Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.7
Temperature, Wet-Bulb. Air temperature as indicated by a thermometer with a
wet bulb. This temperature is less than the dry-bulb temperature, except when
the air is fully saturated with water vapor, or at 100% relative humidity, when
wet-bulb and dry-bulb temperatures will be equal.
Ton, Refrigeration. Refrigeration effect equivalent to 200 Btu/min, or 12,000
Btu/hr.
Vapor. The gaseous state of water and other liquid substances.
Vapor Barrier. An impervious material used to prevent the passage of water
vapor and to prevent condensation.
Velocity Pressure. The pressure caused by a moving airstream, composed of both
velocity pressure and static pressure.
Ventilation. The process of supplying air to any space within a building without
noticeable odors and without objectionable levels of contaminants, such as dusts
and harmful gases, and of removing stale, polluted air from the space. Outside
air is generally used as an acceptable source of ventilation air.
Ventilator, Unit. A type of unit heater with various modes of operation and degrees
or percentages of outside air (frequently used for heating classrooms).
Volume, Specific. Volume, ft 3 /lb, occupied by a unit weight of air.
Water, Makeup. Generally the water supplied to a cooling tower to replace the
cooling water lost by evaporation or bleedoff.
Water Vapor. A psychrometric term used to denote the water in air (actually lowpressure,
superheated steam) that has been evaporated into the air at a temperature
corresponding to the boiling temperature of water at that very low pressure.
13.2 HEAT AND HUMIDITY
People have always struggled with the problem of being comfortable in their environment.
First attempts were to use fire directly to provide heat through cold
winters. It was only in recent times that interest and technology permitted development
of greater understanding of heat and heating, and substantial improvements
in comfort were made. Comfort heating now is a highly developed science and, in
conjunction with air conditioning, provides comfort conditions in all seasons in all
parts of the world.
As more was learned about humidity and the capacity of the air to contain
various amounts of water vapor, greater achievements in environmental control were
made. Control of humidity in buildings now is a very important part of heating,
ventilation, and air conditioning, and in many cases is extremely important in meeting
manufacturing requirements. Today, it is possible to alter the atmosphere or
environment in buildings in any manner, to suit any particular need, with great
precision and control.
13.2.1 Thermometers and Scales
Energy in the form of heat is transferred from one material or substance to another
because of a temperature difference that exists between them. When heat is applied