CPD Programme
O P C A H
cooler operates
supply fan
S
at design loads
air preheater cooler after steam
intake heater humidifier
room cooling with
sensible dehumidification
heating
sensible cooling
cooling and
humidification
R
preheater operates
at design loads
heating and
humidifier
humidification
modulates
steam
cooler modulates
exhaust air exhaust fan
discharge
preheater afterheater modulates
modulates
Figure 3: Basic system component schematic
Figure 4: All-year-round operating regime based on outdoor conditions
degree of sensible heat to the air) where h
AH
the humidifier load, 0.62 x (33.0-17.0) = will also be actuated to reheat the air. This is,
is 27.0kJ⋅kg
-1
. A steam humidifier is then 9.92kW giving a total plant load of 18.8kW. alongside the humidification load, a potential
used to increase the moisture content (with The significant difference between the plant profligate use of energy that, with appropriate
potentially a small increase in air dry bulb and the room loads is by virtue of the need system design, may be substantially reduced
temperature) from g
P
(the same as g
OW
) to to increase the temperature and moisture – this will be discussed in later articles.
g
SH
with the process P→S
H
. content of the cold, dry outdoor air before A graphical interpretation of operating
By examining the psychrometric it can be heated to provide any useful room regimes (preferably combined with frequency
requirements determined for summer conditioning. based climate data) provides an accessible
and winter operation, the initial schematic tool to assist the designer in examining and
of a basic full fresh air, constant volume Year round operation optimising the all-year system operation.
conditioning system can be sketched out as The modes of operation for this simple
in Figure 3. system are shown in Figure 4. This diagram Further reading
(after Legg [3]) indicates the operating modes Air Conditioning Engineering, Jones WP,
Calculating the loads for the system for the annual range of outdoor Butterworth 2001, Chapter 3
The loads may be readily established from conditions. CIBSE Guide B2 2001/5, Section 4.20
the chart where Power (kW) = mass flow rate The preheater is likely to be controlled
(kg⋅s
-1
) x enthalpy change (kJ⋅kg
-1
) = m
.
Δh and using feedback from a downstream duct References
so, for example, the summer cooling coil sensor and is set to maintain a minimum 1. CIBSE Guide B – Heating, ventilating,
design load is 0.62 x (57.0-32.5) = 15.2kW. temperature (likely to be between 5C and 8C) air conditioning and refrigeration. CIBSE,
The summer afterheater load = 0.62 x (35.0- when the system is in operation – this will 2005.
32.5) = 1.55kW, and so the total plant load is only operate in winter. The humidifier should 2. CIBSE Guide A – Environmental Design.
thus 15.2 + 1.55 = 16.8kW. This compares only operate when the outdoor air has a low CIBSE, 2006.
with the room cooling load of 5.2kW sensible moisture content – this is predominantly 3. Air Conditioning Systems – Design,
+ 1.2kW latent = 6.4kW! However, it is not when the outdoor air is cooler (tables of Commissioning and Maintenance.
correct to simply compare the two values as percentage frequencies of occurrence of Batsford, 1991. RC, Legg
the plant load includes: outdoor conditions may be used to determine
• The power to cool the outdoor air (that will the actual periods).
Symbols
be providing necessary ventilation fresh air to For this simple example system a room
Δ = difference
the room) down to the room condition; dry bulb temperature sensor could provide
β = contact factor
• The power to ‘overcool’ the air dry bulb the information to the controller to modulate
C
p
= air specific heat capacity
temperature so that condensation takes place both the afterheater and the cooling coil in
g = moisture content kg·kg
-1
da
to dehumidify the air; and sequence. However the cooling coil will also
h = enthalpy kJ·kg
-1
• Reheat power to bring the air dry bulb be controlled from the feedback from a room
h
fg
= latent heat of evaporation (approx
temperature from the lower dehumidifying humidity sensor – if the room humidity
2450 kJ·kg
-1
)
temperature back up to the supply air rises the cooling coil will be actuated. This
m
.
= mass flowrate, kg/s
temperature. will override the requirement for dry bulb
Φ = load watts
The load in winter will comprise the temperature control and so, if the resulting
P = power watts
heater load, 0.62 x (17.0 -1.0) = 9.9kW and room temperature is too cool, the afterheater
θ = dry-bulb temperature deg C
www.cibsejournal.com March 2010 CIBSE Journal 61
CIBSEmar10 pp59-62 cpd.indd 61 2/25/10 4:26:34 PM
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