Case study: workflow, factors, proposed design, benefits

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.)

(a) Existing workflow

 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

Milk is received, tested, chilled and pumped to the pasteuriser. The operator watches the thermometer and adjusts the steam valve by hand. The milk goes through the holding tube, is cooled, stored and then filled into pouches. Records are written manually.

(b) Critical factors and mitigation

(c) Proposed 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

• Controller: a microcontroller (on-chip ADC, UART, timers).
• Input devices: PT100 temperature sensor, level switch, flow sensor, keypad.
• Output devices: steam valve and pump (via optocoupler and SSR/relay), LCD, buzzer.
• Data converters: 12-bit ADC (0.037 C/count); optional DAC (4-20 mA) for a proportional valve.
• Communication: RS-485 Modbus RTU, 9600 baud, 8N1.
• Handshake signals: ADC start/end of conversion, LCD enable strobe, RS-485 DE/RE and master-slave polling.

/* Assumed: 8-bit MCU, ADC on channel 0 (PT100 amplifier), relay on PORTB.0 */
unsigned int read_adc(void){            /* 12-bit conversion */
    ADC_START(); while(!ADC_DONE());   /* start, wait for EOC */
    return ADC_DATA;                    /* 0..4095 */
}
void control(void){
    float t = read_adc() * 150.0 / 4096; /* temperature in C */
    if (t < 72.5) STEAM_ON();           /* open steam valve */
    else if (t > 74.0) STEAM_OFF();     /* hysteresis control */
    wdt_reset();                        /* watchdog */
}

The control band (72.5 to 74 C) stays above the 72 C minimum holding temperature.

(d) Advantages of the proposed plan

• Technology: digital, programmable, networked and easy to upgrade.
• Production rate: continuous automatic operation and less downtime (assume +10 percent).
• Quality assurance: tight temperature control and records.
• Cost-benefit and ROI: 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.