Case study: new system vs existing (cost, manpower)

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.)

Block diagram of the process control system (existing)

 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 proposed closed-loop control:

 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

Why implement it (cost, manpower, automation)

• Cost: accurate temperature control (±0.5 C) saves steam and avoids over- or under-pasteurisation, so fewer batches are rejected and there are fewer quality complaints. Automatic logging saves the cost of paper records and avoids logging mistakes. Maintenance is cheaper because of the diagnostics.
• Manpower: one operator supervises the whole line from the panel/PC instead of one operator per section, and fewer operators are needed for manual valve control and gauge reading.
• Plant automation: automatic sequencing (start-up, CIP, filling), alarms and interlocks, remote monitoring, trend and production reports, traceability, and an easy path to later 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 after implementation

1. Initial investment and the need for training; operators may resist change.
2. Dependence on supply quality: power cuts and voltage fluctuation (use a UPS/stabiliser).
3. Noise and EMI from motors and ground loops affect the sensors (shielding and grounding are needed).
4. Sensor drift and calibration, and steam and moisture damage; periodic maintenance is required.
5. Software bugs, the need for skilled support or spares, and cyber and data security.
6. Single point of failure (reduced by a watchdog, manual override and redundancy).