Counter-fl ow plus cross-fl ow plate heat exchanger
O R’ R B
Moisture content kg/kgda
Room air dew point X O
Dry-bulb temperature °C
θO R’ θR’ θRdp B θB θR
Figure 3: Plate heat excha nger with condensation in the discharging room air
the device (Pa) and ηfan is the total fan,
drive and motor effi ciency. This will be additional direct electrical power, which is likely to be more costly – and have twice the carbon footprint – of any natural gas heating that is being offset or any cooling savings (owing to the coeffi cient of performance (COP), refrigeration electrical power consumption will typically be less than half the cooling delivered). The heat recovery device will also require a bypass to avoid unwanted heat transfer to the incoming air in summer conditions.
Application of heat recovery model In this CPD module, a simple example building will be used to examine the impact of heat recovery in a very common application of ventilation in the UK (with no cooling). This is an example of a comparative method that may be used – it can be quickly developed in a spreadsheet that can readily be expanded to explore other sensitivities, including NPV analysis. The building is a small, detached store 20m wide, 10.2m deep and 3m fl oor-to-ceiling height, situated adjacent to a busy road on the outskirts of London. The building has triple-glazed windows (and doors) along 50% of the long south-facing wall, and has been constructed within the last two years. The heating and ventilation air is being provided by a mechanical ventilation system to maintain a minimum temperature of 21°C. The occupancy will be
Roof Walls Floor
Glazing Infi ltration rate
Winter outdoor design temperature
Table 1: Example building data 62 CIBSE Journal February 2013
0.18 W·m-2 0.26 W·m-2 0.22 W·m-2 1.80 W·m-2 0.4 hour-1
-4°C K-1
K-1 K-1 K-1
Extract fan
Heating coil
Supply fan
F igure 4: Energy recovery unit integrated into an AHU. Air returns from the rooms in the direction of the red arrow and the outdoor air path is indicated by the blue arrow.
one person, 24 hours a day, and the lighting provides a heat gain of 10 W·m-2
fl oor area,
with no other equipment use. Owing to the materials stored in the building, full fresh air ventilation is required at a rate of at least six air changes per hour. The building’s data required to undertake heat loss calculations are given in Table 1. A heat recovery system is often incorporated into packaged air handling units similar to that shown in Figure 4, designed for mounting in ceiling voids. To examine the building and the
Outdoor air mid band temperature (2K bands)
< -4 -3 -1 1 3 5 7
9
11 13 15 17 19 21
>22 Total Hours
occurring per year
28.93 71.88
193.73 435.67 626.77 814.36 988.80 1070.33 1037.02 972.15 880.98 697.77 431.29 263.86 252.46
8766 hours
Table 2: Frequency of outdoor air temperature for London suburb (24-hour, hourly data), together with building heat loss at mid band temperature, casual gain and resulting banded heating load
www.cibsejournal.com
Building heat loss kW
6.39 6.14 5.63 5.11
4.60 4.09 3.58 3.07 2.56 2.05 1.53 1.02 0.51
0.00 –
Casual gain kW
2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14 2.14
application of heat recovery in detail requires a dynamic simulation package. However, by using binned outdoor temperature data together with the building heat loss coeffi cient and assumed casual gains, a reasonable comparative study may be undertaken. The outdoor temperatures for the suburbs of London (hourly data measured over 20 years) is given in the fi rst two columns of Table 2. This type of data can be readily obtained for most global locations from local meteorological offi ces. The basic building heat loss coeffi cient2
Heating load less casual gains kW
4.25 4.00 3.49 2.97 2.46 1.95 1.44 0.93 0.42 – – – – – –
R gR gB =gO
Counter-flow plus Cross-flow Plate Heat Exchanger
Outdoor air fi lter
Heat exchanger
Return air fi lter
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