Titus 2008 - 2009 Product Catalog | www.titus ... - Texas Air Products

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Direct Digital Control (continued) DDC Distributed Processing Using a concept commonly referred to as distributed processing, DDC controllers can function as standalone devices. In this way if one controller fails, others throughout the system can continue to function unaffected. The controllers are connected over a system communication bus or local area network (LAN) for system wide sharing of information. This information is used to perform sophisticated building control strategies not possible with conventional noncommunicating systems. The network also allows system access locally through a personal computer or remotely via modem over telephone lines (Figure 84). Sizing Basic Terminals from Capacity Tables ENGINEERING GUIDELINES Engineering Guidelines | Terminals, Controls and Accessories Certified Air Terminals To provide engineers with sound power data which can be compared on an even basis, leading air terminal manufacturers joined together under the Air Conditioning and Refrigeration Institute (ARI) to develop an industry standard for rating air terminals and certifying performance data. The result was ARI Standard 880, Air Terminals, and the 880 Certification Program. Standard 880 specifies the procedure, using a reverberant chamber, for developing sound power data. The certification program ensures manufacturers’ equipment performance meets their claims. Compliance with 880 is assured through third party testing. If a manufacturer fails to match claimed performance, the manufacturer must immediately rerate the terminal or lose the ability to use the ARI 880 seal. Another standard, ARI 885, was developed at the same time to assist the engineer in using certified product data. Terminal selection involves a series of trade-offs. The designer needs to try to balance all of the constraining factors and select the terminal which meets overall needs best. Engineers who specify ARI Certified air terminals are assured that the manufacturer’s performance meets the manufacturer’s claims. This is protection for the engineer, the building owner and the building occupant. VAV Controlling HVAC Equipment Local Bus Network Controlling HVAC Equipment Local Bus Network Controlling HVAC Equipment System Interface Local Bus Network VAV VAV VAV VAV VAV VAV VAV VAV Modem Figure 84. System Access via Network Sizing Single Duct Terminals The starting point for sizing single duct terminals is to identify the type and model of controller. This is necessary because some controllers are more accurate at lower velocities than others. Once the type of control is identified, the minimum and maximum primary airflows should be considered against the published cfm range. The trade-offs start here. Some engineers will select terminals near the bottom of the cfm range to reduce sound levels since large inlets reduce face velocity. Others select terminals near the top of the cfm range to hold down equipment costs. Still other engineers believe that one should remain comfortably in the middle to avoid potential control problems resulting from low velocities and sound problems occurring at high velocities. All Titus products operate extremely well within the published cfm ranges. Therefore, low velocity control concerns can be eliminated. This leaves sound and first cost as the key issues. If the terminal is relatively small to begin with and will be located over a kitchen or hallway, sound will probably not be of concern and the designer may choose to slightly undersize the terminal. If, on the other hand, the terminal is located over office space, the designer may slightly oversize the terminal. The selection of an appropriate water coil should also be considered at this time. In some cases, a terminal may need to be increased in size in order to obtain the desired heat output from the coil. With single duct units, the water coil air pressure drop should be subtracted from the duct pressure when determining sound generation. The sound produced by the damper is proportional to the pressure drop across the damper and discharge water coils may reduce that pressure drop. Other significant downstream pressure drops should be considered, and their pressure drop subtracted as well. VAV VAV VAV B B53 Titus 2008 - 2009 Product Catalog | www.titus-hvac.com TERMINAL CONTROLS AND ACCESSORIES
ENGINEERING GUIDELINES B B54 TERMINAL CONTROLS AND ACCESSORIES Engineering Guidelines | Terminals, Controls and Accessories Sizing Basic Terminals from Capacity Tables (continued) Sizing Parallel Fan Powered Terminals Parallel flow (variable volume) fan powered terminals are selected based on their capacity to handle the primary airflow. The same rules which apply to the selection of single duct terminals can be used, except that water coils are not in the primary airstream path, and will not affect sound levels. The pressure drop of the water coils, however, which are on the fan inlet in Titus parallel fan units, must be added to the expected discharge pressure at the fan flow rate when entering the fan curve tables. The fan is selected based on the minimum airflow requirements for the space or the heating load required. In most cases the fan can be downsized from the cooling flow requirement considerably, reducing both first cost and operating cost. The fan is selected from the fan curves. The downstream static pressure of the secondary air may not be the same as the primary air, however. If the secondary airflow requirements are less than the primary air requirements, the static pressure will be reduced. The following equation can be used to determine the static pressure at reduced airflows. (Do not forget to add water coil pressure drops to the fan requirement). Ps 1 = Ps 1 (V 1 / V 2 ) 2 Where: Ps 2 = Primary Air Static Pressure Ps 2 = Secondary (Fan) Air Static Pressure V 1 = Primary Air Velocity V 2 = Secondary (Fan) Air Velocity To select a Titus parallel fan powered terminal, refer to the published fan curves and primary air pressure drop curves, together with the application and sound power data. In the parallel flow type of unit, when the primary air is ON, the fan is typically OFF, and vice versa. As shown in the Figure 86, the primary air and the fan discharge air follow parallel paths into a common plenum. Therefore both airflows will encounter the same downstream resistance at a given flow rate. Since the primary and secondary airflows come from two different sources-and often at two different specified flow rates-the volume vs. pressure relationship in each of these airflows must be checked to ensure adequate flow rates under actual job conditions. Example: Select a Model DTQP for a maximum of 1400 cfm of primary air with 1.00” wg inlet static pressure. The fan airflow required is 1150 cfm. The downstream resistance offered by the duct and diffusers has been determined to be 0.30” static pressure at 1150 cfm. Primary Air: From the chart on page R46, a size 4 with a 12” inlet will handle 1400 cfm of primary air with a minimum static pressure drop of 0.22” through the primary air section. But since the downstream resistance is 0.30” at 1150 cfm, o 1400 2 p 1150 x 0.30” = 0.44” sp Titus 2008 - 2009 Product Catalog | www.titus-hvac.com The overall primary air static pressure drop is 0.22”+ 0.44”= 0.66” sp Since a 1.0” static pressure is available at the inlet, the selection will work. The damper in the primary air section Recirculated Air Primary Air Fan Primary Air Section Figure 85. Schematic Diagram of Airflow in Parallel Flow (Variable Volume) Models Primary Secondary Either / Or Backdraft Damper Downstream Duct and Diffuser To Outlets Figure 86. Actual Arrangement of Components Shown in the Previous Schematic Diagram will do some throttling to hold the maximum air flow to 1400 cfm. Secondary Air (Fan): From the fan curves, a size 4, without coils, terminal will handle 1150 cfm at 0.30” static pressure, with the proper setting of the standard SCR speed control.
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