Letters to the Editor

I submit that much volunteer time, research, and money has been spent to provide the end-user with an ASHRAE Standard that will give good, comparative data on the performance of filters. If the end-user really wants to know “what can filtration do for me,” I would recommend they look at the particle size removal efficiency graphs provided by ASHRAE 52.2.

I would also recommend all filter organizations and end-users become involved with the National Air Filtration Association (NAFA) www.nafahq.org and become fully educated on 52.2 and NAFA’s new air filter Product Certification Program.

Al Veeck Tidewater Air Filter Subsidiary, Flanders/Precisionaire Corp. Chair, ASHRAE TC 2.4

(ASHRAE Technical Committee 2.4 is the committee of Particulate Contaminants and Particulate Contaminant Removal Equipment.)

A tutorial on school ventilation

I read Ms. Turpin’s article [“Schools Vent Frustrations,” June 12, page 1] with much interest and enjoyed Mr. Williams’ analysis and design preferences for ventilation control in schools. I have no doubt that unit ventilators in each classroom and the direct differential pressure control of a central exhaust system is significantly better than what is available in many existing schools.

However, there are several important aspects of outside air control that he missed.

First and most importantly, a mechanically powered ventilator does not, by itself, ensure that code-required minimum ventilation rates are being maintained continuously. Many unit ventilators can be shown not to deliver the amount of outside air that they are rated to supply. They are also very susceptible to wind and stack pressure effects, which in turn retard or multiply the amount of outside air actually being moved.

Also, differential pressure control is not a very reliable input source for pressurization control or for control of intake airflow rates. Outside air intake flow rates in unit ventilators typically range from 50 to 800 fpm, or 0.0002- to 0.05-in. wg pressures. Because there was previously no reasonable way to determine the actual amount of airflow being supplied or the actual amount of outside air introduced at the ventilator, everyone has depended on the manufacturers’ ratings and guessed what was being delivered at partial-load conditions. This has historically caused serious under-ventilation problems.

With most new school and renovation projects, the hvac control system is being upgraded to an electronic one that provides a significant amount of flexibility and data memory. The performance of these systems is totally dependant on the accuracy of remote input data and its programming to satisfy the promises of efficiency and energy savings. This is especially true if it is to be accomplished without negatively impacting mandated ventilation rates. To ignore minimum ventilation rate requirements is to magnify the already burdensome financial risk and direct health costs due to IAQ-related injuries and illness.

Highly accurate and economical measurement products are now available to provide the electronic data input for unit ventilator or ERV intake control. They have only recently been introduced, but are already the basis of state-of-the-art designs in schools. Minnesota statutes now require schools to monitor both outside airflow rates and total supply flow rates to each classroom going into service or renovated after July 1, 2002.

Airflow control is much more stable and reliable than differential pressure control. In Mr. Williams’ example, the exhaust fan speed can be much more easily controlled at a constant volumetric differential, between measured intake flow rates and the ducted exhaust rates. It would provide the greater precision and a more energy efficient means to achieve positive building pressurization. Direct measurement is the only way minimum outside air could be efficiently and reliably controlled at a continuous rate.

Len Damiano National Accounts Manager Ebtron, Inc. Loris, SC

Links

• National Air Filtration Association