Experimental study on the vacuum load of low-temperature thermal desalination plant

Abstract

The objective of this paper is to present a methodology to predict vacuum load theoretically and to compare those results with the experimental values for validation especially the non-condensable gas load and escape water vapour load. The primary objective of this paper is to find out the accumulated effect of non-condensable gases on the vacuum load of low-temperature thermal desalination (LTTD) plant. Suggestions to control the NC gas and escape vapour load on the LTTD process were also discussed in this paper. Determination of an exact amount of vacuum load for the plant is significant, since vacuum system alone shares about 31% of total energy demand of the LTTD plant. Load contribution given by the gas mixtures such as escape water vapour, non-condensable gas and air leak in to the system was experimentally measured by conducting suitable experiments in a running plant of 100 m³/d capacity located in the Island of Agatti, UT Lakshadweep group of Islands, India. Study at Kavaratti plant of same capacity showed that the escape vapour rate was equivalent to 0.3% of the freshwater generation rate of the plant, which was used as an input value for Agatti vacuum load calculation for escape vapour rate. On comparison of the experimental results of non-condensable gas release rate of Agatti plant with the published data and predicted values using methodology, an agreement up to 16 and 7.7% was obtained, respectively, under the same operating conditions. Also, an agreement up to 15% was obtained between the experimental results of Agatti plant with that of observed results of Kavaratti for water vapour escape rate from the main process condenser. It was reported in the literature that for MED desalination system, the extraction of vapour from the evaporator unit linked with a removal of 10–20 units of vapour corresponded to every unit mass of NC gases. But in the present study, for the LTTD process, it was measured that the accumulated effect of NC gases for every unit mass resulted in extraction of 1.6 unit mass of vapour (approx) from the process condenser. This low value could be due to low operating temperature range of LTTD process and use of low-temperature deep-sea cooling water from the ocean in the condenser tubes. From calculation, it was noticed that decrease in molecular weight of gas increased the volumetric vacuum load for the same operating conditions and gas mass flow rates. Also, it was observed that the parameters such as operating pressure, duct loss, mass flow rate of feed water influence the total vacuum load of LTTD plant.

Keywords:

• NC gases
• Escape water vapour
• Air leak rate
• Pumping speed
• Vacuum load
• LTTD

Notes

Presented at Trombay Symposium on Desalination and Water Reuse, Mumbai, India, 22–23 January 2015