Syed Taqi Hussain, while giving tips to improve the quality of controlled air, says that Variable Air Volume box is a smarter way to control indoor space temperature.

The major function of an air distribution system is to provide comfort to building occupants. This article seeks to help specifying and design engineers create better zone comfort.

People are thermally comfortable when their body heat loss equals their heat production, without them sensing any changes in the temperature.

ASHRAE defines comfort as:

• Maintaining a temperature of 22.7°C (73°F) – 25°C (77°F)
• Relative humidity of 25% to 60%
• Maximum velocity in the occupied zone is 50 FPM

HOW DOES AN ENGINEER DETERMINE CFM?

The cooling or heating load (BTU/Hr) in any space determines the quantity of air (CFM) that the space requires to satisfy the heat gain or loss. ∆T (Delta T) is the difference between the supply of air temperature and the room temperature, and is called the temperature differential. As the load changes and goes either up or down, CFM demand too changes. Thus, the need to change the CFM to match the load changes in a given space.

The devices most commonly used to control the load or air volume to a zone are called Air Terminals – Variable Air Volume (VAV).

VAV boxes and their controllers are the most essential components of an HVAC system to achieve indoor comfort. Their proper selection and operation is, therefore, essential for optimum temperature control.

At present, bypass VAV, pressure independent VAV and CAV are the most commonly used types in the market. Apart from the standard types, other types include, parallel fan-powered induction, dual-duct air terminal units.

Bypass terminal units:

These units are designed to achieve variable air volume delivery of conditioned air to a room through a single duct. These are constant volume air distribution systems. Variable air volume control is achieved by directing the airflow either to a room or to a bypass port in direct response to signals from the room thermostat, and is the modulating type.

Pressure independent VAV:

Here, the temperature in the room is controlled by varying the amount of supply of air introduced into the space.

Pressure independent CAV:

Here, the temperature in the room is controlled by varying the temperature of the supply air introduced into the space, keeping the amount of air supplied as constant.

In a pressure independent system, the most important factor for the best performance of VAV is the selection of an accurate flow sensor. In multi-point averaging flow sensor with flow sampling of both velocity pressure and flow, the differential pressure provides precise air flow calculations.

As conditioned air enters the terminal unit, the sensor calculates velocity pressure in W.C (Pv) and converts its value to CFM. A signal is sent to the actuator, which then adjusts the damper shaft to open or close to reach the desired CFM (Min/Max/Constant).

While selecting air terminal units, the information provided on a schedule is critical. Without complete information, you will be forced to make assumptions which will probably lead to product performance problems at the job site.

An improper pressure drop assumption could lead you to believe that you are not able to meet the consultant specification. Apparent NC levels, therefore, may seem to be unattainable.

The most important information which an engineer needs while selecting a VAV unit include:

• Maximum CFM
• Minimum CFM
• Fan CFM (Fan-powered terminals)
• Primary supply pressure
• Sound performance criteria
• Heating requirements

UNDERSTANDING PRESSURES

Primary supply pressure is the system pressure provided by the central air handling equipment to the terminal unit inlet damper.

Minimum pressure is the minimum primary supply pressure required to deliver a given CFM at a given pressure.

Downstream static pressure is the pressure that is measured or specified downstream of the terminal unit, as it is demonstrated in figure 2:

Noise criteria (NC) and values:

The NC value of a terminal unit at a specific flow is absolutely dependent upon the primary supply pressure. If you do not know the primary supply pressure, you cannot estimate the NC. Given below is a list that could be helpful:

• General offices: NC 30 to NC 35
• Open-plan-office areas: NC 40
• Conference rooms: NC 30
• Auditoriums: NC 30
• Lobbies and corridors: NC 40
• Executive offices: NC 25 to NC 30
• Computer/equipment rooms: NC 40 to NC 45
• Classrooms: NC 25 to NC 30

HEATING CALCULATIONS

In order to obtain the desired temperatures in any occupied space and to provide the needed heating capacity, duct heaters are used in line with the VAV boxes. The heater capacity is selected as per the details provided below:

• kW = Power in kW
• CFM: Air volume in cubic feet per minute
• T°2: Temperature of air leaving heater
• T°1: Temperature of air entering heater

Duct heaters can be selected for the best performance as per the criteria given above:

CONTROLS AND CALIBRATION

The majority of controls installed in HVAC systems today are Direct Digital Controls (DDC). The controls can be DDC or application-specific controllers (VAV controllers). It is recommended to have controls supplied by the VAV manufacturer with complete calibration and testing from the factory. This procedure benefits the MEP contractors during testing and commissioning. The boxes are calibrated for min/max/constant flow as per the required schedule. And if used for a BMS project, the MAC address/device number is factory-set with a proper tag on each VAV unit.

Armed with the above information, an engineer or the contracting company in-charge of providing quality indoor space conditions, can serve the clients and end-users better. After all, in the final analysis, the aim of MEP contractors and consultants is to strive to provide perfect indoor space conditions through their products, services and expertise.

The writer is Product Manager, VAV & Controls Energy Industrial Company, Representative – Metalaire, USA. He can be contacted at: taqi@energyind.ae