The maximum temperature that the brine can be heated affects the capital costs and the energy costs of the production of the potable water. The thermal efficiency is then dependent on the temperature range that the flashing occurs.
The sea water enters the condenser tubes in the last stage of the process at an initial temperature, t1. It is heated regeneratively by the condensing product until it exits at a temperature, t2. An external source heats it to t3and enters the first-stage flashing chamber. The brine is then heated to a temperature t4 until the outlet of the flash chambers.
The total heat product can be calculated using the following equations:
Wb x Cp(t3-t4) Q = ----------------- hv (t3-t2) hv q = ---------------- (t3-t4)
where Q = product water(lb/h) Wb = flow rate of brine(lb/h) Cp = specific heat of brine (Btu/lb F) hv = latent heat of vaporization (Btu/lb) q = heat input (Btu/lb product) t1, t2, t3, t4 = brine temperature
(Note: These equations ignore boiling-point elevation, non-equilibrium and heat losses.)
The only method to reduce the external energy requirements is to increase the flashing range (t3-t4). This is because the temperature rise is limited byfactors such as the pressure loss of the vapor and the economics of a reasonable difference in heat-transfer. Increasing the flash range reduces theamount of brine that can be brought through the process, decreasing the flow rate and the capital cost of the plant.