Author: Jack

SINGLE-DUCT – Variable Air Volume

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SINGLE-DUCT – Variable Air Volume

A VAV system, as shown in Figure 10, controls temperature in a space by varying the quantity of supply air rather than varying the supply air temperature. A VAV terminal unit at the zone varies the quantity of supply air to the space. The supply air temperature is held relatively constant: while supply air temperature […]

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SINGLE-DUCT – Constant Volume

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SINGLE-DUCT – Constant Volume

While maintaining constant airflow, single-duct constant volume systems change the supply air temperature in response to the space load (Figure 9). Single-Zone Systems. The simplest all-air system is a supply unit serving a single-temperature control zone. The unit can be installed either in or remote from the space it serves, and it may operate with […]

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AHU – Equipment Isolation

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Vibration and sound isolation equipment is required for most central fan installations. Standard mountings of fiberglass, ribbed rubber, neoprene mounts, and springs are available for most fans and prefabricated units. The designer must account for seismic restraint requirements for the seismic zone in which the particular project is located. In some applications, the fans may […]

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AHU – Outside Air Requirements

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common complaint regarding buildings is the lack of outside air. This problem is especially a concern in VAV systems where outside air quantities are established for peak loads and are then reduced in proportion to the air supplied during periods of reduced load. A simple control added to the outside air damper can eliminate this […]

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AHU – Supply Air Fan

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Either axial flow, centrifugal, or plug fans may be chosen as supply air fans for straight-through flow applications. In factory-fabricated units, more than one centrifugal fan may be tied to the same shaft. If headroom permits, a single-inlet fan should be chosen when air enters at right angles to the flow of air through the […]

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AHU – Humidifiers

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Humidifiers may be installed as part of the central station airhandling unit, or in terminals at the point of use, or both. Where close humidity control of selected spaces is required, the entire supply airstream may be humidified to a lower humidity level in the air handler, with terminal humidifiers located in the supply ducts […]

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AHU – Reheat Coil

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AHU – Reheat Coil

Reheat systems are strongly discouraged, unless recovered energy is used (see ASHRAE Standard 90.1). Reheating is limited to laboratory, health care, or similar applications where temperature and relative humidity must be controlled accurately. Heating coils located in the reheat position, as shown in Figure 1, are frequently used for warm-up, although a coil in the […]

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AHU – Cooling Coil

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In this section, sensible and latent heat are removed from the air. In all finned coils, some air passes through without contacting the fins or tubes. The amount of this bypass can vary from 30% for a four-row coil at 3.5 m/s to less than 2% for an eight-row coil at 1.5 m/s. The dew […]

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AHU – Preheat Coil

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AHU – Preheat Coil

The preheat coil should have wide fin spacing, be accessible for easy cleaning, and be protected by filters. If the preheat coil is located in the minimum outdoor airstream rather than in the mixed airstream as shown in Figure 1, it should not heat the air to an exit temperature above 2 to 7°C; preferably, […]

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AHU – Mixing Plenum

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If the equipment is closely coupled to outdoor louvers in a wall, the minimum outdoor air damper should be located as close as possible to the return damper connection. An outside air damper sized for 7.5 m/s gives good control. Low-leakage outdoor air dampers minimize leakage when closed during shutdown. The pressure difference between the […]

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