CPD Programme
0.2
0.1
Specific 0.2
0.1
0.3 0.3
0.4
enthalpy
0.4
0.5 kJ/kg
0.5
0.6 0.6
0.7
Sensible/Total
0.7
0.8 da
Sensible/Total
0.8
0.9 0.9
1.0 1.0
0.9
heat ratio for water
0.9
heat ratio for water
0.8 0.8
0.7 added at 30C Moisture 0.7 added at 30C
0.6 0.6
0.5
57.0kJ/kg content
0.5
0.4 0.4
0.3 0.3
0.2 kg/kg
0.1 da
0.2
0.1
Specific
46.0kJ/kg
enthalpy
kJ/kg
35.0kJ/kg
O
da
S
Moisture
32.5kJ/kg
11.4g/kg 33.0kJ/kg
31.5kJ/kg
content
R
27.0kJ/kg kg/kg
S
8.8g/kg
da
X 8.0g/kg
C S
C 7.4g/kg
R
w
4.8g/kg
1kJ/kg
S
4.3g/kg
H
2.0g/kg
O
W
P A
H
9.5C 16.0C 23.0C 28.0C -4.0C 8.0C 19.0C 22.0C
12.5C
Dry-bulb temperature deg C
Dry-bulb temperature deg C
Figure 1: Summer psychrometric processes Figure 2: Winter psychrometric processes
The summer system installed systems) a cooling coil will be used room sensible heating load, φ
SH
, the supply
> The summer room sensible/total heat ratio to both cool and dehumidify the air. The air air mass flowrate having previously been
will be 5.0/(5.0+1.2) = 0.81 and since this condition leaving the coil will be determined established from the cooling requirement,
is a coincident sensible cooling and latent primarily by the dehumidifying requirement φ
SC
; or the supply air temperature may be
cooling load, the gradient of the room ratio and the contact factor, β, of the coil. From determined from a requirement of the
line (RRL) is taken from the bottom quadrant the manufacturer a contact factor of 0.85 has particular supply regime (eg low level or high
of the protractor on the psychrometric chart been obtained (based on the flowrate of the level supply).
and is drawn through summer room point air passing through the coil, and the coil size) In this case, having already determined the
R
S
. The intersection of this line with the and from this the coil temperature, (the coil air mass flowrate from the cooling load as
specified value of θ
SC
(ie 16C) provides the ‘ADP’) indicated by point X on the chart may 0.62kg⋅s
-1
the heating supply air temperature
summer supply air point S
C
. As an alternative be determined. will be θ
SH
= θ
R
+ (Φ
SH
/C
p
) = 19 + [1.9/(0.62 x
to using the RRL to determine the supply air So β = 0.85 = (g
O
– g
S
)/ (g
O
– g
X
) so g
X
= 1.012)] = 22C. The winter room sensible/total
point, the room latent load may be used to g
O
- (g
O
– g
S
)/ 0.85 = 11.4 - (11.4 - 8.0)/0.85 heat ratio will be 1.9/(1.9 + 0.8) = 0.70 and
calculate the supply air moisture content = 7.4g⋅kg
-1
da
and hence the point X may be since this is a coincident sensible heating and
from Φ
L
= h
fg
(g
R
– g
S
) and so reading the plotted where the saturation curve intersects latent cooling load, the gradient of the room
value of g
R
from the chart as 8.8g⋅kg
-1
da
or with a moisture content of 7.4g⋅kg
-1
da
. The ratio line (RRL) is taken from the top quadrant
0.0088 kg⋅kg
-1
da
the value of g
S
= g
R
- (Φ
L
/ h
fg
) cooling coil process line is then O
S
→C where of the protractor and is drawn through winter
= 0.0088- [1.2/(0.62 x 2450)] = 0.0080kg⋅kg
-1
da
C is the intersection of the line O
S
to X with room point R
W
. The intersection of this line
or 8.0g⋅kg
-1
da
. Looking at the chart (Figure 1) the supply air moisture content, g
S
and has with the calculated value of θ
SH
(ie 22C)
these two methods provide the same supply an enthalpy, h
C
, of 32.5kJ⋅kg
-1
. provides the winter supply air point S
H
(and
air point – the simple calculation method The air now has an appropriate moisture of course a similar calculation to that used
is probably the most reliable. However, the content to supply the room but, as a result of for the summer design may be undertaken
use of the RRL allows the designer to look at the need to dehumidify the air, the dry bulb using the winter latent load to confirm the
the range of supply air conditions that could temperature is below the required value of supply air moisture content). The supply air
be used if there was flexibility in the design θ
S
. An afterheater is used to increase the enthalpy h
SH
can be read off as 33.0kJ⋅kg
-1
.
supply air temperature. temperature from θ
C
to θ
S
. (The fan will also Point S
H
is clearly both at a higher
To develop the ‘summer cycle’ the outdoor act as a sensible air heater). temperature and moisture content than the
air, O
S
, is plotted (the values identifying O
S
winter outdoor air condition, O
,
W
and so a
having been established from climate data The winter process sensible heater and a humidifier is required;
such as Table A2.6 of CIBSE Guide [2]). In a To outdoor condition, O
W
is plotted (on in this example a steam humidifier has been
full fresh air system, air at O
S
must be finally Figure 2) based on a knowledge of local used. To increase the temperature typically a
conditioned to produce air at S
C
. Looking at climatic data (that can, for example come water or electric coil (or frequently two coils
the chart O
S
has a higher temperature and from Table A2.4 of CIBSE Guide [2]). To - a preheater or frost coil, and an afterheater)
moisture content than S
C
, where (from the determine the supply air point (at winter may be used. In this example one heating
chart) h
O
= 57kJ⋅kg
-1
and h
S
= 35kJ⋅kg
-1
. Hence design), S
H
the supply air temperature, θ
SH
process will be shown from O
W
→P→A
H
,
there is a need to reduce the enthalpy of the must be established. θ
SH
will be determined where θ
AH
is the supply temperature (or
air by (57 – 35)kJ⋅kg
-1
= 22kJ⋅kg
-1
. either from a knowledge of the supply air maybe just slightly cooler as the subsequent
In this simple system (and in many mass flowrate m
.
in combination with the steam humidifier will also add a small
60 CIBSE Journal March 2010
www.cibsejournal.com
CIBSEmar10 pp59-62 cpd.indd 60 2/25/10 4:25:36 PM
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