Basic Documentation
Table Of Contents
- About this Application Guide
- Chapter 1–Introduction
- Chapter 2–Goals of the Laboratory Environment
- Chapter 3–Unique Ventilation Needs of a Laboratory Facility
- Chapter 4–Ventilation Systems Classification
- Chapter 5–Laboratory Facility Exhaust Systems
- Chapter 6–Laboratory Containment Units - Ventilation
- Chapter 7–Room Ventilation, Makeup Air, and Pressurization Control Systems
- Chapter 8–Laboratory Temperature and Humidity Control Systems
- Chapter 9–Laboratory Emergencies - Ventilation System Response
- Chapter 10–Laboratory Ventilation System - Validation
- Chapter 11–Laboratory Ventilation System - Commissioning
- Glossary
- Index
Room Temperature Control by BTU Compensation
A 10 ACH ventilation rate for this size of room will require approximately 450 cfm of
total room exhaust
24
. However, if the sash of the fume hood is fully opened, the total
room exhaust will need to increase to nearly 1,250 cfm to provide the nominal 100
feet per minimum average face velocity at the fume hood.
25
To maintain a negative
static pressure in the room, the supply air will need to be maintained at about 250
cfm less than the total room exhaust. Thus, the room supply air will normally range
from about 200 cfm when the fume hood sash is closed to 1,000 cfm whenever the
fume hood sash is fully open.
Consider a very warm summer day when the fume hood sash is fully closed. The
total room exhaust is 450 cfm and the room supply is at 200 cfm. In order to keep the
room occupants at a comfortable temperature of 74°F under these conditions, the
incoming supply air would probably have to be at its minimum temperature of about
54°F. However, due to the relatively low supply airflow rate of 200 cfm, the room
remains free of objectionable drafts and the occupants are rather comfortable.
However, if someone suddenly opened the fume hood sash all the way, then fume
hood controller would increase the fume hood exhaust rate to 1,250 cfm to maintain
the required face velocity. Likewise, the room controller would have to quickly
increase the supply makeup air to 1,000 cfm.
Almost immediately, five times as much supply air at 54°F begins coming into the
room. This results in a very uncomfortable “cold dump” or temperature swing that is
quickly felt by the room occupants. Eventually the temperature of the supply air
would be increased by the room controller in response to the room temperature
signal from the room temperature sensor, but due to the time lag this will probably
require at least a couple of minutes. Meanwhile the delay in the control action results
in very uncomfortable conditions for the room occupants. Therefore, the conventional
control process of Figure 15 is unsuitable for a small variable air volume laboratory
with VAV chemical fume hoods.
Room Temperature Control by BTU Compensation
A preferred way of ensuring occupant comfort in small VAV laboratory rooms, subject
to significant changes in room airflow rate, is by applying BTU compensation. BTU
compensation is a room temperature control strategy that limits room temperature
upsets due to rapid changes in the ventilation airflow. It not only controls the room
temperature but also the temperature of the supply air whenever large changes in
airflow occur.
24
Total room volume is 2,700 cubic feet (12’ × 25’ × 9’). Ten air changes per hour requires exhausting 450 cfm (2700 ×
10 ÷ 60).
25
A typical 6 foot wide fume hood with an average face velocity of 100 fpm requires an exhaust rate of about 1,250 cfm.
Siemens Building Technologies, Inc. 75