CPD PROGRAMME Without heat recovery No casual gains
Outdoor air mid band temperature
-4 -3 -1 1 3 5 7
9
11 13 15 17 19 21 22
Supply air temp
26.20 25.99 25.57 25.16 24.74 24.33 23.91 23.49 23.08 22.66 22.25 21.83 21.42 21.00 –
Heating
energy over year kWh
1075 2563 6333
12947 16763 19360 20569 19084 15408 11556 7854 4147 1282 0 –
138938 kWh Table 3: Ventilation heating energy with no heat recovery
may be determined from: Σ(AU) + Cv
= Σ(AU) + 0.33 NV =
(20.0 x 10.2) x (0.18 + 0.22) + (10.2 + 10.2 + 20.0 + 10.0) x 3 x 0.26 + 10 x 3 x 1.80 + 0.33 x 0.4 x (20.0 x 10.2 x 3.0) = 255.7 W·K-1 where A = element area (m2 (W·m-2 (W·K-1
), U = U value K-1 ), Cv V = volume of space (m3 ) x (θR
), N = infiltration rate (hour-1 ).
= ventilation conductance ) and
Using the heat loss coefficient, the building
heat loss at each band is evaluated from: (Σ(AU) + Cv
– mid band temperature). The casual gain is shown as a constant
at 2.14 kW (comprising the lighting gain plus one person), and this will offset some of the need for heating throughout the whole heating season. When undertaking such an analysis, there would normally be a diversity factor estimated to account for variations in such things as occupancy, lighting and equipment. The final column of Table 2 shows the heating required in the room at each corresponding band of outdoor temperature. For each band, the required supply air temperature, θS
Heating load = Cp (θR where Cp air, 1.005 kJ kg-1
, may be calculated from: – θS
)
is the specific heat capacity of K-1
. The approximate mass
flowrate of air (in this particular case) is determined by the required ventilation rate of six air changes per hour (and taking the
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Outdoor air mid band temperature
-4 -3 -1 1 3 5 7
9
11 13 15 17 19 21 22
Incoming air temp after HRU
8.50 9.00
10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 –
specific volume of air as 0.83 m3 so
1.22 kg s-1 With 50% effective heat recovery No casual gains
Supply air temp
26.20 25.99 25.57 25.16 24.74 24.33 23.91 23.49 23.08 22.66 22.25 21.83 21.42 21.00 –
Heating energy over year kWh
630
1502 3711 7587 9824 11346 12054 11184 9030 6772 4603 2430 751 0 –
81424 kWh Table 4: Ventilation heating energy with 50% effective sensible heat recovery With casual gains
Supply air temp
24.46 24.25 23.83 23.42 23.00 22.59 22.17 21.75 21.34 – – – – – –
Heating
energy over year kWh
568
1348 3297 6655 8482 9603 9938 8893 6811 – – – – – –
55596 kWh kg-1 ) = 6 x (20.0 x 10.2 x 3)/(3600 x 0.83) = With casual gains
Supply air temp
24.46 24.25 23.83 23.42 23.00 22.59 22.17 21.75 21.34 – – – – – –
Heating
energy over year kWh
1013
2409 5918
12014 15422 17617 18453 16793 13189 – – – – – –
102828 kWh The resulting supply temperature (both
with and without allowing for casual gains) is given in columns two and four of Table 3. The sum of the energy used over a year (relating to each band) is then calculated from: Hours occurring per year x Cp
(θS where θO is the particular outdoor air
mid band temperature. These are then summed to give the total ventilation heating energy used per year – so, in this case, with casual gains being taken into account, this would be 102,828 kWh for a full fresh air system. Having established a base case (illustrated in Table 3) the impact of adding a heat recovery device may be assessed, as in Table 4. As the supply and extract mass flowrates
are equal, the temperature of the outdoor air leaving the device, θB
, which enters
the subsequent heating coil, may be determined from: θB
= θO where θO + εS (θR – θO ) is the particular outdoor
air mid band temperature. This has been undertaken for a heat recovery effectiveness, εS
, of 50% and is shown in
column two of Table 4. (The 50% figure is representative of the seasonal efficiency of an average cross-flow plate heat exchanger.3 The sum of the energy used over a year
) – θO
)
February 2013 CIBSE Journal
63
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