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Case Studies & White Papers:
Demand Based Static Pressure Reset Control for Laboratories

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Laboratories are generally the largest users of energy in any facility. Whether in a pharmaceutical research facility, a university or a research facility, laboratories use much larger amounts of energy than other operations.

The industry has looked at ways to save on energy use by using VAV fume hood control systems as well as low flow fume hoods. These systems took a big bite out of energy use in labs.

In continuing efforts to reduce energy costs and make buildings "greener" and sustainable, Labs21 has recommended reducing the static pressure drop of the devices in the airstream for both supply and exhaust systems in labs.

The goal of this paper is to show the advantages of Demand Based Static Pressure Reset Control for use in laboratories and how it can reduce the system static pressure at which the mechanical systems operate, thus reducing the operating cost and lowering the carbon footprint of the building. Reducing the operating static pressure gives the added benefit of lowering the noise levels in the duct making the lab environment more pleasant to work in.

ASHRAE Standard 90.1
ASHRAE Standard 90.1 - 6.5.3.2.3 provides:
"For systems with direct digital control of individual zone boxes reporting to the central control panel, static pressure setpoint shall be reset based on the zone requiring the most pressure; i.e., the setpoint is reset lower until one zone damper is nearly wide open."
While this standard is written for commercial buildings there is no reason that it shouldn't be applied to laboratories as well. With the high pressure drop of lab control air valves currently being used such as venturi valves, the potential savings is even greater than for commercial buildings.

The theme of the Labs21 conference in 2006 was lowering pressure drop in mechanical systems in labs. Fig. 1 (below) shows where the emphasis was placed for these systems. Lower static pressure systems have the potential for large savings, a strategy which has quite often been overlooked. Labs21, sponsored by the EPA and US Department of Energy Efficiency and Renewable Energy Federal Energy management Program, has put together a best practices guide called "Low Pressure-Drop HVAC Design for Laboratories." The guide offers suggestions for obtaining lower pressure systems in laboratories.

In the guide they list among other things, lower pressure drop VAV control devices. They also mention the ASHRAE 90.1 standard which includes 6.5.3.2.5 above, calling for static pressure setpoint reset.

Figure 1

Note that in Fig.1 the recommended pressure drop for VAV control devices for a "good" design is 0.6" - 0.3" and for a "better" design it is 0.1". A word of clarification is warranted here. Different valve manufacturers designate pressure drop differently. For example, venturi-type airflow valve manufacturers show pressure drop between 0.6" and 3.0". This indicates the operating range of the device not the minimum operating pressure. In other words a venturi requires a minimum of 0.6" to operate and generally would require 1.5" to 2.0" of static pressure to ensure operation over the full range of the device.

On the other hand the AccuValve™ static pressure specification is less than 0.3" pressure drop at the full scale CFM for the valve. This means that if the design airflow is less than the full scale CFM of the unit, the required static pressure would be even less than 0.3". If for example, the design maximum CFM is 20% less than the full scale CFM for the AccuValve™ than the static pressure at design flow would be less than 0.2" of drop across the valve.

Another important point regarding Fig.1 is that by reducing the static pressure of the system, noise from the VAV control devices is reduced and silencers can potentially be eliminated thereby reducing the pressure in the system even further.

System Description
The goal of Demand Based Static Pressure Reset Control is to operate the mechanical system at the minimum static pressure while allowing the controls to maintain a safe environment in the lab. According to the article "Increasing Efficiency with VAV system static pressure setpoint reset" by Steven T. Taylor in June 2007 ASHRAE Journal, utilizing these controls in a commercial building can provide fan savings of 30%-50%. When comparing this approach to the commonly used venturi valve, savings can easily reach these levels or greater in laboratories.

As seen in Fig.2 (below), the static pressure sensor is located about 2/3 of the way down the duct in both the supply and exhaust ducts. During startup the static pressure setpoints are set at 2.5" and the system operates to maintain the fume hoods at a constant face velocity and tracking control in each laboratory to maintain the supply at a fixed differential from the exhaust.

Figure 2

The Building Management System monitors the position of each airflow control valve and determines which valves are most open (i.e. the largest control output). If the control output of the valves that are most open is below 80%, the BMS reduces the duct static pressure setpoint then again looks at the position of each valve determining the valve(s) that are most open. This continues until the most open valve is approximately 80% open. This ensures that the system is operating at the minimum pressure while ensuring that there is enough pressure in the system to allow the controls to operate to maintain a safe lab environment.

It is worth noting at this time that this control scheme cannot be used with a venturi-type airflow control valve. A venturi derives its pressure independence from a spring and plunger in the airstream. The spring and plunger combination is used to compensate for changes in static pressure in the duct. The shaft is "commanded" to a position where it is calibrated for a given flow. The spring/plunger then compensate for static pressure changes in the system. Therefore, regardless of the static pressure in the duct (between 0.6" and 3.0") the valve will always be "commanded" to the same position for a given CFM. Control output or valve position is not indicative of static pressure in the system, so this signal cannot be used to reduce the duct static pressure.

Therefore, safest design for a high pressure drop venturi-type valve is to set the system to a relatively high static pressure to ensure that all valves in the system will operate properly regardless of their location in the duct and the changes in the system as fume hoods open and close. It is also important not to operate close to the minimum allowable static pressure of 0.6" because the friction coefficient of the plunger riding over the cone can change as the shaft becomes coated with exhaust particles thus affecting its responsiveness and accuracy in low static pressure conditions.

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