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Energy Efficiency and Air Handling Units

TThe air handling unit (AHU) is a complicated device involving many interrelated design selections that affect its energy efficiency. The overall efficiency of an AHU is determined by the energy it takes to move a given quantity of air against the system. Three major energy choices for the AHU design must be considered..

Reducing AHU pressure drop
The AHU pressure drop can be reduced by:

Lowering face velocity
Lowering face velocities necessitates larger area coils and filter elements and bigger AHU housings than conventionally used. These larger items increase the AHU's cost, but a resulting decrease in face velocity from 500 fpm to 400 fpm usually makes up for the increased cost over the life of the AHU. Face velocity reductions from 500 fpm to 400 fpm can have a simple payback of about three years. A comparable decrease in face velocity at filters can have a simple payback of one year or less. [Brown, 1990]

Considering filter loading and VFDs
Variable system resistance, primarily resulting from filter loading, can be efficiently corrected for with a VFD on the fan. AHU pre-filter pressure drop will typically vary from 0.5 in. H2O (125 Pa) when "clean" to 1.7 in H2O (423 Pa) when "dirty." The system design point is usually sized for the "dirty" condition. However, on average, about 90 percent of the design head and energy is actually required. The energy saved by a VFD will pay for the drive in two to five years, depending upon system capacity. [Brown, 1990]

Adding bypass dampers
Some AHU components are only utilized during a portion of the year; therefore, bypass dampers should be used to reduce parasitic energy losses when these components are not needed. [Brown, 1990]

Ductwork System Effect
The heart of the HVAC system is the fan. Improper fan design or installation can cause serious degradation of performance because of "system effects." System effect factors for connections between fan and duct system are presented in "Fans and Systems" (AMCA, 1973), the ASHRAE Fundamentals Handbook [ASHRAE, 1997], and the DFDB [ASHRAE, 1994]. System effects cause changes in a fan's aerodynamic characteristics that result in an entirely new fan performance curve. They appear when inlet and outlet connections cause non-uniform inlet and outlet flow and swirl at the fan inlet.

An engineer designing ductwork should provide uniform straight flow conditions at the fan inlet and outlet. An elbow at a fan's inlet causes turbulence and uneven flow into the fan opening. An elbow at a fan's discharge causes a non-uniform backpressure. An engineer can only estimate how much pressure will be lost as a result of system effects by reviewing the air distribution system design.

System effects are rarely measured during system start-up procedures. They become evident only when not enough air is delivered to the building. Start-up personnel must then speed up the fan by changing pulleys or even increasing the fan's motor size in order to get design air-flow rates.

System effects can be reduced or largely avoided by a few easy precautions for centrifugal fans, as follows:

With a free inlet fan (no inlet duct), a minimum of one half the fan wheel diameter, D, clearance should be maintained from the inlet to the wall or structure.
An elbow entrance to the fan should be at least two times the inlet diameter distance from the flex connector at the fan, C=0.15 (remember, the fan entrance is a high-velocity duct).
A 2D cone to the inlet flex (vibration isolator) should be used to decrease friction losses (C=0.10).
If this 2D inlet cone is attached to a rectangular, vertical duct, its dimensions should be two times the fan's inlet diameter by one half the fan's inlet diameter and extend one diameter past the center line of the fan's inlet (Figure 1).
A duct expansion from the fan discharge should be at a 10° angle up on a top horizontal discharge fan (Figure 2). Down angles and increases in the width of the duct are less efficient.
A straight duct for a distance of three to six duct diameters from the fan discharge should be used in order to develop a full dynamic head. Branching and turning sooner causes system effect losses.

Incorporating the most efficient fan "system"
The fan "system" includes the following:

Fan type
Centrifugal fan efficiencies range from 50 percent to about 70 percent depending upon blade configuration and vane-axial fan efficiencies range from 80 percent to over 90 percent. However, the energy engineer must keep in mind that all fans add heat to the air stream. The air stream temperature typically rises between three degrees F and six degrees F for inefficient fan types. Also, actual installed efficiency is typically worse than the manufacturer's rating data indicates, dropping the fan's efficiency by 10 to 30 percent in some cases.

Drive mechanism
Direct drive fan/motor combinations are the most efficient at 100 percent. Standard V-belt drives are about 93 percent efficient when first installed but drop over time. Synchronous belt systems offer an energy-efficiency increase to about 96 percent for the life of the belt.

Motor efficiency
Premium efficiency motors that have full-load efficiencies of greater than 94 percent depending upon horsepower are preferable. It is also important to consider the part-load motor efficiency and its operation with a VFD in VAV systems.

Sound Power Requirements
Acoustical attenuators, used to reduce a fan's sound, increase the pressure drop of the AHU. Therefore, engineers should consider the disadvantages of high-efficiency but noisy fan that will require acoustical attenuation devices. These devices can negate the energy savings from face velocity reductions.

Evaporative cooling
Evaporative cooling is an alternative to conventional compressor-type cooling systems. Direct and indirect evaporation systems or a combination can supplant or displace conventional cooling systems. Evaporative cooling systems have been used to condition facilities since before refrigerated cooling was invented. There are two types of evaporative cooling techniques: indirect and direct. The indirect approach removes sensible energy only from the air stream with an exchanger that has the air stream flow on one side of a plate and water on the other. The direct approach removes energy from the air stream adiabatically by adding moisture to the air stream. These approaches can be combined to provide cooling and humidification that displaces energy from a conventional compressor-type HVAC system. Annual energy savings of more than 15 percent for a one-stage, direct system and over 38 percent for a two-stage indirect/direct system can be realized when compared to a conventional HVAC system. Brown (1993) says, "The use of direct evaporative cooling to humidify increases the opportunity to recover heat by 36 percent and reduce energy consumption during the heating season." One- to two-year paybacks are possible. The justification of evaporative cooling should not be based solely on the ability to replace mechanical cooling but should be considered as an opportunity to displace mechanical cooling/humidification energy. [Brown, 1992; Brown, 1993]

Evaporative cooling should always be considered for all make-up air systems, regardless of geographic region, as noted in the descriptions of the approaches below:

Indirect evaporative cooling is applicable when there is a large process equipment-cooling load, the HVAC system operates on a continuous basis, and a direct evaporative cooling opportunity exists. [Brown, 1993]

Direct evaporative cooling can be justified on the basis of eliminating the need for a steam humidification system, enhancing economizer operation, reducing humidification energy used in conjunction with heat recovery, and controlling the temperature of a space independent of controlling its humidity. Direct evaporative cooling for both make-up air and recirculation-type systems should always be considered when the outdoor wet-bulb temperature is 53°F (11.7°C) or less for 3,500 or more hours per year. Direct evaporative cooling of air conditioning condensers, which are normally simply air-cooled, can reduce electrical energy and demand consumption by 20 to 40 percent. [Knebel, 1997] [Brown, 1990; Brown, 1993]

Typically, a two-stage evaporative cooling system comprises a direct evaporative cooler downstream of an indirect evaporative cooler. The two coolers can be combined in a myriad of arrangements to increase energy efficiency

When mixing of the facility's exhaust air with make-up fresh air is permitted, economizers can save energy in both cooling and heating modes.

Coil placement
Brown (1993) points out that placement of the fan upstream from the cooling coil and evaporative cooler has two energy advantages. In cooling mode, fan heat enhances operation of the cooling coil, and, in heating mode, the same fan heat is used to provide humidification energy and to displace preheat energy input. [Brown, 1993]





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