CPD Programme
> In HVAC it is unlikely that Process Calculations
a simple ‘latent’ only cooling To explore the calculations
process will be present. However associated with the processes
‘combined’ latent and sensible discussed so far consider the
cooling will regularly take place cooling coil shown in Figure 8,
in the same piece of equipment - where all the data has been read off
for example in a cooling coil that a CIBSE Psychrometric chart. The
has a surface temperature below psychrometric values were based on
the dew-point temperature of the on-coil and off-coil dry-bulb and
the incoming air. This combined wet-bulb temperatures as indicated
process will produce an off coil in the diagram. For any type of
temperature somewhat higher psychrometric process the rate of
than that of the actual temperature energy transfer to or from the air
of the coil (known as the Apparatus may be simply determined from
Dew Point, ADP and shown as the chart using the relationship
point X on Figure 5). Power (kW) = mass flow rate
How close the dry-bulb (kg/s) × enthalpy change (kJ/kg)
temperature of point B approaches = m
.
Δh
the ADP will depend on the
Figure 1 – Outline Psychrometric Chart
So P = 0.75 × (66.0 – 40.5) =
effectiveness of the cooling coil. 19.1kW
This will relate such parameters as This may be split into the power
the number and shape of coil rows, used to provide the sensible and
the spacing and type of fins on the latent cooling and this is given
coil, and the air velocity. by m
.
(h
Y
– h
B
) and m
.
(h
A
– h
Y
)
This is commonly combined in respectively.
the term known as ‘contact factor’, For the same system the Contact
β – where the contact factor is a Factor, β may be established from
measure of how close the air will β = (θ
A
– θ
B
) / (θ
A
– θ
X
) = (30 –
get to the coil ADP and can be 16.5) / (30 – 10) = 0.67 (or 67 per
seen as the fraction of the distance cent)
between points A and X of the and the Bypass Factor will be
distance A to B ie: contact factor = (1 – 0.67) = 0.33.
AB/AX. The rate of water vapour
By virtue of the way that ‘similar removed from the air may be
triangles’ work AB/AX = (g
A
– g
B
)/ obtained from
(g
A
– g
X
) = (θ
A
– θ
B
)/ (θ
A
– θ
X
). m
.
× Δg = 0.75 × (0.0140 – 0.0098)
The term ‘bypass factor’ is = 0.0032 kg/s (or 3.2 g/s)
sometimes used and is simply (1
Figure 2 – Psychrometry of Sensible Heating
These calculations may be
– contact factor). reorganised to determine unknown
The process that would typically result Air Conditioning Engineering book). The variables for any of the processes illustrated in
from a steam humidifier (almost pure latent concept of a ‘contact factor’, in this case this article. A future article will combine these
heating) is indicated in Figure 4 – it can known as saturation efficiency is still apparent processes to examine system psychrometry.
reasonably be considered as a vertical line as being the fraction of the approach to the © Tim Dwyer
following a constant dry-bulb temperature. saturation curve.
A steam humidifier will produce a small Where two air streams combine Further Reading
amount of sensible heating since the adiabatically (as they normally would) the Air Conditioning Engineering, Jones WP,
higher temperature of the device (the tube) resulting air will have a psychrometric state Butterworth 2001, Chapter 3
carrying the steam into the air will increase that is related to the proportions of each of ASHRAE Fundamentals 2009, Chapter 1
the temperature of the air and there will be the combining air mass flowrates. This is
a very slight sensible heating effect from the indicated in Figure 7 as air stream A mixing Symbols
steam itself. with air stream B to produce mixed air at Δ = difference
So-called Adiabatic (constant energy) condition M. The ratio of the lengths AM to β = contact factor
Humidification as shown in Figure 6 BM is simply the same ratio as the quantities g = moisture content kg⋅kg
-1
da
practically characterises the processes of of air mass flowrates (and in this particular h = enthalpy kJ⋅kg
-1
atomising spray and ultrasonic humidifiers case would indicate that the greater mass
.
m = mass flowrate, kg/s
(as well as the rarely used recirculating spray flowrate is from branch B, m
.
B
). P = power, watts
and wetted pad humidifiers). By convention The resulting psychrometric states may μ = percentage saturation %
the process is shown as being along the wet therefore be determined as, for example, θ = dry-bulb temperature °C
bulb temperature line (for discussions on m
. .
M
θ
M
= m
.
θ' = wet-bulb temperature °C (normally sling)
the validity of this see Chapter 3 of Jones’
.
A
θ
A
+ m
B
θ
B
and m
.
M
g
M
= m
.
A
g
A
+ m
B
g
B
60 CIBSE Journal October 2009
www.cibsejournal.com
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