Chapter hvac engineering Fundamentals: Part 1 Introduction

78
Chapter Three
ing). This whole matter is further complicated by the knowledge that
outdoor air quality may not be acceptable in many indoor environ-
ments, so that special treatment to remove contaminants may be nec-
essary (see Chaps. 5 and 21).
Thus, while the ventilation load is not a part of the space load, it is
reflected in the air-handling unit and central plant capacity. For heat-
ing, it is the minimum outside air quantity multiplied by the design
temperature difference and the proper air factor. Thus,
q
⫽
CFM
⫻
1.08
⫻
(
t
⫺
t
)
(3.7)
h
oa
i
o
where
q
h
⫽
ventilation load for heating, Btu / h
t
i
⫽
design inside temperature, heating
t
o
⫽
design outside temperature, heating
CFM
oa
⫽
outside-air quantity, ft
3
/ min
1.08
⫽
air factor, Btu / h / [(ft
3
/ min)
䡠
⬚
F] (for standard air—must
be adjusted for higher elevations)
For cooling, the ventilation load is the minimum outside air quantity
multiplied by the design enthalpy difference. Thus,
q
⫽
CFM
⫻
(
h
⫺
h
)
⫻
0.075
⫻
60
(3.8)
t
oa
o
i
where
q
t
⫽
total cooling load for ventilation, Btu / h
h
o
⫽
enthalpy at design outside conditions, Btu / lb
h
i
⫽
enthalpy at design inside conditions, Btu / lb
0.075
⫽
air density, lb / ft
3
(for standard air—must be adjusted for
higher elevations)
60
⫽
min / h
The sensible cooling load is calculated from the design temperature
difference. Thus,
q
⫽
CFM
⫻
(
t
⫺
t
)
⫻
1.08
(3.9)
s
oa
o
i
where
q
s
⫽
sensible cooling for ventilation, Btu / h
t
o
⫽
outside design temperature, cooling
t
i
⫽
inside design temperature, cooling
If the AHU capacity is calculated from a psychrometric chart anal-
ysis, as shown in Chap. 4, the ventilation load is automatically in-
cluded.
Notice that interior zones, with no heat loss, can make use of outside
air for winter cooling if the air-handling system is so designed. This
will result in some reduction of the ventilating heating load.
Design Procedures: Part 1
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