.