Catalogue

16 UV-PRC004-EN
Selection Procedures
Selecting a Unit Ventilator
Trane horizontal classroom unit ventilators provide air delivery and capacities necessary to meet
the requirements of modern school classrooms. They are available with the industry’s widest
selection of coils to precisely satisfy heating, ventilating and air conditioning loads with the best
individual type of system. Unit ventilator selection involves three basic steps.
• Determine the classroom/space unit cooling and/or heating loads.
• Determine the unit size.
• Select the coil.
Capacity Required
The first step in unit ventilator selection is to determine room heating and air conditioning loads.
The calculation of this load is essential if the equipment is to be economical in first cost and
operating cost.
Adequate ventilation is mandatory in classroom air conditioning design. The amount is often
specified by local or state codes and, in air conditioned schools, may be either the same or less than
that specified for heating systems. The usual requirement is between 15 and 25 cfm of outside air
per occupant, based on the intended use of the room. For instance, a chemistry laboratory normally
requires more ventilation for odor control than a low occupancy speech clinic.
Ventilation is an important concern and should be accurately determined to assure good indoor air
quality. Purposely oversizing units should be avoided, since it can cause comfort and control
issues.
Unit Size
Unit ventilator size is determined by three factors:
• Total air circulation.
• Ventilation cooling economizer capacity required.
• Total cooling or heating capacity required.
Total air circulation, if not specified by code, sho
uld be sufficient to ensure comfort conditions
throughout the room. This is usually from six to nine air changes per hour, but can vary with room
design and exposure. Often rooms with large sun exposure require additional circulation to avoid
hot spots.
Ventilation cooling capacity is determined by the amount of outside air delivered with the outside
air damper fully open, and the temperature difference between the outside air and the classroom.
In air conditioning applications, ventilation cooling capacities should maintain the comfort setting
in the classroom whenever the outside air temperature is below the unit or system changeover
temperature.
Example:
Ventilation cooling capacity = 1.085 x cfm
t
x (T
1
- T
2
)
cfm
t
= Total air capacity of unit with outside air damper open 100%
T
1
= Room temperature
T
2
= Outside air temperature
In classrooms with exceptionally heavy air conditioning loads, unit size may be determined by the
total cooling requirement. Good practice dictates 375 to 425 cfm per ton of hydronic cooling
capacity. Normally, however, Trane classroom air conditioner coils have sufficient capacities.
UV-PRC004-EN.book Page 16 Monday, February 4, 2013 11:01 AM