Trane TRG-TRC014-EN - Manuals

TRG-TRC014-EN 1 notes period one What Is Variable Air Volume? A variable-air-volume (VAV) air-conditioning system varies the volume of constant-temperature air that is supplied to meet the changing load conditions of the space. For the purpose of comparison, we will look at a traditional constant-vo...

TRG-TRC014-EN 3 period one What Is Variable Air Volume? notes Rearranging the equation and using the constant supply airflow of 1,840 cfm [0.87 m 3 /s], the supply air temperature required to balance a lesser 20,000 Btu/hr [5,862 W] space sensible load is: Therefore, as the space sensible load drops...

4 TRG-TRC014-EN notes period one What Is Variable Air Volume? However, because this type of system can respond to the demands of only one thermostat, it can serve only those building spaces with similar cooling requirements. If a building has many spaces with diverse cooling needs, each must be serv...

TRG-TRC014-EN 5 period one What Is Variable Air Volume? notes The terminal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces. Each space has its own heating coil to temper the air to satisfy the space load. Of course, any heat added to meet the p...

6 TRG-TRC014-EN notes period one What Is Variable Air Volume? In contrast, a variable-air-volume (VAV) system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions. Similar to the constant-volume system, at design ...

TRG-TRC014-EN 7 period one What Is Variable Air Volume? notes However, at part load, the VAV system produces a balance by modulating the volume of constant 55°F [12.8°C] supply air. In this example, the air volume required to balance the lesser 20,000 Btu/hr [5,862 W] space sensible load is: That is...

TRG-TRC014-EN 9 period one What Is Variable Air Volume? notes Contrasting the three systems discussed in this example, the simple, constant-volume, variable-temperature system consumes constant fan energy while providing refrigeration energy savings at part load. However, it can only deliver comfort...

10 TRG-TRC014-EN notes period one What Is Variable Air Volume? There are some basic building characteristics that favor the application of VAV systems. We will discuss three: n Variable thermal load profiles in the spaces. n The need for multiple, independently-controlled spaces. n The existence of ...

TRG-TRC014-EN 11 period one What Is Variable Air Volume? notes Another feature of the VAV system is the ability to efficiently satisfy the comfort requirements of many different spaces within the building. As we saw from the comparison of the VAV and terminal reheat systems, the VAV system is the mo...

12 TRG-TRC014-EN notes Next, we will examine the individual components that compose a VAV system. A simple VAV system includes the following: n Central air handler with a variable-volume supply fan, a cooling coil, possibly a heating coil, controls, filters, a mixing box, and a return or relief fan ...

TRG-TRC014-EN 13 period two Components of a VAV System notes VAV Terminal Units A VAV terminal unit is a sheet-metal assembly installed upstream of its respective space diffusers. The unit consists of an air-modulation device, control hardware and, depending on the system application, possibly a hea...

14 TRG-TRC014-EN notes period two Components of a VAV System The simplest of all VAV terminal units is the single-duct, cooling-only terminal unit. It consists of an airflow modulation device with controls packaged in a sheet-metal enclosure. The unit can only modulate the primary airflow to the spa...

TRG-TRC014-EN 15 period two Components of a VAV System notes determined by the design cooling load of the space and the minimum setting is normally determined by the space ventilation requirement or minimum airflow for proper diffuser selection. Most cooling-only units are applied to spaces that hav...

16 TRG-TRC014-EN notes period two Components of a VAV System VAV reheat terminal units provide supply-air tempering or space heating by reheating the cool primary air. This is accomplished by adding an electric, or hot-water, heating coil to the discharge of a cooling-only unit. The VAV reheat termi...

TRG-TRC014-EN 17 period two Components of a VAV System notes During the heating mode, the primary airflow must often be greater than the minimum setting of the unit for the cooling mode. When warm, buoyant air is supplied from the ceiling, a certain velocity is required to effectively deliver it dow...

20 TRG-TRC014-EN notes period two Components of a VAV System In the cooling mode, the primary airflow is reduced as the cooling load in the space decreases. The total supply airflow to the space remains constant, a combination of cool primary air and warm plenum air. If the space cooling load drops ...

TRG-TRC014-EN 21 period two Components of a VAV System notes Most designers also believe that series units offer improved acoustical conditions in the space. The constant sound of the series unit, with the fan operating whenever the space is occupied, is generally preferred to the on-off sound gener...

TRG-TRC014-EN 23 period two Components of a VAV System notes As before, this graph indicates how the air supplied to the space by the terminal unit varies as the space loads change. The vertical axis indicates the total airflow (cool primary air plus warm primary air) supplied to the space. The hori...

24 TRG-TRC014-EN notes period two Components of a VAV System To deliver a constant volume of supply air to the space in a two-fan, dual-duct system as the cooling load in the space decreases, the amount of cool primary air is reduced and the amount of warm primary air is increased, maintaining a con...

26 TRG-TRC014-EN notes period two Components of a VAV System VAV Terminal Unit Controls Next we will explore the control of VAV terminal units. Before discussing the commonly-available control technologies, it is important to first understand a fundamental control concept: pressure-dependent versus ...

TRG-TRC014-EN 27 period two Components of a VAV System notes duct static pressure, airflow to the space can vary even when the immediate modulation device does not change position. A pressure-dependent VAV control scheme uses the space temperature sensor to directly control the position of the modul...

28 TRG-TRC014-EN notes period two Components of a VAV System Accurate measurement of primary airflow is required to enable pressure-independent control. Most terminal units accomplish this with a multipoint airflow sensor mounted on the inlet. This sensor measures a pressure difference between the p...

TRG-TRC014-EN 29 period two Components of a VAV System notes A pneumatically-controlled terminal unit has an airflow-modulation device operated by a pneumatic actuator consisting of an inflatable bladder and a return spring. The return spring positions the damper to a “normal” state (the position it...

30 TRG-TRC014-EN notes period two Components of a VAV System An electronically-controlled terminal unit has an airflow-modulation device operated by an electronic actuator that can drive the modulation device open or closed. Electrically-actuated modulation devices typically do not have a spring to ...

TRG-TRC014-EN 33 period two Components of a VAV System notes Diffusers Each VAV terminal unit is commonly connected to a downstream, sheet-metal duct that is then connected to the remotely-located diffusers by flexible ducts. Diffusers distribute the supply air effectively to the conditioned space. ...

34 TRG-TRC014-EN notes period two Components of a VAV System Linear slot diffusers are generally preferred for VAV air distribution. They use a principle known as the Coanda effect to distribute air to the conditioned space. The Coanda effect occurs when air is discharged at a relatively high veloci...

TRG-TRC014-EN 35 period two Components of a VAV System notes Because variable-air-volume systems are installed in, and distribute air along the surfaces of, ceilings, it is essential that the diffusers be compatible with the ceiling design. The T-bar ceiling is possibly the most popular design. It i...

TRG-TRC014-EN 37 period two Components of a VAV System notes Supply Duct System The supply duct system transports the primary air from the air handler to the VAV terminal units and then on to the space diffusers. A successful design achieves the following: n Minimizes the static pressure and associa...

38 TRG-TRC014-EN notes period two Components of a VAV System Designers commonly use two methods to engineer the supply duct system: n Equal friction n Static regain Equal friction duct systems are designed for an equal static-pressure drop per foot of duct. This results in a static pressure that is ...

TRG-TRC014-EN 39 period two Components of a VAV System notes Other publications contain more complete details related to duct design, but here are a few general recommendations: n Keep the duct layout as simple and symmetrical as possible n Place main duct runs and, when possible, branch runs and te...

40 TRG-TRC014-EN notes period three System Configurations To permit a complete understanding of the VAV system, we must look at it from a system level. In this section we will explore a few common space types and how VAV systems address the cooling and heating needs associated with them. For simplic...

TRG-TRC014-EN 41 period three System Configurations notes and equipment. Interior spaces on the top floor of a building often need to be treated as a perimeter space due to the heat gain/loss from the roof. Each space is typically served by an individual VAV terminal unit, allowing independent cooli...

42 TRG-TRC014-EN notes period three System Configurations If the heat loss of the perimeter space exceeds 450 Btu/hr per linear foot [430 W per linear meter] of outside wall, an under-the-window, or baseboard, heating system is typically used. With this much heat loss, supplying a high quantity of w...

TRG-TRC014-EN 43 period three System Configurations notes If the heat loss of the perimeter space is less than 450 Btu/hr per linear foot [430 W per linear meter] of outside wall, downdrafts are less problematic and heated air supplied through ceiling diffusers can provide a satisfactory solution. A...

46 TRG-TRC014-EN notes period three System Configurations Interior Spaces Because an interior space is surrounded by spaces at the same temperature, it does not experience the same heat gain and loss fluctuations as a perimeter space. Therefore, an interior space typically requires some degree of co...

TRG-TRC014-EN 47 period three System Configurations notes Typically, either VAV reheat or fan-powered terminal units are used to provide the tempering needed to balance the reduced cooling load. Changeover/Bypass VAV System Many smaller buildings cannot afford commercial, or applied, equipment. Ofte...

TRG-TRC014-EN 49 period three System Configurations notes In a typical commercial building, the occupied mode occurs during daytime operation. The building must be ventilated and the comfort cooling or heating temperature set points must be maintained in all occupied zones. The occupied mode is char...

50 TRG-TRC014-EN notes period three System Configurations In a typical commercial facility, the unoccupied mode occurs at night. The building does not require ventilation because it is not occupied, and the temperature in the perimeter spaces must be prevented from getting too cold (perhaps 60°F [15...

52 TRG-TRC014-EN notes period four Fan Modulation To accommodate the variable-volume requirements imposed by a VAV system, the system supply fan must be selected and controlled in such a manner that it is capable of modulating over the required airflow range without entering an unstable area of oper...

TRG-TRC014-EN 53 period four Fan Modulation notes plot performance on the basis of the static pressure component only. Performance curves published for some packaged air handlers will include the effects of the dampers, filters, and coils that are in the unit. In this case, the static pressure capac...

54 TRG-TRC014-EN notes period four Fan Modulation When the system resistance curve is superimposed on the fan curve, the intersection predicts the airflow and static pressure at which the system will balance (operating point “ A ”). Running the fan in this example at 1,000 rpm will satisfy the desig...

TRG-TRC014-EN 55 period four Fan Modulation notes This “riding the fan curve” method of modulation can be used with any centrifugal fan. However, it is most energy efficient when applied to forward-curved (FC) fans. The configuration of the input power curves of the FC fan are such that input power ...

56 TRG-TRC014-EN notes period four Fan Modulation Fan Modulation Curve The fan modulation curve illustrates the static pressure requirement of the VAV system over the range of airflows. This requirement consists of a fixed component and a variable component. The VAV system requires a minimum amount ...

TRG-TRC014-EN 57 period four Fan Modulation notes To achieve this balance, a simple control loop is used. First, the static pressure is sensed from a particular location in the system. Second, a controller compares this static pressure reading and the set point of the system. Finally, the fan capaci...

58 TRG-TRC014-EN notes period four Fan Modulation The system static pressure controller senses this higher pressure and sends a signal to the supply fan to reduce capacity. Modulating the fan capacity results in a new fan performance curve and the system balances at an operating point ( B ) along th...

60 TRG-TRC014-EN notes period four Fan Modulation Inlet vanes modulate the capacity of a fan by “pre-swirling” the air in the direction of fan wheel rotation before it enters the wheel. By changing the angle of air entry into the fan, the modulating inlet vanes lessen the ability of the fan wheel to...

62 TRG-TRC014-EN notes period four Fan Modulation Finally, the capacity of vaneaxial fans can be modulated by adjusting the pitch angle of the actual fan blades. This diagram shows a set of fan blades that can swivel to modulate the capacity of the fan. This method of fan modulation is typically onl...

TRG-TRC014-EN 63 period four Fan Modulation notes In response to reduced airflow and rising system static pressure, the variable-pitch vaneaxial fan responds similarly to a centrifugal fan with either inlet-vane or variable-speed capacity control. By changing the angle of the fan blades, the perform...

64 TRG-TRC014-EN notes period four Fan Modulation System Static-Pressure Control Each of these fan-capacity control methods requires a signal from a controller that monitors static pressure, using a sensor located somewhere in the supply-duct system. This controller compares the sensed pressure to a...

TRG-TRC014-EN 65 period four Fan Modulation notes included in the supply duct, this method ensures that the sensor is on the fan side of the damper so that the duct is protected from high pressures. Also, depending on the layout of the duct system, this method may eliminate the need for multiple duc...

66 TRG-TRC014-EN notes period four Fan Modulation The optimized static-pressure control method combines the location- related benefits of fan outlet control with operating cost savings that exceed those of supply duct static-pressure control. A single static-pressure sensor is located at the fan out...

68 TRG-TRC014-EN notes period five Application Considerations When designing a VAV system for a building, there are several considerations to address: n System-level ventilation n Freeze protection for coils n Part-load space humidity control n Building pressure control This is by no means the entir...

TRG-TRC014-EN 69 period five Application Considerations notes System-Level Ventilation During operation, a typical VAV system delivers a mixture of outdoor air and recirculated air to the multiple spaces it serves. The volume of supply air that enters a space is controlled by a temperature sensor in...

70 TRG-TRC014-EN notes period five Application Considerations With all three components, DDC controls on the VAV terminal units and air handler, a method to measure outdoor airflow at the air handler, and a communicating building automation system (BAS), a VAV system can use the multiple-spaces equa...

TRG-TRC014-EN 71 period five Application Considerations notes Freeze Protection for Coils As mentioned, proper VAV system ventilation often leads to a high percentage of outdoor air at part-load conditions. Streams of air at vastly different temperatures are difficult to mix and may cause the mixed ...

72 TRG-TRC014-EN notes period five Application Considerations Coils can be protected from freezing in many ways, such as: n Draining the coils during cold weather n Keeping the water in motion n Adding glycol to the water n Using face-and-bypass dampers to limit the amount of cold airflow through th...

TRG-TRC014-EN 73 period five Application Considerations notes Minimum space ventilation requirements often dictate minimum primary airflow settings on VAV terminal units. If the space cooling load requires less airflow than this minimum, the space may overcool (to B = 68°F [20°C] in this example ins...

74 TRG-TRC014-EN notes period five Application Considerations Building Pressure Control Maintaining a slightly positive pressure inside buildings, especially in humid climates, increases comfort and helps avoid indoor air quality (IAQ) problems related to microbial growth and outdoor contaminant sou...

76 TRG-TRC014-EN notes Now, we will review some of the main concepts from this clinic on variable-air-volume (VAV) systems. A variable-air-volume (VAV) air-conditioning system varies the volume of constant-temperature primary air to meet changing load conditions in the space. VAV systems can provide...

TRG-TRC014-EN 77 period six Review notes A simple VAV system is comprised of the following components: n Central air handler n Supply duct n VAV terminal unit with thermostats and supply diffusers for each independently controlled space n Thermostat and unit controller for each terminal unit n Retur...

78 TRG-TRC014-EN notes period six Review A simple building has two types of spaces: perimeter and interior. These two space types require different types of VAV terminal units to satisfy their requirements. It is important to realize that many VAV systems include a combination of different types of ...

TRG-TRC014-EN 79 period six Review notes Finally, when applying a VAV system to a building, there are several considerations that must be addressed. They include: providing the correct system-level ventilation, adequately protecting the coils from freezing, controlling space humidity over a wide ran...

TRG-TRC014-EN 83 Questions for Period 1 1 Given a space sensible load of 30,000 Btu/hr [8,793 W], a space dry-bulb temperature of 78°F[25.6°C], and a primary air dry-bulb temperature of 58°F [14.4°C]: a What supply airflow is required to balance this load? Assume the space sensible load is reduced t...

84 TRG-TRC014-EN notes Quiz 11 What method of static-pressure control provides the greatest power-savings potential for a VAV system? Questions for Period 5 12 What industry standard outlines a method for determining the proper quantity of ventilation air to be introduced at the system-level air han...

86 TRG-TRC014-EN 1a 1,382 cfm [0.65 m 3 /s] 1b 63°F [17.2°C] 1c 829 cfm [0.39 m 3 /s] 2 The reduced airflow across the cooling coil at part load causes the refrigeration system to throttle back in order to stabilize the supply air temperature condition. IN turn, this results in a reduction in refrig...

TRG-TRC014-EN 87 changeover/bypass VAV system A system that allows variable supply air to the spaces but uses a constant-volume central supply fan. This is accomplished by using a large damper to bypass the excess air. This is common in smaller buildings requiring individual space comfort control. C...