For a successful performance of the evaporator, the instrumentation must be well established and all the critical parameters should be indicated on instruments to avoid malfunctions.
Some important instrumentation is:
- Flow meter for the raw milk
- Temperature of the raw milk
- Preheating temperature
- Pasteurizing temperature
- Heating and boiling temperature and pressures of each effect
Steam pressure available:
- Steam pressure to the thermo-compressor
- Cooling water temperature
- Warm water temperature
- Conductivity of condensate
- Density (solids content)
These instruments should be in a control panel centrally placed, preferably in a control room, to make operation easy. As to the automation and degree of automation there are many opinions. However, as the aim is a steady run without fluctuations the following control loops are recommended:
- Feed flow
- Pasteurization temperature
- Steam pressure for thermo-compressor
- Solids content
- Vacuum/last effect boiling temperature
The feed flow, i.e. the raw milk, is controlled automatically via a flow meter, which can either a conventional type, or better based on inductive magnetic measurement.
In order to be sure that the final powder has always been exposed to the wanted pas-teurization temperature, this temperature is controlled automatically in most cases. The system is simple and well known from plate heat exchangers where the steam pressure controls the milk temperature by means of a simple steam pressure reduction valve.
Steam pressure for thermo-compressor (TVR)
In order to avoid fluctuations in the steam pressure, this should be kept constant. By altering the steam pressure to the TVR it will change the amount and pressure/temperature of the "mix vapour". The evaporation rate of the first effect, and consequently of the whole evaporator, will change accordingly.
In order to produce a first class product the solids content should be kept constant. It should also be kept constant in order to avoid overconcentration resulting in viscosity increase with the risk of burnt deposits in the tubes or even blocking of the tubes.
The solids content can be regulated in different ways, but the first thing to consider is how to measure the solids content. This can be done by measuring the refractive index or the density.
The refractive index in milk concentrate is determined by the refractive index of lactose, minerals, fat and even the proteins and is a function of the concentration of each of the mentioned components. This means, however, that the measured sum is subject to changes according to seasonal variations in the milk composition. The factor (refractive index x F = % solids) is therefore not constant, and frequent changes of the factor should be made based on drying oven tests in the laboratory. The refractive index is converted to an electronic or pneumatic output suitable for the controller.
Another method is to measure the density of the liquid, as this will change directly proportionally with the solids content. The density of the concentrate is measured passing the concentrate through a fast vibrating U-tube. The vibrations will be reduced proportionally to the mass passed through. Thus it is possible to get an output which can be used in the controller.
It is now possible to discuss what to do with the output from the controller, i.e. what parameter to change to obtain a constant solids content:
The alteration of the milk flow means that the final amount of concentrate will alter. With high evaporation ratios (whey for instance) a small change in the flow would mean a tremendous change in the solids and amount of final concentrate, see Fig. 31. This method, even though being simple, is not recommended, as it may easily result in deposits due to coverage problems.
Fig. 31. Density control by means of feed flow
Steam pressure to the thermo-compressor
Regulating the steam pressure to the thermo-compressor is therefore more used, see Fig. 32. However, as the total residence time in an evaporator is 8-20 min. it takes a long time to get a response from a change. In large evaporators an extra control loop is therefore often placed after the second or third effect.
Fig. 32. Density control by means of steam pressure/flow
Back-mix of liquid pasteurized milk to the concentrate
The third solution is to run the evaporator at a somewhat higher solids content (1-2% higher) than necessary in the last effect but one, and then dilute with pasteurized milk prior to entering in the last effect. In that way there will be no fluctuation in the solids, as the response is quick, and overconcentration in the last effect is avoided, see Fig. 33.
Fig. 33. Density control by back-mixing liquid pasteurized milk or condensate
Vacuum/last effect boiling temperature
Fig. 34. Vacuum control, regulating the amount
of cooling water in a mixing condenser
The vacuum in the condenser and thereby in the last effect is controlled by the amount of cooling water or in some cases by the socalled "bleeding system", where more or less air via a valve is led into the condenser, i.e. the system runs with surplus water. The two systems are shown in Fig. 34 and Fig. 35.
Fig. 35. Vacuum control, bleeding system in a mixing condenser