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Frequently Asked Questions

We get a lot of questions about the AccuValve® and why it is better than a venturi or any other valve. Here is a list of some of the questions we have fielded over the years.

Why is a low pressure drop valve important to me?

A low pressure drop valve can mean significant savings in operating costs over the life of the building. Using an airflow-based valve instead of a mechanical pressure-based valve can reduce duct static pressures by greater than 1.0" wc. This will result in fans running at lower brake horsepower and can mean savings of over $425,000.00 over a 20 life of a building with 100,000 CFM of exhaust.

It's the 21st century – why would I want to use a mechanical pressure-based valve in a critical environment application like a laboratory?

There is no longer a good reason to use a 40 year old mechanical design like a venturi valve in your critical environment application. When VAV control of laboratories was first being installed, a mechanical device such as a venturi valve had advantages over closed loop control due to limitations of airflow sensing and the high cost of digital controls with the required scan rates to accomplish the speed of response required for this application.

As with any industry many improvements have been made which has rendered mechanical open loop control an outdated method of control of a critical environment system. Some of these improvements are:

Airflow Sensing — venturi suppliers state that airflow measurement is not reliable or that it does not have the turndown required in a laboratory environment. While 25 years ago with only pitot and orifice plates available this had some validity, however the introduction of digital vortex shedding airflow sensors have eliminated this concern. Offering high accuracy, large turndowns and resistance to harsh environments the vortex shedding airflow sensor is ideally suited for laboratory environments.

Speed of Response — One positive of an open loop mechanical system was the high speed of response and 25 years ago it could not be matched by the digital controllers in a cost effective way. With the ever improving speed and lower cost of microprocessors this has also become a nonissue for closed loop systems which now offer superior performance and reliability to a mechanical pressure independent system.

Nonlinear Damper — 25 years ago the venturi was compared to a single blade damper. The single blade damper has a very limited turndown because of the inherent nonlinear action of the device. In 2006 the AccuValve was introduced which by using a dual chamber system with linearizing linkage it eliminated the hunting associated with a single blade damper allowing for much faster control action which is stable over a much wider range.

In short with the AccuValve there is no reason to continue to use an outdated mechanical pressure independent valve which requires high duct pressures to operate.

Why does a venturi valve require significantly higher duct pressures to operate than an AccuValve?

The venturi valve is a mechanical pressure-based airflow valve whereas the AccuValve is airflow-based. Because the venturi is mechanical pressure-based it requires pressure in the duct to create the pressure independence without measuring airflow. The venturi valve in essence uses the fan horsepower to "power" the mechanical spring and cone assembly. The AccuValve measures airflow so it does not require pressure to operate.

What is the difference between a pressure-based valve and airflow-based valve?

A pressure based damper like a venturi requires pressure in the duct to operate. The mechanical system of cone, spring and slide bearings requires pressure in the ductwork to achieve its pressure independence. This results in using the fan brake horsepower to create the pressure independence. A venturi is specified to operate between 0.6" to 3.0" wc. This does not mean that it has a 0.6" pressure drop – instead it means that it requires a minimum pressure of 0.6" to operate. A venturi typically requires 1.5" to 2.0" of system pressure to insure its pressure independence.

An airflow-based valve such as an AccuValve does not require pressure in the system to operate. Instead it measures airflow directly and will drive the valve blades to achieve the required airflow regardless of the pressure in the system. In the event there is not sufficient pressure in the duct to achieve the required airflow it will read low airflow and an alarm will occur. The AccuValve is specified with a maximum pressure drop of 0.30". It does not use fan horsepower to operate meaning significantly less pressure requirements.

I am controlling CFM why wouldn't I want to measure CFM?

A very good question. We have always believed that if a parameter is important you should measure it. Twenty-five years ago, when laboratory VAV controls were in their infancy, the airflow technology to reliably measure CFM in the harsh environments of laboratories were limited and expensive. Vortex shedding airflow measurement overcame the technical as well as the cost issues and there is no longer a reason not to measure airflow. It is by far the safest way to control airflow in a laboratory environment. Bottom line – to control temperature, you measure temperature; to control humidity, you measure humidity, therefore we feel it is important to measure airflow, which of course is the most critical parameter in a lab.

Why is it important to an owner to have Demand Based Static Pressure Reset Control in labs?

Laboratories are generally the largest users of energy in any facility. Whether in a pharmaceutical research facility or a university 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. ASHRAE Standard 90.1.-6.5.3.2.3 addresses this issue to which the AccuValve design has incorporated its capabilities. Demand Based Static Pressure Reset Control (DBSPRC) offers dramatic savings in operating cost by minimizing the static pressure in the duct thereby allowing the supply and exhaust fans to operate at lower brakehorsepower. To utilize DBSPRC the airflow valve must incorporate airflow measurement and closed loop control. Through the measurement of true airflow, the valve will modulate to the proper airflow volume. This will occur regardless of the static pressure in the duct. Information on the valve position will then be communicated to the Building Management System which will use that information to reset the static pressure in the duct to the minimum setpoint possible to maximize energy savings. An open loop mechanical venturi valve is not acceptable because it relies on a calibrated spring and plunger which only drives to a specific valve position and relies on the spring to compensate for pressure changes. Therefore valve position is not indicative of required static pressure in the system and a venturi valve cannot be used for DBSPRC.