The directive from the owner was to upgrade in phases the existing refrigeration plant for higher efficiency and safety, involving reasonable investment. Many a times plant upgrade or modernisation leads to complete change of plant whereby heavy investment and payback estimated at 3 to 4 years are estimated. Hence, it was decided that plant improvement be based on green design aiming to achieve highest efficiency and safety management plan. The objective was to keep RoI period less than a year plus achieving minimum 15 per cent energy savings. The refrigerant used for plant is pure Ammonia (earns a high environmental mark with ODP & GWP being Zero) which breaks down to its natural components of nitrogen and hydrogen in a relatively short time. The facilities’ refrigerating system is operated through natural (gravity) flooded feed.
Noted Prevailing Problems
A systematic study was made to accordingly implement the above. The primary step was to prepare a HAZOP report and investigate operational plant problems on day to day basis. We proceeded by recording plant operation parameters and operating hours. After careful study, following problems were observed in the existing refrigerating system:
1. The chiller rooms and freezer rooms are not able to achieve desired temperature in spite of long running hours of compressors.
2. All unit compressors including standby were required to remain in operation in order to meet the required refrigeration capacity.
3. The compressors were running full load at all times.
4. The compressor suction pressure and plant evaporating temperatures did not correlate. The suction pressure was much lower than the corresponding room temperature.
5. The compressor discharge pressure was high considering the ambient conditions.
6. Complete plant had to be operated manually.
7. The air cooler coils for chiller (cold) rooms and freezer rooms were frosted.
8. Although the cold room and freezer rooms were designed at – 25°C room temperature, however the best temperature achieved would be below -14°C.
9. The defrosting system was manual and was never able to defrost the coils completely. 10. Plenty of oil was getting accumulated in ACU or freezer coils.
11. Plant safety management was non-existent.
12. The level control system was bypassed and operators were manually throttling the valves on receiver supply line.
13. Freezer air cooler coils were starved for liquid supply, but operators were afraid of liquid surge to compressor.
14. Operators were kept occupied by operating various valves and using all efforts to check operation, temperature and liquid level.
15. Due to small leakages through flange joints, strong ammonia odour was continuously felt in the machine room.
16. The chiller rooms and freezer rooms were located around the plant and many times the loader would be trapped in the room, thus, preventing the trapped person to freely communicate with plant operators.
Energy-Efficient Measures & Novel Solutions
Above conditions lead to high energy consumption and loss of production while overall plant was unable to perform at designated requirements. Also considering that the allocated budget to rectify the above was quite limited and in view of time constrain, it was decided to concentrate on achieving desired room temperature, increase plant efficiency and improve plant safety at shortest period of time.
Another constraint faced was limited job site availability of highly skilled and certified manpower to operate the plant. Hence, it was required to provide an automatic operating system which can be easily handled by professional plant operators.
The following functions were performed to update plant operation, smoothly and trouble-free:
1. Installed dual safety (relief) Valves on all pressure vessels with required pressure ratings.
2. Calibrated compressor safety cut out, repaired and re- connected for safety.
3. Installed (on each compressor) easy to use automatic compressor control system with energy monitoring.
4. Installed fully automatic air purge on condenser and liquid receiver circuit.
5. Installed automatic hot gas defrosting system (replacing the existing manual defrost) on all air cooling units in chiller rooms and freezer rooms.
6. Installed temperature monitoring and control devices for all cold rooms and freezer rooms.
7. The automatic level control system was serviced and put into use.
8. Installed the reflex type level gauges and removed the glass tube one.
9. Installed an integrated automatic ammonia leak detection system.
10. Replaced all flange type valves were replaced with 40 bar weld in-line valves. The valves were chosen with back seating facility.
11. The chiller room alarm system with built-in battery back-up was installed on machine room doors for trapped loader and generate alarm in plant room. This unit was incorporated with inbuilt battery backup so that it can work independently in the event of power outage.
12. The overall above system improvement is anticipated to provide a Low Life Cycle Analysis (LLCA) and low carbon emissions where the refrigerating system gets greener and the energy savings add to the bottom line footprints.
Above performance was successfully implemented within 10 working days. This was done together with the valuable assistance of plant operators and one certified welder, without any downtime or disturbing status quo of plant operations. Once the required changes were in place and analysis performed against baseline and prevailing industry standards, the positive results achieved on the same was evaluated as follows:
1. The compressor discharge pressure reduced significantly to 160 PSI from 220 PSI.
2. The automatic operation of compressor units removed operator interference and resulted in smooth loading / unloading of each compressor. Additionally, overall energy requirement for compressor was reduced significantly.
3. The safety valves and release system ensured increased safety at plant and no discharge of ammonia in plant in case safety valve pops up.
4. The automatic ammonia leak detection and alarm system increased plant safety meeting OSHA’s PSM requirements extending operator flexibility in working around the plant.
5. The automatic hot gas defrost system replaced manual defrost operation.
6. The defrost time was reduced to 15 minutes instead of 45 minutes.
7. The increase in cold room or freezer room temperature during defrost reduced to 2°C from 10°C.
8. The cold room or freezer room design temperature of 25°C was successfully achieved.
9. The time required for freezer operation reduced 25 per cent.
10. The number of compressors required reduced. The standby compressor remain as standby, was never required to operate.
11. Reducing compressor running hours by 25 per cent.
12. The automatic level control system made sure that ACU or freezer coils are flooded thus, preventing liquid slop- over to the compressor.
13. The automatic level control system avoided operator’s interference by throttling valves on receiver supply line.
14. Oil accumulation in ACU and freezer units was eliminated.
15. Frosting on ACU and freezer units eliminated.
16. The online data logging and remote monitoring system installed helped customer to monitor the plant on mobile phone while travelling abroad and enjoy his holidays.
17. The temperature control system made sure that required temperatures are maintained continuously. No under shooting or overshooting observed. All temperature was maintained within ±2°C.
18. The weld in lines valves eliminated the leakages through flange joints of the valves.
19. The back-seating facility in the valve assured operators that now they don’t have to pursue in tightening valve glands.
Plant piping before modification
After observing a trouble-free plant operation for a period of one year, an estimated 30 per cent energy saving was derived with an overall payback recovered within four months. Additionally, it also improved product (ice cream) quality and production capacity as compared to (prior to above improvement) previous year.