A tube and shell heat exchanger for heat recovery from an industrial pilot plant is to be
designed, with a view to minimise its payback time. The outdoor air is drawn into the plant
while the indoor air is discharged, through the heat exchanger, to the environment. The use
of a heat exchanger, however, can only be justified if the energy savings are commensurate
with the capital and operational costs of such a device.
Currently the pilot plant is in operation 40 hours per week (8 hours per day, 5 days per
week). The volume of the pilot plant is 600 m3
. The plant is heated by oil which is burned in a
boiler (oil specific heat: 45 MJ kg-1
; oil density: 0.80 kg L-1
; oil price: £0.70 L-1
). The heating
efficiency of the boiler is 80%. The heating season is six months (from 20 October till 20
April) with a mean outside air temperature of 6oC during this period.
The complete system consists of the tube and shell heat-exchanger, a supply air fan and an
exhaust fan. Assume equal air flow rates on both sides of the heat-exchanger. Construction
material is aluminium (price: £1.6/kg). Overheads of 400% apply to the costs of aluminium
used in the heat-exchanger to obtain the total price of the heat-exchanger (including
manufacturing, assembling and installation costs).
The design specifications are as follows:
1. A temperature of 20
oC should be maintained inside the pilot plant.
2. The air in the plant should be refreshed with a rate of 3600 m3 h
-1
(6 times per hour), as
recommended by industrial standard 6281.
3. The allowable pressure drop in the extract side is 300 Pa. An air fan is already installed in
the facilities (no additional costs).
4. An electrically driven air fan should be installed to force fresh air across the tube bundle.
Electricity costs £ 0.08 per kWh.
5. Tube dimensions: 4 mm inner diameter, 6 mm outer diameter.
Objectives:
1. Suggest the best heat exchanger design based on the specifications provided. Estimate the
annular energy savings that would be achieved by such device.
