Lab.Control Type

①Constant Volume 

The first laboratories with mechanical ventilation, and many laboratories today, were designed as CV systems. As the name implies, the air volume supplied and exhausted from a CV laboratory does not change.

CV laboratories are the best option for ventilation driven laboratories, where the air changes per hour rate determines the required supply air volume. In ventilation-driven laboratories, the air changes per hour ventilation rate provides enough air to ensure the expected cooling loads and the exhaust make-up air needs are met. Therefore, there is no need to supply more air to the space. 

◆The CV control sequence has a number of advantages. These include

• Easy to design.

• Minimizes cost of controls.

• Few controls to maintain. 

■The CV sequence of operations has a number of potential disadvantages, including:

• Mechanical equipment must be sized for full-flows, increasing first and operating costs of the fans, chillers and other capital equipment.

• Difficulties relocating equipment. Moving equipment within a building may change the HVAC system pressure distribution, resulting in a requirement to rebalance systems without pressure-independent controls.

• Limited future expansion, because there may not be sufficient system capacity to allow additional equipment.

• Limited opportunities to warn users of unsafe system operations. Monitors and controllers are not inherent to CV systems. Adding monitors to comply with recent standards adds cost. Older systems without monitors cannot warn users of unsafe conditions, potentially risking exposure. 

②2-Position 

Initial efforts to reduce the operating expenses associated with the high continuous operations of constant volume control focused on reducing airflow in unoccupied laboratories. Under this scenario, full airflows are present only when laboratory users are present. Reduced airflows are possible when the laboratory is unoccupied because users are not present to create airflow obstructions at fume hoods.

As with constant volume laboratories, use the 2-Position control sequence for ventilation-driven laboratories do not use 2-Position control if exhaust or cooling loads require more supply air. 

◆The 2-Position control sequence offers similar advantages and disadvantages as the constant volume control sequence. Benefits include:

• Easy to design.

• Low cost of controls when compared to a variable air volume system.

• Decreased flows during unoccupied hours can reduce operating costs when compared to a constant volume system. 

■Weaknesses of the 2-Position control system:

• Mechanical equipment must be sized for full-flows, increasing first and operating costs of the fans, chillers and other capital equipment.

• Difficulties relocating equipment. Moving equipment within a building may change the HVAC system pressure distribution, resulting in a requirement to rebalance systems without pressure-independent controls.

• Limited future expansion, because there may not be sufficient system capacity to allow additional equipment.

• Limited opportunities to warn users of unsafe system operations. Monitors and controllers are not inherent to 2-Position systems. Adding monitors to comply with recent standards adds cost. Older systems without monitors cannot warn users of unsafe conditions, potentially risking exposure. 

③Variable Air Volume (VAV)

VAV laboratories reduce supply and exhaust volumes to the minimum required to maintain temperature, ventilation and safe fume hood face velocities. When fume hood sashes are lowered, VAV fume hood controls reduce the air exhausted to maintain constant velocity of air through the sash opening while VAV laboratory controls correspondingly reduce supply air to balance the space. Similarly, VAV room controls increase general exhaust volume to balance the laboratory when increasing the supply air volume to cool the space.

VAV laboratories offer a number of advantages when compared to constant volume and 2-position labs. Advantages include:

• Reduced energy costs because less air will be conditioned, supplied and exhausted when loads decrease.

• Use unoccupied mode to decrease supply and exhaust airflows to further save energy expenses.

• Applying diversity decreases maximum design airflows, resulting in smaller capital equipment such as fans, ductwork, and air handlers.

• Pressure-independent VAV controls adapt to system changes when equipment is moved or added, requiring rebalancing only in areas directly affected by the change as opposed to the entire building.

• VAV controls alarm if fume hood face velocity and room pressure differential or balance reach potentially unsafe levels.