Case study: current control and recommendation

Assumed case study: a milk pasteurisation and packaging section of a dairy plant (HTST process: raw milk tank, heat exchanger, steam-heated holding tube, cooling, filling). The existing system uses analog panel indicators, bimetal thermostats, a manual steam valve and operator logging. (If your own visit was to another plant, replace the process names; the structure of the answer stays the same.)

Current control mechanism

 Raw milk tank --> pump --> heat exch. --> holding tube
   (level sw)      (motor    (steam valve    (72 C, 15 s)
                    starter)  manual)           |
                                        cooling --> filling
 Panel meters / thermostats  <-- operator reads, adjusts

The operator reads the thermometer and the panel meter and opens or closes the steam valve by hand to hold 72-75 C. A bimetal thermostat gives a crude over-temperature trip, and the pump runs by push-button. Level is observed visually and data is written in a logbook. There is no feedback loop, no alarms and no records.

Recommendation

Replace manual control with a microcontroller-based closed-loop system:

 PT100 --> Amp/SigCond --+
 Level sensor ----------+--> MUX/ADC (12-bit)
 Flow sensor ---(pulse)-+         |
 Keypad ----------------+---> [Microcontroller] <--> LCD 16x2
                               |  ARM/AVR + flash   |
   Opto + SSR/relay <----------+  watchdog, timers  +--> RS-485
     |                         |                          (Modbus)
 Pump / steam valve / alarm    +--> EEPROM log         PC / SCADA
 Power supply: 24 V + 5 V + UPS

• A PT100 sensor with a signal-conditioning amplifier and a 12-bit ADC for temperature.
• A controller using ON/OFF control with hysteresis (or PID) to drive the steam valve through an optocoupler and relay/SSR, plus a flow-diversion valve that sends milk back if the temperature is below 72 C.
• An LCD and keypad for set-point and alarms, and an RS-485 link to a PC for logging.

Key figures (calculated): a PT100 amplifier scaled 0-150 C to 0-5 V and a 12-bit ADC give a resolution of 150/4096 = 0.037 C per count (1.22 mV per count at 5 V), enough for the ±0.5 C pasteurisation limit. An 8-byte Modbus-style frame at 9600 baud takes 8 × 10 bits / 9600 = 8.3 ms, so every point can be updated several times per second.

Why management should accept it

• Better product quality and food safety (consistent pasteurisation, traceable records for standards such as HACCP).
• Savings in steam, labour and rejected milk, so the payback is short.
• Less dependence on operators and fewer breakdowns, with the possibility of expansion.

Cost-benefit (assumed figures): investment Rs 12,00,000 (controller panel, sensors, valves, SCADA PC, installation, training). Monthly savings: manpower Rs 60,000, steam/energy Rs 35,000, reduced rejection Rs 20,000 = Rs 1,15,000. Payback = 12,00,000 / 1,15,000 ≈ 10.4 months; yearly return = 13.8 lakh on 12 lakh, about 115 percent.

Problems expected after implementation

Operator training and acceptance, calibration and maintenance of sensors, electrical noise and power quality, software faults, spare-part availability, and the need for a manual bypass when the controller fails.