The aim in a heat recuperator is to transfer the heat contained in the
outlet air, having temperatures of 80-95șC, from the spray dryer and to utilize
it e.g. to preheat the drying air. But it is also possible to use the
recuperated heat to heat water for cleaning purposes or air for heating rooms.
In the following only preheating of the drying air is discussed.
In principle, there are two different recuperating systems:
- Air-to-Air
- Air-Liquid-Air
Both systems are incorporated after the fines separator. However,
incorporating a bag filter prior to the heat recuperator increases the
efficiency, as deposits on the heat surface cannot be completely avoided even
with correctly selected air velocities in the dust-loaded air. It is possible
to operate the recuperator several days without cleaning, but should it prove
necessary to clean the equipment, this is done by means of a built-in CIP
system.
If the heat recuperator is designed to cool the outgoing air below the
condensation temperature (depending upon moisture content per kilo dry air),
also the condensation enthalpy is used for preheating. In this case a bag
filter is installed prior to the heat re-cuperator to avoid deposits in the
intermediate zone between the dry and wet area. Even bigger savings are then
possible than the ones described below, which are all calculated without
condensation.
Air-to-Air Heat Recuperator
In the heat recuperator type air-to-air, see Fig. 98, the drying air is
preheated by means of the outgoing air passing counter-currently over the heat
surface of the recuperator. This surface is formed as a number of tubes, inside
of which the outgoing warm air is passing while the cold air is passing on the
outside.
Fig.
98 Heat recuperator type air-to-air
The incorporation of this equipment in an existing plant may prove difficult
and ex-pensive, as it may require large and long air ducts from which part of
the recuperated energy is lost due to radiation, if the ducts are not
insulated. In new installations it is easier to incorporate this type of heat
recuperator, as the arrangement can be optimized with short air ducts.
The temperature to which the air can be preheated depends upon the
temperature of the outgoing air. Therefore, this type of heat recuperator is
most beneficial in combination with a one-stage spray dryer where the
temperature of the outgoing air is high. The figures mentioned below are based
upon a one-stage plant as mentioned in the table on page 139.
Ambient air preheated from 10șC to 52șC
Outgoing air cooled
from 93șC to 51șC:
Without Recuperator:
| Fuel consumption: |
175 kg/h |
| Electric energy consumption: |
120 kW |
corresponding to 1,595 Kcal/kg
powder
With Recuperator:
| Fuel consumption: |
140 kg/h |
| Electric energy consumption: |
135 kW |
corresponding to 1,305 Kcal/kg powder
or a net saving of 18%.
Air-Liquid-Air Heat Recuperator
Another system, more flexible regarding the installation, is the
air-liquid-air heat re-cuperator. This system is divided in two heat
exchangers, in between which a heat transfer liquid is circulated, for example
water. If, due to low air temperatures during winter, it may be expected
that the temperature of the water gets below zero, an anti-freeze agent is
added to the water. As the heat transfer co-efficient is higher for air-liquid
than for air-air, this system is more efficient than the air-to-air heat
recuperator despite the fact that two heat surfaces are needed.
The heat transfer surface placed in the outgoing air is formed as a bundle
of tubes inside which the dust-loaded air is passed. On the outside of the
tubes the water streams counter-currently. The heat transfer surface placed in
the inlet air is a normal finned tube heat exchanger. Water is recycled by
means of a centrifugal pump.
If a recuperator of this type is installed in a
one-stage spray dryer, the below mentioned heat savings are possible:
Ambient air preheated from 10șC to 60șC
Outgoing air cooled
from 93șC to 44șC:
Without Recuperator:
| Fuel consumption: |
175 kg/h |
| Electric energy consumption: |
120 kW |
corresponding to 1,595 Kcal/kg
powder
With Recuperator:
| Fuel consumption: |
130 kg/h |
| Electric energy consumption: |
142 kW |
corresponding to 1,225 Kcal/kg powder
or a net saving of 23%.
If indirect oil- or gas-fired air heaters are used, the heat transfer liquid
can - after the passage through the exhaust air heat exchanger - be passed
through a heat exchanger placed in the combustion air duct, whereby even
further savings can be achieved.