Archive for June, 2019

AHU – Economizers

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An economizer uses outside air to reduce the refrigeration requirement. A logic circuit maintains a fixed minimum of ventilation outside air. The air side economizer is an attractive option for reducing energy costs when the climate allows. The air-side economizer takes advantage of cool outdoor air to either assist mechanical cooling or, if the outdoor […]

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AHU – Outdoor Air Intakes

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Resistance through outdoor intakes varies widely, depending on construction. Frequently, architectural considerations dictate the type and style of louver. The designer should ensure that the louvers selected offer minimum pressure loss, preferably not more than 25 Pa. High-efficiency, low-pressure louvers that effectively limit carryover of rain are available. Flashing installed at the outside wall and […]

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AHU – Return Air Dampers

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The negative pressure in the outdoor air intake plenum is a function of the resistance or static pressure loss through the outside air louvers, damper, and duct. The positive pressure in the relief air plenum is, likewise, a function of the static pressure loss through the exhaust or relief damper, the exhaust duct between the […]

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AHU – Relief Openings

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Relief openings in large buildings should be constructed similarly to outdoor air intakes, but they may require motorized or selfacting backdraft dampers to prevent high wind pressure or stack action from causing the airflow to reverse when the automatic dampers are open. The pressure loss through relief openings should be 25 Pa or less. Low-leakage […]

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AHU – Automatic Dampers

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Opposed blade dampers for the outdoor, return, and relief airstreams provides the highest degree of control. The section on Mixing Plenum covers the conditions that dictate the use of parallel blade dampers. Pressure relationships between various sections must be considered to ensure that automatic dampers are properly sized for wide open and modulating pressure drops.

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AHU – Relief Air Fan

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In many situations, a relief (or exhaust) air fan may be used instead of a return fan. A relief air fan relieves ventilation air introduced during air economizer operation and operates only when this control cycle is in effect. When a relief air fan is used, the supply fan must be designed for the total […]

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AHU – Return Air Fan

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A return air fan is optional on small systems but is essential for the proper operation of air economizer systems for free cooling from outside air if the return path has a significant pressure drop (greater than about 75 Pa) It provides a positive return and exhaust from the conditioned area, particularly when mixing dampers […]

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AHU – Air Conditioner

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AHU – Air Conditioner

To determine the system’s air-handling requirement, the designer must consider the function and physical characteristics of the space to be conditioned and the air volume and thermal exchange capacities required. Then, the various components may be selected central system—equipment must be adequate, accessible for easy maintenance, and not too complex in arrangement and control to […]

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AHU- Humidification

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AHU- Humidification

The methods used to humidify air include • Direct spray of recirculated water into the airstream (air washer) reduces the dry-bulb temperature while maintaining an almost constant wet bulb in an adiabatic process [see Figure 3, Paths (1) to (3)]. The air may also be cooled and dehumidified, or heated and humidified by changing the […]

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AHU- Cooling Process

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AHU- Cooling Process

The basic methods used for cooling include • Direct expansion, which takes advantage of the latent heat of the fluid, as shown in the psychrometric diagram in Figure 2. • Fluid-filled coil, where temperature differences between the fluid and the air cause an exchange of energy by the same process as in Figure 2 (see […]

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