One of the costliest items in a laboratory construction project is the air conditioning system. Labs need more air than almost any other type of facility and supplying that air can be expensive. The decisions you make, or let others make for you, in the design phase of the project are going to have a major impact on your construction budget and on the eventual operating costs of your facility.
A quick review of the two major types of air conditioning systems used in the laboratory will help save you money and ensure that your lab meets both your operational and fiscal requirements.
Two Major Types of Systems
Air conditioning systems for laboratories can be classified into two categories: Constant Volume (CV) and Variable Air Volume (VAV).
Constant Volume System
A constant volume air system is one that exhausts a constant amount (volume) of air through the lab fume hoods while at the same time supplying a constant volume of air into the laboratory.
In a constant volume system the amount of air exhausted and supplied is constant regardless of fume hood usage or sash position. The same amount of air is supplied to the room because the same amount is always being exhausted.
Advantages of a Constant Volume System
- Since a basic assumption of the design is that all hoods will be in operation at all times and that they will be exhausting at a constant rate, it is likely that the equipment specified for a CV system is of sufficient size to provide enough air for a safe, comfortable lab.
- If the supply equipment of the AC system has been designed to accommodate expansion (increase in the number of hoods, room size, etc.) the addition of the new hoods is easily accomplished without an uncomfortable lack of supply air or the need for additional supply equipment.
- The constant volume system is simple. It is simple to design, install and maintain since the equipment is conventional industrial air conditioning equipment. The start-up and testing, installation, and maintenance can be performed by conventional experienced HVAC (Heating, Ventilation, Air Conditioning) contractors. Constant Volume air systems are tried and true; they are the antithesis of cutting edge technology.
- The initial cost of a CV system is typically lower than a VAV system. Since VAV requires more controls and more sophisticated equipment they tend to be more expensive.
Disadvantages of a Constant Volume System
- The disadvantage is energy costs. CV systems exhaust a lot of conditioned air. Conditioned air equals energy costs. Depending on the size of your lab, the number of hoods, and the way in which your lab’s air conditioning costs are accounted for, this cost can be considerable.
- There have been some attempst to modify the CV system in order to reduce energy costs. The most notable attempt being the auxiliary air system. This system brings in unconditioned air form outside the building and supplies it directly in front of the fume hood sash opening. The hood then exhausts this unconditioned air and only a portion of the lab’s conditioned air. Unfortunately, due to the temperature of the undconditioned air (hot in the summer, cold in the winter) auxiliary air systems can make for an uncomfortable lab.
- Since auxiliary air hoods and additional supply fans also add to the cost and maintenance of the lab, these systems are decreasingly popular.
Variable Air Volume Systems
Variable Air Volume (VAV) systems vary the amount of air being exhausted and supplied to a lab based upon the usage of the fume hoods.
If the hood sash is open and more air is required to pass through the hood the VAV system increases the exhaust flow from that hood and at the same time increases the amount of air supplied into the lab.
If the hood sash is closed the VAV system decreases the exhaust flow and reduces the amount of air supplied to the lab.
According to the manufactures of VAV equipment, this increase and decrease of the air exhausted is accomplished while maintaining a constant 100 fpm across the face of the hood.
VAV systems differ in how they measure the amount of air to be exhausted. Some systems utilize a sensing device built into the side wall of the hood to measure the air volume drawn into the hood. As the volume increases the sensor activates and increases the exhaust. A decrease in the air volume results in a reduction of the exhaust.
Other systems use a sash position sensor. This sensor tracks the location of the hood sash and increases or reduces the exhaust depending on the sash location.
Besides a sensing and control mechanism, most VAV systems require a special VAV style hood. This type of hood only allows air to flow into the hood from the sash opening. This is different from a more common hood style known as a bypass hood. A bypass hood allows air into the hood through a grille when the sash is closed. This additional air allowed into the hood is undesirable when using a VAV system.
Other items of equipment that may be used in conjunction with a VAV system include an automatic sash positioning unit and a sash alarm. The sash positioning unit sensor opens a fume hood sash whenever a person is detected to be in front of the hood and closes the sash when the person walks away. The sash alarm emits an audible or visual alarm or both whenever a sash has been open for a selected length of time. This alarm reminds users to close the sash.
A key element to the success of a VAV system is found not in the equipment but in the design criteria used to develop the system. The crucial idea in this development being “diversity”. Diversity, as it applies to laboratory ventilation, is the assumption that only a certain percentage (typically less than 50%) of all hoods in a lab will be operating at their maximum capacity at any one time. In other words, not all hoods will have their sashes fully open at all times thereby exhausting the maximum amount of air possible at all times.
If not all the hoods are exhausting their maximum amounts of air at one time then the air conditioning equipment supplying the lab need not be sized to accommodate this demand. The equipment can be sized to handle a smaller volume of air supplied and exhausted. This downsized equipment is typically less expensive and thereby results in a cost savings.
When utilizing diversity in design it is important to be cautious. The reason for this is that since the system is sized to account for a percentage of the hoods to be in full operating capacity at one time, if this percentage is exceed the supply to the lab may be insufficient.
Advantage of a Variable Air Volume System
- The primary advantage of a VAV system is the cost savings resulting from reduced energy consumption. Since the system exhausts less conditioned air it requires less conditioned air with resulting lower energy costs.
- Another advantage is the reduction in size of the HVAC equipment. Since less air is required the equipment can be down sized. This is a distinct advantage if space is at a premium. A subsequent reduction in equipment cost is also achieved.
Disadvantages of a Variable Air Volume System
- The initial equipment cost for a VAV system is typically higher than a CV system. This is due to the cost of design, equipment and installation required of a VAV system. Many systems can achieve a payback due to reduced energy costs. However, if the initial project budget is tight this payback may be insufficient incentive to make the initial capital outlay.
- VAV systems are also more complex with dampers and additional controls. In duct dampers can deteriorate with negative effects on the overall system. Additional controls also add increased maintenance concerns.
- VAV systems are also very dependent on accurate diversity calculations. If the diversity percentages built into the system are “wishful thinking” and not accurate reflections of how the lab hoods are utilized the entire lab air system may be inadequate.