Innovative Energy Engineering

Airside Design


Fans provide the total pressure required to move the air. Manufacturers provide performance curves and selection software to accounts for actual altitude and temperatures, which impact air density and fan performance. Under all operating conditions (flow, pressure, rpm) the fan should be outside the surge region and at highest possible static efficiency and lowest possible break horse power (bhp). Manufacturer-recommended inlet and outlet conditions need to be obeyed or fan performance suffers due to system effect.

Modern fans are driven directly by speed-adjustable motor and use computer-designed airfoil blades for higher efficiency and less noise. Older less efficient designs employ backward curved, backward inclined and forward-curved fans and single speed belt-driven motors. Centrifugal fans typically are more efficient when high static pressures are required (e.g. air handlers with coils, filters and other equipment). Vaneaxial fans with straightening vanes typically are more efficient when lower static pressures are required (e.g. return or exhaust fans).

Housed fans should discharge directly into a straight duct. The first turn should be on the side of the fan inlet or projecting in the direction of fan rotation. When discharging into a plenum system effect can cost as much as 250 Pa (1 in-wc). Plenum outlets should be as far away from the fan as practical to minimize pressure drop. Outlets should have bellmouths for round ducts and rectangular ducts should not be larger than the opening.

Duct Design Guidelines

Air, unlike water, is a compressible fluid, which impacts its interactions when velocities change. Total pressure is the sum of static and dynamic (velocity) pressure. In diffusers (velocity decreases ) dynamic pressure decreases with the square of velocity. Static pressure increases by the same amount (static regain). In a nozzle (velocity increases), dynamic pressure increases with the square of velocity and static pressure is reduced accordingly. For positive pressure ducts downstream of a fan (e.g. supply ducts) the static pressure lost due to friction can be re-gained by reducing velocity. In real world systems with conversion losses and friction total pressure decreases. The lost pressure is converted to small amounts of heat and noise. Efficient designs minimize those conversions.

Pressure can be measured by measuring the rise of water (i.e. inH2O) or mercury (i.e. mmHg) in a tube or by using sensors. Static pressure is measured by inserting a tube into the duct (opening 90° to flow direction) and using the atmosphere as a reference (tube open to space). Total pressure is measured using the same setup, except the inlet tube is 0° to the airflow (air pushing into the tube opening). Velocity pressure is the difference and indicates velocity.


Equipment and spaces need to be protected from dirt. 4" pleated filters in angled arrangement minimize pressure drop and extend replacement intervals. MERV 8 filters protect equipment. MERV 11 is recommended for finished occupied spaces. MERV 11 filters are much cheaper than MERV 13 filters while providing almost same filtration. Pre-filters increase pressuredrop and equipment size unnecessarily. Bag filters collapse in VAV systems and require excessive space.

Air Economizers and Building Pressure

Airside economizers use colder or less humid OA to pre-cool the air to eliminate or minimize the cooling coil load. The economizer can be enabled based on dry-bulb temperature when OAT is 8°C-12°C (15°F-20°F) colder than Return air (RA). This occasionally can lead to slight energy waste when OA is more humid as the dry-bulb economizer does not take humidity into account. In theory enthalpy economizers solve this problem as they compare enthalpies of OA and RA and therefore always would save energy. But in reality enthalpy sensors are not accurate enough, seldom get calibrated or replaced as required and end up using more energy. Therefore drybulb economizers are recommended.

The amount of OA entering the building needs to be relieved to control building pressure to acceptable levels. In theory building pressure should be slightly positive in summer and slightly positive in summer to prevent building damage due to moisture. In practice pressure varies in each zone and is difficult to actually measure. 4 outdoor sensors are required to average wind direction, and multiple indoor sensor since spaces themselves are pressurized or under-pressurized. Most installations use only one outdoor sensor and one indoor sensor and make measurement more a guess. It is best to balance all ventilation and exhaust flows around the ERV and to control the relief fan and damper only during economizer operation attempting to achieve neutral pressure. Relief in VAV systems should be provided by relief dampers or relief fans in ducted return systems. Use of return fans is is appropriate in constant volume systems, but in VAV systems those are less efficient.

Spaces with different contamination levels require pressure control to prevent contamination. Measuring flow into and out of the space can control critical spaces, such as labs. In less critical spaces where an exact pressure difference is not required (i.e. keep garage negative to office), some added exhaust flow is sufficient to accomplish the goal reliably.

Variable Air Volume - VAV

Variable Air Volume systems are an advancement of less efficient Constant Volume (CV) systems. CV systems cool a constant airflow and re-heat the flow in each zone to "modulate" cooling. Fan energy use is high and re-heat inherently wastes cooling and heating energy. VAV systems modulate the airflow in each zone by means of actuated dampers and air-flow meters. The fan speed is adjusted to meet a pressure point. This pressure setpoint is re-set based on actual demand for flow (e.g. damper positions). discharge air temperature can also be re-set to reduce reheat need. Fan, refrigeration and re-heat energy are drastically reduced compared to CV systems. Under part load noise will be reduced and with load diversity the fan, AHU and refrigeration plant can be downsized. If designed and controlled properly, VAV systems can be more efficient than most of other system types.

Heating performance of VAV systems is limited since most diffusers are optimized for cooling and supply from above. Buoyancy of warm air prevents proper heating and ventilation with warm air. Increasing DAT can reduce heating performance. If DAT is 10°C (15°F) above room temperature, the outside air Rate needs to be increased according to ASHRAE 62.1. In general, heating only should be performed by VAV systems in moderate climates. Perimeter or in-floor radiant heat is required if the wall load is above 250 Watt per linear meter (250 Btu/h per linear foot) or when ceilings are high. In heating season the air supplied by the AHU is a mixture of warm Return Air (RA), and preheated (by ERV) Outside Air (OA). Unlike older systems without ERV, this keeps temperatures high enough to not require heating. As long as the space heater (e.g. room radiator) is sized to account for the ventilation load, there is no need to add heating coils in AHU or terminal devices. This reduces pressuredrop and eliminates freezing. In systems with water Cooling-Coil (CC) , a freeze-stat needs to be located downstream of the CC or the coil drained during heating season.

Traditionally cooling supply is at around 13°C (55°F). This may not be optimal for all situations and based on latent load and sensible loads design temperatures could be 10° - 14°C (50°F - 58°F) or even outside this band. The space comfort temperatures can also be adjusted to optimize airflows. Higher space temperature at lower humidity feels as comfortable as lower space temperature at higher humidity. In addition supply temperature should be re-set based on actual cooling demand. Based on number of zones calling for cooling the supply temperature will be increased or decreased within a band. Upper and lower limits of this band depend on ambient temperature. In order to be able to cool interior zones in cold weather, those space air flow rates could be designed for a higher supply temperature. It is recommended to separate AHU for zones that most likely require cooling or heating at the same time. If a DOAS feeds dehumidified air into the AHU, DAT reset can be more aggressive.

In zones with highly varying occupancy (i.e. conference rooms) minimum flow-rate should be reset based on space CO2 level. Spaces with operable windows should disable the zone when a window is open (provide heating only when temperature is near freezing). Motion sensors should put the zone in a "standby" mode when no motion is detected. This should reduce minimum flow rate to "0" (as read on flow-station, which still allows 5-10% flow) and widen the deadband by 1-2°F. Minimum flowrates for both cooling and heating season should be properly calculated. Many designers just assume a 30% minimum flow, which isn't necessarily correct.

Sometimes fan-powered VAV boxes are used to improve ventilation and heating performance. Modern motors and controls reduced, but not eliminated some of their inherent problems. Fan-powered boxes add significant cost due to the fan, motor, added controls and electrical supply. They add maintenance (filter, repair) in occupied spaces. Noise levels and energy consumption increases due to the constant volume nature and the less efficient smaller fan and motor.

The cheapest VAV terminal devices are Pitot-tube style VAV boxes. They require field calibration (with all inaccuracies based on unskilled trades), have low accuracy at low flow conditions and bad inlet conditions and are susceptibility to dirt and tube leaks. Once these VAV boxes fail or become inaccurate (which is only a question of time), the VAV system loses its ability to measure airflow, malfunctions. This can be unnoticed for a while because the system could work as a very inefficient constant volume system. A better solution are air valves with Vortek flowstations. At a slightly higher price the air valve does not require field-calibration, can be used in dirty air flows, measures very low flows and is less sensitive to inlet conditions.

Plenum return typically uses less space, less fan energy, and is cheaper to install as it requires less ductwork. For relief in economizer operation often a relief damper suffices instead of a relief fan. Relief dampers can be located in many locations in the building, which increases flexibility. A case for ducted return can be made if the return plenum is likely to experience a lot of infiltration or if the it consists of combustible material (i.e. wood trusses) and contains non-plenum-rated equipment and wiring.

Chilled Beams - CB

Chilled Beams induce a secondary flow of space air through a chilled water cooling coil. This reduces the need for air supplied by the AHU and can reduce fan energy and duct sizes somewhat. Limitations include the lack of air-flow regulation and the lack of dehumidification. CB in most cases are constant-volume systems. When misapplied, space humidity can form condensation at the coils and it "rains" in the room. Controls preventing the condensation, could stop the systems in humid areas. In general CB systems are not appropriate for office or other applications with high latent loads and the opportunity to vary ventilation or where infiltration can be high. When misapplied, energy consumption will be higher than VAV systems at a higher up-front cost. Trane and Taylor Engineering studied CB applications in offices and found an energy disadvantage. Many of the zone and AHU control strategies of VAV systems can't be implemented, decreasing efficiency even further.

In order to take advantage of the option to use higher chilled water temperature, a second chiller is required. This is because one chiller still needs to chill the water to low temperatures for DOAS dehumidification. This is practical in very large systems only.

We have investigated CB systems in varying applications in Wisconsin and Minnesota. Mechanics, operators, designers and commissioning agents were interviewed The systems were of varying levels of sophistication and installed at very high levels of detail and commissioning. The common theme is that they can operate reliably if the AHU is operated 24/7, which eliminates any energy savings compared to VAV. When operated intermittently, humidity or "rain" problems can occur easily. In addition trouble-shooting and control optimization require much more qualified personnel than regular systems. Controls require humidity sensors that need to be calibrated frequently. Unlike VAV systems, CB systems are not forgiving. A failure in the DOAS, or less capacity of a chiller, or a sensor failure can shut down the system or cause humidity and rain problems. CB systems can be beneficial in applications with high ventilation requirements and high sensible cooling loads (e.g. hospitals) or in arid climates.

Under Floor Air Distribution - UFAD

UFAD deliver air from under the floor and typically use that under-floor space as a plenum. UFAD require a very tight under-floor plenum, which often is not practical. The floor slab can raise air temperature, reducing performance. Dirt will fall into the underfloor diffusers and land in the plenum. There are some advantages of UAD in spaces with high ceilings.

Additional Information:

only search this site

Information about: