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
Glossary
pressure dependent
Variable air volume (VAV) room temperature control system where the temperature directly
drives the damper. There is no airflow measurement and no explicit flow control. The airflow
varies, but it is not controlled to any particular value. (Also see pressure independent.)
pressure drop
The loss in pressure that occurs mostly to friction when air flows through an HVAC element
such as a length of duct, duct fitting, damper, etc. For airflow to take place in a duct or
through a ventilation system component, some pressure drop must occur. However, a good
ventilation system design minimizes the amount of pressure drop necessary to achieve a
specific airflow because in so doing, less energy cost (mainly in fan electrical energy) is
incurred and less airflow sound is typically generated.
pressure independent
Variable air volume (VAV) room temperature control system in which the temperature drives
an airflow setpoint. A separate airflow regulator (control loop) drives the damper forcing the
measured airflow to the setpoint. This system requires airflow measurement. (Also see
pressure dependent.)
proportional control
Closed loop control arrangement whereby the magnitude of the controller’s output signal is
proportional to the amount of variation of the controlled variable from the setpoint. This
variation is referred to as the error. Thus, if a temperature is 4°F from its setpoint, the
controller’s output signal would be twice what it would be if the temperature was only 2°F
from the setpoint. Under most conditions, proportional control cannot give a control output
that can keep a process exactly at the setpoint since no control output can exist without some
variation (error) from the setpoint. Therefore, some minimum error (referred to as the offset)
will always exist in a true proportional-only control system.
proportional plus integral gain (PI)
A feature used in addition to proportional-only control to remove the offset component. After
the proportional control action takes place and the control output signal remains stable, the
integral control function imposes a bias on the control output signal of sufficient magnitude to
force the remaining offset to nearly zero. PI control needs some time to complete its function
since the controller’s proportional control action must be completed and the process
stabilized before the magnitude of the offset can be determined. Then the integral gain
function can shift (bias) the output signal until the offset becomes zero.
proportional plus integral plus derivative gain (PID)
Functionality used in state of the art control systems to decrease the response time of the
overall control process. The addition of the derivative gain function to PI control results in the
controller providing a control output signal that is proportional to the rate by which the error is
changing. Instead of the control output merely being proportional to the error (as with
proportional only type control), the magnitude of the control output is significantly increased
or decreased to reduce the time needed to attain the desired setpoint.
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