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CPD Programme
Figure 4 - Tempered Spray Figure 5 - Face and Bypass of a Cooling Coil
dehumidified. If applied appropriately, face and shown in Figure 6. In comfort air
and bypass can usefully reduce the energy conditioning, the line will be in the upper
used in ‘reheat’ processes commonly applied quadrant of the protractor when there is a
0.2
0.1
to provide close control of supply temperature room heating load, and in the lower quadrant
0.3
and humidity. when there is a room cooling load. The
0.4
This process is shown in Figure 5. For a room ratio line is used to indicate a set of
0.5
bypassed cooling coil the position of the mixed possible room conditions when the supply
point M will be determined by the proportions air condition is known, or conversely a set
0.6
of the air that has passed through the coil, B, to of suitable supply air conditions when the
0.7
the air that has simply bypassed the coil A. design room condition has been plotted. 0.8
Sensible/Total heat
The actual length of the line is determined
0.9
ratio for water
The room process calculations by the actual room load and the supply air
1.0
added at 30°C
The conditioned space is itself represented as mass flow rate – this will be fully applied in a
0.9
a process on the psychrometric chart and has subsequent article.
0.8
a dedicated section on the standard CIBSE
0.7
Ratio
Psychrometric Chart to assist in determining Filters and fans 0.6
= 6.00/9.23
the gradient of the ‘room ratio line’, RRL. There is often uncertainty as to how filters and
= 6.5
The value of this sensible/total heat ratio is fans in an air handling system are represented
0.5
determined from on a psychrometric chart. Practically a filter
0.4
(Room Sensible Load)/(Room Sensible would not affect the heat content in the air – it
0.3
Load + Room Latent Load) would of course have a pressure drop across it
0.2
0.1
where the loads (normally in watts) have and so consume air power. So the filter is not
been previously obtained from a load normally included in a psychrometric plot. The
calculation undertaken by hand (eg the fan will sensibly heat the air by about 0.1K for
Figure 6 - Sensible/Total Heat Ratio
CIBSE Admittance Method) or by using each 100Pa of total fan pressure (WP Jones,
computer modelling software. It is important 2001, Air Conditioning Engineering, 5th Edition) Further reading
that these loads are truly room loads – these and so can be shown on a psychrometric chart Air Conditioning Engineering, Jones WP,
are sometimes confused with central plant as a simple, sensible heating process. Butterworth 2001, Chapter six
loads that include the fresh air load that, if All the psychrometric processes that ASHRAE Fundamentals 2009, Chapter one
supplied from a central plant, is not a room have been described in this short series
load. (This confusion commonly arises from a of articles may be combined to determine⋅ Symbols
misunderstanding of the output of computer the component requirements for an air Δ = difference
generated load calculations). conditioning system, to determine its g = moisture content kg⋅kg
-1
da
So, for example, if a room sensible heating control strategy and to predict its operation. h = enthalpy kJ⋅kg
-1
.
load was 6kW and the room latent load was By linking in data reflecting historic outdoor m = mass flowrate, kg/s
3.23kW the room sensible/total heat ratio air conditions the psychrometric analysis can P = power watts
would be predict comparative energy consumptions θ = dry-bulb temperature deg C
6/ (6+3.23) = 0.65. for different systems. A future article θ' = wet-bulb temperature deg C (normally
The slope of the RRL may then be determined will develop these combined systems. sling)
from the ‘protractor’ printed on the chart © Tim Dwyer 2009
52 CIBSE Journal December 2009 www.cibsejournal.com
CIBSEdec09 pp50-53 cpd.indd 52 11/19/09 3:59:08 PM
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