Generally the design considerations governing constant volume air conditioning systems can be applied to the design of air handling and distribution equipment for variable volume systems. Whilst the subjects of heat loads and air quantity or temperature calculations are fully dealt with in such publications as the ASHRAE guide, the following general procedure is recommended:

1. Cooling and heating load calculations are made for every module or zone in the building so that individual maxima and simultaneous maximum cooling/ heating requirements and air quantities can be established.
2. The design temperature of supply air at the terminal (allowing for all normally calculated extraneous gains such as fan heat and duct wall heat pick-up) is arrived at by conventional psychometric calculations.
3. Individual maximum module or zone air volumes are calculated using the maximum room sensible heat gain figure and the temperature difference between supply air temperature at the terminal and design space temperature. These air volumes are then used to select air diffusers.
4. Maximum simultaneous cooling and air supply requirements are then computed in order to establish the capacity of air handling units and central cooling equipment. Care must be taken in design that ducting is adequately sized to convey the correct amount of air to every module or zone under all conditions.

OTHER DESIGN CONSIDERATIONS TO BE NOTED ARE:

Control of duct Pressure

The control of supply air duct static pressure within the design limits of the terminal units is an essential requirement of variable volume systems and such control is easily achieved by any or a combination of the following methods, depending on the size and configuration of the air distribution system:

1. Control of supply air duct fan output by means of a static pressure regulator operating in conjunction with its capacity modulation device. In smaller systems this device need be no more than fan motor speed control although acceptable performance will probably be achieved with a face and by-pass damper configuration.
2. Dividing the air distribution systems into the most conveniently selected low pressure supply duct zones, supplied from low/medium pressure main ducts or risers, by the fitting of branch duct dampers operating in conjunction with branch supply duct static pressure regulators.

Duct Static Pressure Variations

Design and manufacturing discrepancies in supply duct systems are largely compensated for by terminal volume adjustment in response to thermostat action. Tolerances of - 10% to +20% in duct static pressure will not materially effect correct functioning of variable volume terminal units. Under all terminal output conditions duct pressure variations will be compensated for by temperature controllers which react to an incorrect supply of air by terminal volume regulation. At minimum flow, variations in duct static pressure are normally slight as duct pressure losses at low air flow are almost non-existent.

Recommended Branch Duct Pressure

The determination of design branch duct pressure should be governed by the noise levels to be satisfied, the area the conditioned air should serve, or throw and the duct pressure needed to deliver the air flow required by thermal load calculations. All these details are provided in the relevant sections of the manual.

Recommended Main Duct or Riser Pressure

For economical fan operation, this pressure should be just high enough to accommodate main duct or riser pressure losses and design tolerances. With careful riser duct design it should not be necessary to require riser or main duct pressure to be more than 0.4 in.w.g. above branch duct pressure.

Positioning of Static Pressure Sensors

In all cases static pressure sensors for branch ducts should be fitted in the branch ducts themselves and not in the feeder ducts to individual terminals nor in the terminal units themselves. Generally it is recommended that static pressure sensors in branch ducts be fitted half-way between the duct pressure control damper or supply fan and the end of the relevant duct so that the most representative pressure is sensed.

Branch Duct Velocities

In the case of branch ducts serving terminal units it is recommended that conventional criteria be used in the determination of branch duct velocities, and it is assumed that, depending on noise level requirements these will be in the range of 700 - 1200 ft/min. In some cases slightly higher branch duct velocities are allowed because of the inherent noise attenuation properties of the terminal units with associated flexible duct connections. Generally a maximum of 1500 ft/min is recommended. In the case of smaller air handling units of the fan/coil type incorporating light duty centrifugal fans, volume modulation is most easily achieved by fitting supply duct restrictors and bypass dampers. This system is also used when packaged air handling equipment is selected.

Cooling plants of the direct expansion type - generally cheaper to install and operate than chilled water generating plants - can be used for virtually every application. Limitations of this type of plant are imposed only by capacity. In the case of smaller plants where cooling is controlled in only one or two steps, the temperature sensor for cooling control should be placed in the "warmest" room. This avoids short-cycling of the compressors, which would result if the off-coil temperature were to be measured and controlled. Generally individual reciprocating compressor cooling stations are limited to a capacity of 45 tons refrigeration, above which the economics of centrifugal or screw type chilled water generators begin to influence the choice.

High Velocity System

Building space limitations may dictate the use of high velocity systems, obviously at an operating cost premium due to the resultant higher fan duties. At the point of change from high velocity, pressure reducing dampers are fitted. Each damper unit requires a sound attenuator to reduce high velocity duct and damper noise generated through pressure reduction.

Rickard worldwide & history / Variable Geometry VAV Systems / Products & Specs / Home / Maintenance & Installation / News+Updates / Contact us / Links / E-Commerce / Intro /