Performance evaluation of a small size TVC desalination plant

This paper is concerning the performance evaluation of small size thermal vapour compression (TVC) desalination units. The plant consists of two identical vapour compression units, each of a nominal capacity of 375 m3/d of distillate water. Despite the fact that these units are fairly new (two years in operation at time of evaluation), some operating problems were experienced. In this study, some important design factors and operating conditions are reviewed and the plant operating data were used to evaluate the unit performance. Conclusions derived from this evaluation are presented with emphasis on plant economy.

8. Quality of product water The quality of water produced by any desalina- In the light of results obtained by this study regarding the performance of the desalination Table 2 Operation cost analysis (at AF = 0.55) for unit A $/m3 distillate 1. Interest and amortization 2. Taxes and insurance 3. Steam consumed charges 4. Electricity consumed charges 5. Repaired and maintenance charges 6. Labour charges 7. Chemical charges 8. Overheads charges Total annual charges 1.38 0.42 1.28 0.19 0.14 0.16 0.07 0.16 As percent of product cost 36.27 11.09 33.65 4.97 3.70 4.26 1.79 4.26 3.7947 100.00 Table 3 Reference operation cost analysis (at AF = 0.95) for unit A $/m3 distillate 1. Interest and amortization 2. Taxes and insurance 3. Steam consumed charges 4. Electricity consumed charges 5. Repaired and maintenance charges 6. Labour charges 7. Chemical charges 8. Overheads charges Total annual charges As percent of product cost 0.84 0.26 1.28 0.19 0.09 0.10 0.07 0.10 2.9153 28.85 8.82 43.80 6.47 2.94 3.39 2.33 3.39 100.00 plant, the following conclusions could be drawn: 1. The units have experienced operating problems soon after start of operation, repeated shutdowns of the units resulted in loss of production and increase of costs. 2. The overall performance ratio of the units was found to be 70–80% of the design value. This was relatively low and had negative impact on plant economy. 3. The production cost of distillate water found to be $2.915/m3 of distillate water. A significant proportion of the cost increase was due to fixed costs and steam costs. 4. Inspection and observation of the units revealed that after about two years of operation the plant remained in good condition indicating the suitability of materials used in the construction of the units. In particular, no pitting of heat exchanger tubes has been reported. Other process equipment such as brine pipe, pumps and water boxes were found to be less resistant to corrosion problems. 5. For economical reasons, it is essential to operate both units at full capacity. 6. The use of additional steam during the cold season (winter operation) can be avoided by installing a heat exchanger to heat up the incoming cold seawater feed with hot reject brine.