Module 5. Pasteurizers

Lesson 18

18.1 Introduction

Continuous operation of HTST pasteurizer depends on the monitoring of temperatures, and thereby controlling the flow of milk in diverted or forward flow condition. Further, it is a legal requirement to record the temperature and time of operation of pasteurizer in many of the countries. Most of the instrumentation is now based on digital form and are much more compact as compared to earlier versions of Pneumatic controls.

Commonly used equipment for control are

1. Hot milk temperature indicator and controller

2. Cold milk temperature indicator

3. Hot milk and cold milk temperature recorder

4. Flow Diversion Valve

5. Visual and audio alarms

18.2 Diagram of Pasteurizer Controls


Fig. 18.1 Pasteurizer controls

The temperatures of hot milk or cold milk are sensed by suitable sensors. Common among them is the Resistance Temperature Device (RTD), which sends an electrical signal, proportional to the temperature sensed. This signal is received and compared with the Set Point of temperature by the Temperature Controller. Depending on the deviation of the temperature of milk from the Set Point temperature, an error signal is sent, which will range from 4 to 20 mA. The error signal is converted into a pressure signal (of compressed air), by Electro-pneumatic Converter. This signal which is in the form of pressure will operate the final element, like Diaphragm of Steam control valve.

In addition to the control of steam supply to hot water, the signals of sensors are also received by Temperature transmitter, which in turn sends a pneumatic signal to pen drive of Thermograph. The Thermograph is a circular graph, with temperature indicated on concentric circles; the radial lines indicating the time. The graph is placed on a disc, which rotates one full circle either in 12 or 24 hours. The graph forms a legal record of the time temperature combination applied to the processing of the milk.

The controller components are very much standard, and uniform, so that they can be replaced, once their specifications are known. The motive supply air must be 6 kg/cm2 and which is then reduced to the required pressure depending on the size of the valves to be operated. The air must pass through filter to avoid moisture and oil.

18.3 Flow Diversion Valve

The temperature sensor of hot milk also conveys signal to the Temperature controller, where, the signal is compared to the Set Point. If the temperature of milk is less than the Set Point, the electrical signal goes to the Solenoid valve, which stops the compressed air supply to FDV. The compressed air, which will be acting against the spring pressure, is now absent. The FDV valve has three openings, one for supply of milk, and two for outgoing milk. When the compressed air is acting, the FDV will be in Forward flow position, while, the absence of air, will push the valve plunger in the FDV to Diverted flow position. It must be noted here that, whenever electric power interruption is there for the plant, the FDV will be in Diverted flow position.

The signal that triggers the Diverted flow condition of FDV is also fed to the visual signal as well as to a hooter, to alert the plant personnel.


Fig. 18.2 Flow diversion valve

Last modified: Wednesday, 3 October 2012, 8:54 AM