We live in an era of global warming. Sustainability, carbon foot print, energy conservation and efficiency are topics commonly discussed and of greatest concern. A majority of construction and manufacturing sectors in the world have already started to feel the pressure from government and other regulatory bodies to use energy efficient products. Building HVAC is one such sector which has caught the attention of keen ‘environmentalist’ architects and engineers as an ideal target to promote ‘Green’ products.
In the middle-east, air-conditioning load in buildings constitute more than 60% of the total energy consumption. This may be attributed to the harsh climatic conditions, especially during peak summer where demand for power is more than twice as compared to winter. Many HVAC products are being introduced to improve system performance and energy efficiency. Variable air volume (VAV) terminal is one such product being promoted in the field of building HVAC.
What is a VAV terminal?
A VAV terminal modulates the supply airflow at constant temperature into an occupied space based on cooling demand. As the load increases the supply of cool air increases and as the load decreases the air supply also decreases. It consists of a thermostat, actuator, controller, damper and airflow sensor (for pressure independent type only).
Application Example: Consider a ducting system connecting FOUR Reception halls served by only 1 AHU (see Fig-1). In scenario 1, A/C duct branch serves reception hall 1 & 2 without VAV. i.e., volume of supply air cannot be varied. In scenario 2, the A/C duct branch serves Halls 3 & 4 with VAV. i.e., volume of air supply can be varied.
Cooling load in all these 4 Halls need not be necessarily the same. It depends on several factors like occupancy level, space size, internal and external heat loads and so on.
Observation is as below:
Scenario 1 ( Branch without VAV)
- Rcptn Hall 1 : Crowded – insufficient air supply for cooling-too warm – discomfort experienced.
- Rcptn Hall 2 : Less crowded – air supply more than required too cold – discomfort experienced.
Scenario 2 ( Branch with VAV)
- Rcptn Hall 3 Crowded – air supply increased to attain set point temperature – optimal temperature maintained-occupants comfortable.
- Rcptn Hall 4 : Less crowded – limited air supply to maintain set point temperature – optimal room condition-occupants comfortable.
VAV Types:
Airmaster manufactures two basic types of VAV air terminals:
- a) Bypass VAV
- b) Pressure Independent single duct air terminal.
a) Bypass VAV: Only sufficient amount of air will be supplied to the occupied space just enough to meet the demand (i.e., set point temperature). Excess supply air will be by passed into the ceiling plenum or return air duct. This type of VAV device neither measures nor maintains air flow between desired cfm limits.
It is generally chosen where a Constant Volume Air handler’ is employed by the designer. The volume of air entering into the space is a function of static pressure of the upstream air duct.. In other words the air flowing into the room fluctuates depending on the duct static pressure which may lead to occupants discomfort if it is not balanced properly.
b) Pressure Independent VAV: This type of VAV is designed to control the volume of air entering into occupied without being affected by pressure variation in the duct.
Working : It is equipped with a airflow probe that measures and limits the air flow between cfm ranges that are factory calibrated. The input to the actuator controller is derived from the thermostat. Actuator throttles the damper just enough to meet the cooling load. For example, if the room temperature has increased by1°C above set-point temperature, the thermostat calls for more cooling. The actuator controller receives signal from thermostat to open the damper to let in more air till the room temperature matches set-point.
The volume of air entering into the space does not depend on pressure inside the duct. Even if the duct pressure fluctuates, the flow sensor communicates this change to the actuator controller in order to maintain a steady air flow to achieve comfortable conditions in the occupied space.
Note: A variation in this model would be to add heat to the air whenever required. This process of “Reheat” can be achieved by complementing the pressure independent VAV box with Reheat coil (Duct Heater).
How is energy conserved?
When the cooling load in an area decreases, the actuator controller receives a signal from thermostat to reduce air volume by closing down the damper to predefined minimum airflow. As a result the duct static Pressure (SP) elevates. Pressure sensors inside the duct communicates with the fan controller (also known as Variable Frequency Drive, VFD) about the increase in duct SP. The fan speed is thus reduced to sustain the designed duct SP thereby saving fan energy.
Reduction in volume of air handled also reduces the load on compressor. Therefore through the process of controlling fan speed, a remarkable amount of energy is conserved.
In the case of a bypass VAV, the bypassed air mixes with return air thereby reducing the resultant air temperature before entering the Air Handler. Energy savings can be achieved in terms of reduction in energy required by compressor to cool the return air.
Advantages of VAV air terminal:
- Energy efficient air conditioning system.
- Occupants are more comfortable as optimum temperature is always maintained.
- Multiple zones can be controlled with the same air handler.
- Low AHU maintenance as load on fan and compressor is reduced.
- Lower energy bills and low carbon foot print
