Condenser Fouling in a 100 TR Cold Store
A 100 TR ammonia cold store maintains a fish chill room at −10°C evaporating temperature. The evaporative condenser is designed for 40°C condensing temperature with 27°C wet-bulb ambient. Under clean conditions the measured compressor power is 110 kW at full load and the plant COP is 3.2. After two seasons, the condenser tubes are scaled; the condensing temperature at the same load rises to 46°C, while the evaporating temperature remains −10°C. The maintenance contractor quotes ₹1.2 lakh for chemical cleaning and a 48-hour shutdown. The plant manager instead proposes adding a second compressor of 90 kW input, citing reliability. The store operates 8,000 hours per year; energy is charged at ₹8.50/kWh. Assume the plant's second-law efficiency relative to the Carnot cycle remains unchanged between the two condensing temperatures. 1 TR = 3.517 kW.
1. Calculate the Carnot COP at 40°C and 46°C condensing, the second-law efficiency, and the estimated COP at 46°C.
Read the worked answer
Evaporating temperature = 263.15 K. At 40°C condensing (313.15 K), COP_Carnot = 263.15/(313.15 − 263.15) = 5.263. Second-law efficiency = 3.2/5.263 = 0.608.
At 46°C condensing (319.15 K), COP_Carnot = 263.15/(319.15 − 263.15) = 263.15/56 = 4.699.
Assuming the same second-law efficiency, COP_actual = 0.608 × 4.699 = 2.857. This is a drop of about 10.7% from 3.2. The calculation shows that the loss is not a compressor fault; it is the thermodynamic penalty of rejecting heat at a higher condensing temperature.
2. Quantify the annual energy and cost penalty and compare it with the cleaning quotation. What is your recommendation?
Read the worked answer
Cooling capacity = 100 × 3.517 = 351.7 kW.
At COP 3.2, compressor power = 351.7/3.2 = 109.9 kW. At COP 2.857, power = 351.7/2.857 = 123.1 kW. Extra power = 13.2 kW.
Annual extra energy = 13.2 × 8,000 = 105,600 kWh. Annual extra cost = 105,600 × 8.50 = ₹8.98 lakh, approximately.
The cleaning cost is ₹1.2 lakh. Simple payback = 1.2/8.98 × 12 ≈ 1.6 months, even before counting the 48-hour shutdown. Cleaning is clearly justified. Also check condenser water quality, scaling control, and approach temperature after cleaning; a restored approach of about 13 K above 27°C wet bulb is consistent with the design 40°C condensing condition.
3. Is adding a second compressor technically sound as the primary response? Discuss the condenser, part-load operation, and reliability.
Read the worked answer
No. A second compressor does not remove the cause of high condensing pressure; it adds more heat rejection through the same fouled condenser and can make the condensing temperature worse. At the same evaporating temperature, the existing compressor is already penalised by the 46°C condensing condition, and a second machine would run at the same poor COP unless the condenser is restored or enlarged.
If the store needs redundancy, that is a separate reliability decision and should be justified by load growth or failure consequences, not by fouling. The sequence should be: clean the condenser, verify approach temperature, check purge and non-condensable gases, review water treatment, and then measure the restored COP. Only after the condenser is clean should the plant evaluate whether additional compressor capacity is needed for pull-down or redundancy.