• Preheating the Drying Air with Condensate
• Preheating the Drying Air with Vapour from the Evaporator
  

Preheating the Drying Air with Condensate

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.


Fig. 96 Preheating by means of condensate

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.

Preheating the Drying Air with Vapour from the Evaporator

The vapour from the last effect of the evaporator (temp. 45-50ºC) is usually passed through a tubular heat exchanger, where the vapour is cooled by means of the cold incoming product, which is reheated. The precooled vapour is passed on to a condenser, where it is condensed using cooling water either from a cooling tower or a natural water reservoir.

It is an obvious idea to utilize the drying air instead of cooling water in a heat exchanger for condensing the vapour simultaneously preheating the drying air.
In the four-effect evaporator operating in conjunction with the two-stage spray dryer at a drying temperature of 230ºC, as described on page 145, there is approximately 800 kg/h vapour with a temperature of about 47ºC available. However, by means of 31,500 kg/h air at 10ºC it is only possible to condense 400 kg/h.

Therefore, an additional condenser, operated with water, is necessary. Another inconvenience of this system is that the additional condenser should have sufficient capacity to cope with the extra duty in case the ambient temperature increases. Further, during start-up of the evaporator it is necessary to have at least the fans of the spray dryer operating to get stable conditions in the evaporator. Therefore, this way of saving energy in the spray dryer is not used in practice, but only included, as there is a patent describing it as a possibility.