There have been many changes in building system design in recent years. Contractors who want to compete in the tight construction market have to be aware of new trends and need a good understanding of how the systems they bid on and install operate long after they have packed up their tools and moved on to the next job.
The system often chosen by many architects and engineers in new offices and buildings is a variable air volume system (VAV) with re-heat capabilities. As the name suggests, a VAV system varies the amount of air delivered to maintain space conditions. A VAV system has the lowest energy costs to operate and, when combined with a building automation system, offers excellent temperature control to the spaces being conditioned. These benefits far outweigh the drawbacks of increased installation costs and system complexity, many engineers say.
System size and capacityIn the past, many buildings were served by constant volume (CV) air-handling units (AHU). Design-day (the hottest/coldest day of the year when maximum heating/cooling is required) air-flow requirements were calculated for each of the rooms in the building and the total air-flow required was used to determine the size of the air handler required. Once the system was balanced to the design air flow, the system was left to run and deliver conditioned air through the building at a constant rate. Energy costs were maintained by shutting the system down at night. It was simple, effective and inexpensive to install.
However, the traditional CV system ignores an important issue: The volume of air required to ventilate and condition a building is not constant. These requirements will change depending on the time of day, the time of the year, internal cooling loads and room occupancies. For example, the amount of air required to cool a space on the east side of a building will peak in the morning or early afternoon and will gradually decrease as the day continues. At the same time, the air-flow requirements for spaces on the west side of the building will be lowest in the morning and gradually increase during the day.
The CV system is sized based on the peak air flow of each space while a variable air volume system is sized for the peak air flow at a given point in time. The size of the equipment will always be less for a VAV system then it will be for a CV system. By reducing the size and capacity of the equipment, the power required to operate the equipment is reduced due to the decrease in horsepower. By reducing the quantity of air flow, the heating, whether the system uses steam or hot water, and cooling requirements are reduced as well. This means a smaller boiler and chiller plant can be installed to meet the building requirements.
Modular air-handling unitsA variable air volume air-handling unit is constructed of modules that bolt together to form the complete piece of equipment. Each of these modules contains the various components, including fans, heating coils, cooling coils and filters, that are necessary to heat, cool and dehumidify the air delivered to the spaces served by the air-handling unit. See Figure 4. (Typical VAV AHU).
Similar to a constant-volume system, a VAV system is designed to deliver discharge air at 55?F from the air-handling unit at all times. This air is delivered to the spaces through the duct system and is varied through a terminal re-heat box or VAV box, located near the space being conditioned. The VAV box maintains the temperature set points in the space, usually 75?F and 50% relative humidity in the summertime and 70?F in the wintertime without any humidity control. Some specific installation may require winter humidity control. See Figure 1 (AHU schematic).
Supply and return fansEach air-handling unit will have a supply fan to deliver conditioned air to the space, and a return fan to return air back to the AHU for reconditioning. These fans are usually of different sizes, capacities, static pressures and horsepower. The quantity of return air brought back to the air-handling unit will almost always be less then the quantity of air supplied to the space, due to the exhaust requirements and pressure relationships required by the space design criteria. As well, the static pressure requirements on the supply fan are greater then the return fan due to additional components located in the supply-fan side of the system. Unlike the fans in a CV system, the speed and air flow of these fans will be constantly changing due to the changes in air flow requirements necessary to meet the space conditioning requirements at any given time.
Adjustable frequency motor control
Each fan will be equipped with an adjustable-frequency motor control (AFMC). This is the electrical device that controls the speed of each fan in response to changes in space, or zone requirements throughout the building. Each fan will require an individual AFMC due to the difference in supply and return fan performance characteristics. Measuring the rpm of each fan will not accurately determine the actual air flow delivered by each fan due to difference in fan performance. See Figure 4 (Typical AFMC).
Air-flow monitoring stationsAir-flow monitoring stations (AFM) are used to measure the quantity of air that each of the fans is delivering at any time. By measuring the actual flow rate, the station allows the adjustable- frequency motor control to adjust the speed of each fan up and down to maintain the required air-flow rates of the supply and return fans. The supply-air flow adjusts to meet the space conditions while the return-air fan changes to meet the supply air-flow rate, minus the constant exhaust flow rate. The difference between the two fans will be constant, but the total air-flow rates will vary.
The heating coil is used in the winter to bring the return/ventilation air mixture up to the 55-degree discharge air temperature. Capacity of the coil depends on the amount of outside air brought into the space and will be sized based on the maximum quantity of ventilation air that will be brought into the AHU on a winter day. Steam or hot water can be used to heat the air. Steam should be used when large amounts of cold ventilation air are brought into the system to prevent coil freezing.
Cooling coilThe cooling coil is used to lower the temperature of the mixture of return and ventilation air that is returned to the air-handling unit. Chilled water is pumped through the coils and lowers the temperature of the air stream down to below the dew point (55?F). In cooling the air below the dew point, moisture will condense out of the air stream. It is then collected in a drain pan and dumped. In humid climates, the air has to be cooled to below the dew point, in order to dehumidify and then re-heated as necessary to maintain space conditions.
Relief-air openingwill vary, usually anywhere from the minimum ventilation rate to 100% of the return air. The quantity of relief air leaving the building matches the amount of ventilation air, minus exhaust requirements, brought into the building.
Ventilation air openingVentilation air is a portion of the air supplied that is brought into the space to satisfy code requirements. This amount will also vary, depending on the type of facility being served, from 0.5 cfm per sq. ft. to 100% fresh air, and will range from the minimum ventilation rate plus exhaust flow rates, to 100% of the supply-air quantity.
Economizer modeCommercial buildings will usually have a cooling load all year round and mechanical cooling is provided until the outside air temperature falls to 55?F. At this point, the air-handling unit will operate on what is commonly called economizer mode. When the outdoor air temperature falls to 55?F, the AHU will draw in 100% outside air to maintain a 55-degree discharge air temperature.
As the outside air temperature falls, the relief-air, ventilation-air and return-air dampers will adjust to blend return air with outside air to maintain the 55-degree discharge air temperature. This is known as "free" cooling, as the ambient air is used to cool the building. When the return air dampers are fully closed, the AHU is bringing in 100% outside air and is running in full economizer mode. See Figure 1 (AHU schematic).
FiltersFilters are installed in the air-handling unit to clean the conditioned air before returning the air to the space being cooled. How much dirt is removed depends on the system's design criteria. Filters typically range from 30% efficiency to 90%-95% efficiency. In hospitals or clean rooms, High Efficiency Particulate Air or HEPA filters are often used.
Low-end filtration systems usually have a single low-efficiency filter installed, often as low as 30% or 40% efficient. When higher filtration rates are required, the air-handling unit will have two sets of filters. A low cost, 30%-efficient filter will be used as a pre-filter, with the higher efficiency, higher-cost filters used as a final step. In this arrangement, the low cost, disposable 30%-efficient filters will be changed more frequently then the higher cost, final filters, lowering maintenance costs.
Magna-helix gaugeA magna-helix gauge is mounted across the filter bank and is used to measure the pressure drop across the filters. When filters are clean, the pressure drop across the bank is low. As the filter gets dirty, the pressure drop across the filter increases. Filter manufacturers have a maximum recommended pressure drop for each filter. Once the pressure drop increases to this maximum value, it is time to change the filters.
Filter status can be monitored on a regular basis by maintenance staff or, as is becoming more common, the gauge can be connected to the building automation system by a differential pressure switch. Once the pressure drop reaches the maximum point, the automation system sends an alarm to the building operator who notifies maintenance staff for scheduled filter replacement.
Access doorsEach of the components requires periodic maintenance. Access modules are installed in the air-handling unit to allow the maintenance staff a means of working inside the air handler. Fan modules have an access door installed in the module itself while other components may require access both upstream and downstream, in which case access modules will be installed where maintenance will be required. The modules usually come in small, medium and large sizes. The size used depends on the type of maintenance required, or on the preference of the maintenance staff.
Terminal re-heat boxThe conditioned air is distributed to the spaces by a terminal re-heat box, or variable air volume box, where both the volume and temperature of the air delivered to the space is varied. The box is constructed of metal and is lined with a sound absorbing material. There is an air-flow monitoring station on the inlet side of the box, an adjusting damper inside the box and a reheat coil on the discharge of the box.
The damper can be controlled either pneumatically or electrically with direct digital controls. The most popular type of box is a pressure independent box. It will deliver a set quantity of air, or flow rate, independent of the system pressure upstream of the box. When day cooling is required, the box will open up to its maximum flow rate and deliver 55-degree air to the space.
As cooling requirements fall, the box will reduce the amount of air delivered to the space and will continue to fall until the box reaches a minimum set point. This minimum set point is determined by the design engineer and will be between 30% and 40% of the box's maximum set point. In comparison, a constant volume system also requires re-heat, but the amount of hot water necessary for re-heat is higher, due to the higher amount of air delivered to the space. If the amount of 55-degree air is still overcooling the space, the control valve on the re-heat coil will open, allowing hot water to flow through the coil and raise the discharge temperature of the air. The positions of these components are continuously changing in response to the requirements of the space.
For a system that looks fairly simple on the surface, the variable air volume air-handling unit is really a complex installation, with continuously changing flow rates and temperatures helping to maintain a comfortable environment for office workers.