Lab Introduction

This room is a 600 square foot room with two 8-foot chemical fume hoods. Each fume hood requires 1750 cfm of make-up air to capture and contain fumes when the fume hood sash is fully open. If this room were to be operated as a constant volume room, the cost to condition and move the make-up air through the room would be over $15,000 per year. By making this room a VAV room, the hoods will capture and contain better, and the cost to condition and move the air through the room would be about $6,000 per year.

Sequence of Operations

Introduction

Two conventional variable air volume fume hoods are each designed to exhaust a volume of air which provides a constant face velocity (typically, 100 fpm) at the sash opening regardless of sash position. As each sash opening increases or decreases, the volume of air exhausted through its associated hood exhaust valve changes proportionately, thereby maintaining a constant average face velocity at the sash opening. [All fume hoods are set up to furnish the desired face velocity at some minimum sash opening which corresponds to the amount of open area required for the minimum hood exhaust volume to achieve the desired face velocity. As the sash area decreases below this minimum opening, the minimum hood exhaust volume remains constant, thereby increasing face velocity.] This conventional variable volume control approach only provides for a reduction in airflow volumes, and an associated increase in energy savings, if each fume hood sash is closed to its minimum opening. Unfortunately, getting the sash to its minimum opening is completely dependent on operator compliance to close the sash before leaving the hood.

Lower operating face velocities (i.e., less than 100 fpm) have not traditionally been accepted in laboratories. A major impediment to this acceptance has been the knowledge that the presence and dynamic movement of the operator in front of the fume hood creates turbulence that can pull vapors out of the hood at face velocities under 100 fpm. However, without an operator present, a fume hood is capable of providing excellent containment at much lower face velocities (i.e., 60 fpm or greater). This has been recognized in current OSHA guidelines which recommend a range of operating face velocities from 60 to 100 fpm.

With the installation of the Phoenix Controls Zone Presence Sensor (ZPS), each variable air volume fume hood can be placed into a standby mode of operation based on actual hood usage. This standby operation reduces the hood exhaust volume, thereby providing a lower, yet safe, face velocity (i.e., 60 fpm), regardless of sash position, whenever the operator is away from the hood.

In this application, an exhaust valve (EXV) is used to control the exhaust volume out of each fume hood. Each hood exhaust valve is configured with a factory-mounted pressure switch to detect low static pressure across the valve. Each fume hood is equipped with a fume hood monitor which generates an alarm to alert the operator to low static pressure and flow alarm conditions.

During initial commissioning, each fume hood monitor is calibrated for both the standard (operator present) and standby (operator absent) modes of operation. In each mode the fume hood monitor is calibrated to maintain the relationship between sash opening and exhaust air volume so the respective face velocity is obtained. [As described above, all fume hoods are set up so the minimum sash opening and the minimum hood exhaust volume provide the standard face velocity. This minimum hood exhaust volume is typically not setback to a lower level, even during standby operation.]

The make-up air volume entering the zone is controlled with two separate master and slave make-up air valves (MAV) which, although installed in separate branch ducts, operate as a single make-up air unit. The minimum make-up air volume is sized to satisfy the minimum ventilation rate (calculated from air changes per hour). Due to a minimum ventilation volume that is greater than the hoods' total minimum exhaust demand, the make-up air minimum is clamped to a volume that is large enough to satisfy the ventilation rate. The make-up air valves track the total fume hood exhaust volume minus the desired room offset volume until this minimum make-up air volume is reached.

During conditions when the zone experiences a high internal heat gain (caused by season, time of day, people, lights, equipment, etc.) additional supply air is required to cool the space when all fume hood sashes are at their minimum opening. The zone's pneumatic thermostat commands the make-up air master/slave valve chain to open in response to a demand for cooling, regardless of the hoods' total exhaust demand, thereby accomplishing temperature override control.

Neither the large minimum ventilation volume nor the cooling override volume can all exhaust through the fume hoods when all the sashes are at their minimum opening. Therefore, an exhaust valve (EXV) is added to the system to remove the zone's general exhaust (GEX) volume. The general exhaust valve operates inversely to the total hood exhaust volume when the total hood exhaust demand is less than either the make-up air minimum ventilation volume or the temperature override volume.

This control approach works to maintain the minimum ventilation volume, to accomplish temperature control for cooling and to maintain zone pressurization control. In addition, the AFV control approach reduces the hood exhaust and conditioned make-up air volumes to provide an increase in energy savings during standby operation in non-temperature override conditions.

Independently, the zone thermostat controls the reheat coil to provide zone temperature control.

Hood Operation

Sash Movement
As each fume hood's sash opening increases or decreases, the sash sensor signal to the related fume hood monitor shall change proportionally. (A sash sensor and monitor are mounted on each fume hood.)

ZPS: Operator Present
With an operator at the fume hood, each Zone Presence Sensor (also mounted on the fume hood) shall detect the operator and shall send a 0 Vdc (operator present) user status signal to its associated fume hood monitor. This 0 Vdc user status signal shall switch the monitor into its standard operation mode. Based on the combination of user status and sash position inputs, the fume hood monitor shall apply a multiplier of "1" to the sash signal to generate a 0-10 Vdc standard operation command signal. This 0-10 Vdc linear, calibrated command signal shall control its associated hood exhaust valve, thus maintaining a constant average, standard operation, face velocity at the fume hood opening. During standard operation, each fume hood shall operate as a conventional variable air volume hood with a conventional face velocity (i.e., 100 fpm).

ZPS: Operator Absent
When the operator walks away from the fume hood and out of the ZPS detection zone, each Zone Presence Sensor shall detect the absence of the operator and shall send a +12 Vdc (operator absent) user status signal to its associated fume hood monitor. This +12 Vdc user status signal shall switch the monitor into its standby operation mode. Based on the combination of user status and sash position inputs, the fume hood monitor shall apply a multiplier equal to "(standby face velocity)/(standard face velocity)" to the sash signal to generate a 0-10 Vdc setback command signal. This 0-10 Vdc linear, calibrated command signal shall control its associated hood exhaust valve, thus maintaining a constant average, standby operation, face velocity at the fume hood opening. During standby operation, each fume hood shall operate with a lower, yet safe, face velocity (i.e., 60 fpm).

Room Control

Hood Exhaust Volume
Each hood exhaust valve shall generate a 0-10 Vdc feedback signal, proportional to the valve's airflow in cfm, and shall send this signal to the make-up air controller.

The make-up air controller shall calculate the total hood exhaust volume by summing the feedback signals from both hood exhaust valves, and shall generate a 0-10 Vdc total hood exhaust signal.

Pressurization
The make-up air controller shall maintain a constant, adjustable net negative offset between the zone's total exhaust and make-up air volumes. This offset shall not vary with changes in exhaust volume magnitude and represents the volume of air that enters the zone from the corridor or adjacent spaces.

To achieve a negative room offset volume, the make-up air controller shall subtract the quantity of offset from the total hood exhaust signal. The resultant 0-10 Vdc signal is the make-up air for hood demand signal and represents the volume of make-up air required to satisfy the total hood exhaust demand with respect to the desired room offset volume.

Supply and Temperature
The override clamp (on the make-up air controller) shall clamp the make-up air valves to a minimum volume that is large enough to maintain the minimum ventilation volume.

On a rise in zone temperature, the electronic thermostat [or DDC controller] shall send a thermal demand signal to the make-up air controller that is within a 0 to 10 Vdc range. This thermal demand signal shall be proportionate to the supply air volume required to condition the lab. The ETI option (mounted on the make-up air controller) shall scale the thermal demand signal into a 0-10 Vdc signal at the supply cfm/volt scale factor of the make-up air controller.

The make-up air command signal shall be generated by comparing the override minimum clamp, the make-up air for hood demand and the scaled thermal demand signals, and selecting the higher of these three.

Independently, the zone thermostat shall control the reheat coil.

General Exhaust Volume
The make-up air controller shall generate a 0-10 Vdc signal to control the zone's general exhaust valve. This command signal shall equal the algebraic difference between the make-up air and total hood exhaust volumes, plus the desired room offset volume. The make-up air controller shall command the general exhaust valve to open when additional exhaust volume is required to maintain zone pressurization. Should the total hood exhaust volume increase, the make-up air controller shall decrease the command to the GEX valve accordingly, thereby providing pressurization control of the zone.

Static Pressure Fluctuations
As the static pressure in the exhaust and supply duct systems fluctuates, the pressure independent cone/spring assembly of each Phoenix venturi valve shall modulate to maintain a fixed set-point volume within one second.

Low Static Pressure
When the differential static pressure across each hood exhaust valve drops below the valve's minimum operating differential static pressure, the differential pressure switch (mounted on each hood exhaust valve) shall open, causing its associated fume hood monitor to generate an audible and visual flow alarm, indicating that the valve is outside of its control range. Upon a valve jam condition (i.e., feedback signal does not equal command signal) the fume hood monitor shall also generate a flow alarm. A mute button shall silence the audible portion of the alarm. When system conditions return to normal, all alarms shall automatically clear.

Fail-safe Condition
The valves in this application have been configured to fail in the following manner. Under loss of pneumatic air or power, each hood exhaust valve and the general exhaust valve will fail to their maximum mechanical limits, and each make-up air valve will fail to its minimum scheduled position. This zone fails in a negative pressurization mode with an increased offset volume.