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By William J. Coad, PE
Chairman of the Board
McClure Engineering Associates
St. Louis, Mo., USA
William J. Coad, PE, 2001-2002 ASHRAE President travelled to India in February 2002 to inaugurate ACREX 2002 in Bangalore. This paper was presented by him at the symposium held during the event.
In designing systems for new buildings, the design of the air system cannot be separated from the selection of the psychrometric control system. Generic systems can be categorized as:
Since the vast majority of systems at this time are some form of VAV systems, this paper will include a detailed discussion on VAV system configurations.
The basic air handling system configuration for a VAV system is shown in Figure 1. Referring to the figure, air to be conditioned enters the left side, is filtered, cooled and supplied at the desired supply air temperature (usually about 55°F) to the supply air duct system. Wherever it is desired to have a control zone, the duct is tapped, and the air is supplied to the zone through a VAV terminal. The terminal, as shown in Figure 2, is simply a damper and sometimes an acoustically lined chamber to which the zone ductwork is connected. The damper is controlled by a room thermostat. This simple VAV terminal is called a pressure dependent terminal because at any given damper position, the airflow quantity (CFM) will change if the upstream pressure changes.
This dependence of the CFM upon upstream pressure makes the control of these systems quite unstable. So, to prevent this problem, most VAV terminals now used are designed to be pressure independent. Referring to Figure 3, the damper position is controlled by a constant volume controller which has a sensor which senses the velocity pressure in a duct of known area. The controller then calculates the CFM and positions the damper to maintain a constant CFM. In this regard, the controller is actually a constant volume controller, and if the upstream pressure changes, the damper will continually readjust to hold the flow constant. The thermostat then simply resets the setpoint of the controller up or down as more or less air is desired.
Another feature of the constant volume controller is that the maximum flow quantity and the minimum flow quantity can be set – the maximum to the design flow for the zone and the minimum to the minimum amount required for effective air distribution or the ventilation requirement – whichever is greater.
If the minimum flow quantity exceeds the air flow that would be necessary to maintain the space dry bulb setpoint at any time the building is occupied, the space will overcool. A typical example of this would be an interior office or conference room. Under conditions of partial occupancy and low loads, the room temperature can approach the supply air temperature. (This has been referred to as the "ice box effect").
To prevent this type of overcooling, several techniques have been employed. One is the variable air volume reheat terminal. A variable air volume reheat terminal is shown diagrammatically in Figure 4. The reheat coil has been added to the terminal, and the control sequence is that with a drop in room temperature below setpoint, the volume damper will modulate to its minimum value, and a further drop in temperature will cause the reheat coil valve to start modulating open.
An additional feature of this terminal is that it can heat the airstream to a temperature above the room temperature and thus provide heat for systems with a heating load. A system employing VAV-reheat terminals is called a VAV-reheat system.
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Another combination system that can be used is basically a dual stream system but the more commonly used name for it is a fan-powered terminal. The fanpowered terminal mixes recirculated air from the ceiling plenum with the primary conditioned air stream when the quantity of air from the primary conditioned air duct drops below that necessary to maintain effective ambient air velocity or mixing.
There are two types of fan-powered terminals – series and parallel. A simple series terminal is shown in Figure 5. The primary conditioned air at design supply air temperature is directed through the fan and into the supply air duct to the space. A spring loaded (or motorized) damper prevents the air from going backward into the ceiling return air plenum. With the motor off, air blows through a fan wheel and the wheel will rotate in the reverse of its operating rotation; and if it is then turned on, a single phase motor will continue to turn in that direction.
So with the series powered terminal, it is necessary to interlock all of the terminals with the main fan to assure that they are all operating before the air handling unit fan is allowed to run.
The sequence of operation is that the fan output (CFM) is usually matched to the design supply air quantity. Then, as the thermostat requires less cooling, the primary air is reduced by the reset of the volume controller, and the fan draws its additional air from the ceiling plenum, thus mixing ceiling plenum recirculated air with primary supply air. Note that as the space sees it, this system is a constant volume system with constant space circulation rate but as the air handling unit sees it, it is a variable volume system.
If the ventilation air is being supplied as a component of the primary supply air, then the minimum supply quantity is set by the required ventilation rate. If this quantity of primary supply air will tend to overcool the space, one solution is to add reheat to the terminal as shown in Figure 6. This is called a series fan-powered terminal with reheat. The control sequence is that (1) the fan runs continuously and the primary supply for full cooling is at its design CFM value; (2) as less cooling is required, the control volume damper throttles to reduce air flow and the fan causes some recirculated air from the plenum to mix with the primary air, essentially increasing the temperature of the air supplied to the room; (3) after the volume controller reaches its minimum flow required for ventilation, on a call for a further reduction in capacity the reheat coil starts modulating open.
Parallel fan powered terminals, like series terminals, are available with or without reheat.
A parallel fan-powered terminal with reheat is shown diagrammatically in Figure 7. Note with this configuration the primary supply air bypasses the fan, and the fan circulates only the recirculated air stream. The sequence of operation for these devices is that: (1) At design cooling load, the design quantity of primary air flows through the device and into the space air distribution system, the reheat coil is closed and the fan is off. (2) As the space load reduces, the volume controller reduces the quantity of primary air. (3) When the CFM of primary air reduces below that quantity required for effective ambient air velocity and/ or effective mixing, the fan turns on, mixing recirculated plenum air with primary air, thus raising the supply air temperature. (4) On a further drop in space load, the reheat coil starts modulating open.
Generically, both the series and parallel fan-powered terminals are VAV-dual stream and those which include a reheat coil are VAV-dual stream-reheat systems.
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For the purposes of this discussion, “ventilation air” is air introduced into the space from the outdoors and removed from the space to the outdoors for the purpose of removing contaminants from the space. The contaminants are those which are generated within the space, and the 'removal' is generally achieved by dilution of the contaminated space air through mixing it with the non-contaminated air from the outdoors. Thus, for effective ventilation to occur requires:
Contaminants generated in occupied spaces are many and varied. They include such things as biological effluents and odors; carbon dioxide; cooking odors; cleaning solvents; washing compounds; offgassing of volatile organic compounds from finishing materials and furniture; environmental tobacco smoke; combustion products from open flames; microbes, molds and mildews, etc. ASHRAE has conducted extensive research over the past 100 years and attempted to correlate ventilation rates with different types of spaces and space occupants.
The latest published data for ventilation rates is available in ASHRAE Standard 62-1989. Although the research leading to this data reported these rates as being adequate for spaces with moderate smoking, because of the carcinogenic nature of tobacco smoke, it is generally recommended that if there is to be smoking indoors, significantly more ventilation air should be provided. Many building codes have been adopted which require the ventilation rates published in Standard 62-89.
Most constant volume systems have historically been configured to mix the outdoor ventilation air with the return air prior to the conditioning apparatus as shown in Figure 8. When this is done, the amount of ventilation air is determined by first establishing the amount of ventilation air required by each space served by the air handling unit, second establishing the amount of conditioning supply air required for each space, and third, applying the multiple space equation from ASHRAE Standard 62-89.
The outdoor air quantity so determined is normally expressed as a percentage of the total air supplied. In studying the multiple space equation, it becomes evident that if different rooms have different percentages of ventilation air required, the amount of ventilation air required in the supply air stream will always exceed the sum of the amount required in the individual rooms.
Referring to Figure 8, the mixing chamber shown at the inlet to the conditioning unit is a static device, and when properly designed, installed and balanced it will always provide the same percentage of outdoor air to total air – thus if the total air flow is constant and at design flow, the ventilation air will also be constant and at design flow.
So, used with constant volume systems, the static mixing chamber provides ample quantities of outdoor air whenever the fan is running. (The use of the mixing chamber does result in poor indoor humidity control with heat-cool-off and dual stream systems in warm, humid climates at reduced space sensible loads because when the cooling coil is off or is being bypassed, moist outdoor air is being delivered directly to the space.)
However, when used with variable air volume systems, since the static mixing chamber is a constant percentage device, as the total supply volume decreases, the amount of ventilation air decreases. If the contaminant source does not reduce linearly with the sensible space load (which it seldom does), the static mixing chamber will not work acceptably with variable air volume systems.
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This leaves the system designer with two options for ventilating with variable air volume systems:
Dynamic mixing simply means that the absolute amount (CFM) of outdoor air is "controlled" in some way to be constant and/or independent of the quantity of supply air.
Numerous methods of system design and control have been used to achieve required ventilation with VAV systems, some of which are:
The first three options, although commonly used, become extremely complex and costly and exacerbate the problem of poor humidity control in warm humid climates because of the continually increasing percentage of outdoor air in the mix. The last option provides the simplest and least costly of the four and, properly applied, provides the best performance in warm humid climates.
With the last option, the building is provided with two different types of air handling units, the Space Temperature Control (STC) unit and the Ventilation Air Conditioning (VAC) unit. With this option all space humidity control is usually handled by the VAC unit. Then the STC unit, shown diagrammatically in Figure 9, is a relatively simple unit which includes filters, cooling coil and fan. Recirculated air from the space is circulated through the return air system to the unit where it is filtered, sensibly cooled and recirculated to the space. The cooling coil can be chilled water or direct expansion, but for air quality control, to prevent the likelihood of microbial growth, the coil should be dry, providing sensible cooling only. The drain pan is included as an emergency device to remove moisture on the cooling coil during startup or resulting from a malfunction of the VAC unit. The only two controlled devices on this unit are the chilled water valve and the fan speed.
Another control feature is offered by this option. Since the cooling coil is not used for humidity control, the discharge air temperature control can be reset upward by the zone requiring the most cooling. In most VAV system applications, this will result in the optimum available air flow at all times, negating the need for reheat or fan-powered terminals in all zones except those where space heating is needed at some times of the year.
The VAC unit is usually considerably smaller than the STC unit, since it is only sized for the ventilation air flow rate. Fundamentally, this unit is typically configured as shown in Figure 10. The unit is a 100% outdoor air unit with the following basic components for a unit in a temperate climate (cold winters and warm-humid summers).
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Other refinements to the VAC unit are numerous. Refinements such as heat recovery from the exhaust air, desiccant dehumidification, precooling/reheat cycles, etc.
Depending upon the geometry of the building, occupancy schedules, etc., there could be one VAC unit per recirculating STC unit or a single VAC unit with multiple STC units serving multiple spaces.
The conditioned air leaving the VAC unit can either be supplied through a separate ducting system to the spaces or supplied to the inlet return air stream entering the recirculating STC units. In the first configuration, the air is supplied to the spaces through a usually small diffuser or diffusers designed to provide the mixing required for effective dilution of the contaminants. A flow diagram for a system with 5 spaces is shown in Figure 11.
In the second configuration, the air is supplied to the recirculating air stream entering the STC unit as shown in Figure 12. With this option, shutoff dampers can be provided at the point of connection to shut off the supply of conditioned outdoor air when the STC unit is turned off. To assure design ventilation rates at all times, the damper assembly can be provided with a constant volume controller which could also be reset if controlled ventilation rates are desired.
The system described, when properly designed, offers the best indoor air quality control available, with maximum simplicity and usually at a lower cost than the options, while providing the following two additional benefits:
For assured good indoor air quality, some other design aspects to keep in mind are:
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