Comparison of cooling applications used in Oil Cooling Systems in the Plastics Sector
The plastics industry, from which the sample application explained in the previous section was selected, is one where cooling applications are at great demand and where Free Cooling applications are commonly used. For this reason, informed selection of methods to be used in cooling applications is required.
The cooling of moulds used in manufacture is important particularly for product quality. The moulds need to be kept within specific temperature ranges according to the type of plastic. Surface roughness is high and discolorations may be observed for products manufactured in moulds that are not properly cooled. Furthermore, inadequate cooling leads to extension in the opening and closing times of moulds and diminishing of production capacity. Since in cases where the ambient wet bulb temperature exceeds the temperature of the required cooling water the remaining cooling systems are not able to meet the cooling demands, the chiller group needs to be brought online to aid in mould cooling.
For machines to operate in a problem free and efficient manner, the oil used must be cooled properly as well. Otherwise, performance drops and abrasion increases. In this case, in addition to an increase in energy loss, the lifespan of the machine will shorten as well. These problems can be avoided by proper cooling of oil. Cooling water around 29-35ºC is required for oil cooling. For economic assessment of oil cooling systems, operational costs should also be taken into consideration as well as initial investment cost. In selection of cooling alternatives, the choice that offers the greatest mid- term and long-term advantages as compared to others should be determined.
Results of the analysis made for the comparison of open and closed circuit water towers and Wet-Dry Cooler System capable of meeting the same requirements have been given below. 630 kW of heat is taken from the condenser of a cooling device. The inlet temperature of the 70%-30% water/glycol mixture into the cooler is 30ºC, and the outlet temperature of the same is 26ºC. The ambient temperature and relative humidity have been taken as 32ºC and 38% respectively. For the analysis the unit price of water and the unit price of electricity have been taken as 1.98 €/m3, and 0.09 €/kWh respectively including VAT and similar additions. For the calculation of capital expenditure the annual interest rate applicable to the Euro has been taken as 10% and it has been assumed that all three systems will be used for 15 years (It is also important to bear in mind that the lifespan of even towers of galvanised coating have an economic life of 5 to 10 years due to their vulnerability to corrosive gases and other reasons).
THE SYSTEM EXPENDITURE
|
Open Circuit Water Tower
|
Closed Circuit Water Tower
|
Wet-Dry Cooler
|
Initial Investment Cost (€)
|
4,350
|
9,500
|
23,500
|
Water Expenses (€/year)
|
31,300
|
31,300
|
4,600
|
Electricity Expenses (€/year)
|
5,900
|
7,020
|
11,260
|
Maintenance Costs (€/year)
|
510
|
640
|
510
|
Interest Expenses (€/year)
|
570
|
1,250
|
3,090
|
Annual Total Cost of Operation (€/year)
|
38,280
|
40,210
|
19,460
|
Table 4. Economic comparison of water towers and the Wet-Dry Cooler System
Annual total cost of operation is the sum of water, electricity, interest expenses and maintenance costs. According to the above graph, the Wet-Dry System amortisation period the difference in initial investment cost with the open circuit water tower in 1 year and the difference in initial investment costs with the closed circuit water tower in less than 1 year. At the end of these periods, the Wet-Dry Coolers become economically more favourable than other systems. Since the figures used for the calculation of expenses will vary depending on the location and time of operation of the facility, the above table should be used only as a general comparison. In places where water is plentiful and cheap, the initial investment cost may be a more important criterion than water expenses. However, in places where water is scarce and expensive the selection of the system offering the greatest economy of cooling water will be beneficial.
In the following table a chiller group meeting the 560 kW cooling requirement is compared to a Wet- Dry Cooler System capable of the meeting the same requirement in terms of monthly (over 30 days) energy expenditure. The fluid passing through the cooler needs to be cooled from 35ºC to 31ºC. The ambient temperature and relative humidity have been taken as 33ºC and 48% respectively. It has been assumed that both systems operate 16 hours/day. The unit price of electricity has been taken as 0.09 €/kWh including VAT and other additions.
The Chiller Unit
|
The Wet-Dry Cooler System
| |
Compressor power: 136 kW
|
Power drawn from 16 fans:
32 kW
|
Power drawn from 10 fans: 20 kW
|
Total power: 168 kW
|
Total power: 20 kW
| |
Monthly energy expenditure:
80,640 kWh
|
Monthly energy expenditure:
9,600 kWh
| |
Monthly expenditure:
7,258 Euros
|
864 Euros
| |
Table 5. Economic comparison of the Water Cooling Group (Chiller) and the Wet-Dry Cooler System
According to the preceding table, in case the Wet-Dry Cooler System is used, the monthly gain can be calculated as 6,395 Euros. It should be kept in mind that when only coolers are taken into consideration, to the exclusion of water installation and similar investments, the investment cost of the used chiller group is approximately 61,355 Euros and that the Wet-Dry Cooler can be obtained in exchange for an investment of 27,600 Euros. In conclusion, the Wet-Dry Cooler System is more advantageous than the chiller group both in terms of initial investment cost and operational costs.
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