From an evaporator with TVR recompression a substantial
amount of condensate is discharged with a temperature of 50-70ºC depending upon
from which stage of the evaporator it originates.
A simple method is to utilize this energy for preheating the
drying air. The condensate from the first effect of the evaporator with
temperature of about 70ºC is normally returned to the steam boiler, where it is
used as feed water. This is advantageous because it is warm and soft. But the
condensate from the last effects can be used. The temperature is typically
56ºC, see Fig. 96.
From a
four-effect evaporator operating in conjunction with a two-stage spray dryer at
a drying temperature of 230ºC, as described on page 139, about 12,000 kg/h
condensate at 56ºC can be obtained.
The drying air is preheated to 47ºC reducing the fuel oil
consumption to 170 kg/h, however, a bigger condensate pump and supply fan are
needed due to the bigger pressure drop across the system. Below mentioned
figures illustrate the total net saving:
Ambient air preheated from 10ºC to 47ºC
Condensate cooled from 56ºC to 32ºC:
Without Preheater:
|
Fuel consumption: |
205 kg/h |
|
Steam consumption: |
167 kg/h |
|
Electric energy
consumption: |
152 kW |
corresponding to 1,280
Kcal/kg powder
With Preheater:
|
Fuel consumption: |
170 kg/h |
|
Steam consumption: |
167 kg/h |
|
Electric energy
consumption: |
159 kW |
corresponding to 1,090
Kcal/kg powder or a net saving of 15%.
Another advantage is that the condensate is cooled to 32ºC
and can thus be led direct into a bacteriological waste water cleaning plant,
where too high temperatures are not allowed.
If we compare with the saving in a one-stage dryer it will
amount to about 32%.
In MVR
evaporators with preheating/heat treatment of the milk from 5ºC to 72ºC, the
applied energy through the compressor is just enough for the evaporation and a
discharge temperature of the concentrate at ≈50ºC and the condensate at ≈10ºC.
Only if a heat treatment higher than 72ºC - by using additional steam - is
wanted, the temperature of the condensate will be high enough, so that it can
be used for preheating the drying air.