Gained Output Ratio

Gained Output Ratio or GOR is a measure of how much thermal energy is consumed in a desalination process.

The simplest definition is as follows; How many kilograms of distilled water are produced per kilogram of steam consumed? Typically the value of GOR ranges from 1 to 10 kg/kg. Lower values are typical of applications where there is a high availability of low value thermal energy. Higher values, even up to 18, have been associated with situations where local energy values are very high, when the local l ocal value or need for water is high or a combination of both. GOR should be considered at the design stage of a desalination system when the quantity and economic value of energy and water can be used to compare the capital and operating costs of units with different GORs. Typically higher GOR systems cost more but consume less energy and therefore have lower operating costs (at least the energy component of operating cost is lower). Some other considerations; For multiple effect (ME) systems the GOR is directly related to the number of effects Since: GOR= 0.8 n. More effects directly increases GOR and For systems using thermo-compressors thermo-compressors the GOR is also impacted by the pressure of the steam. Higher pressure steam will recycle more process vapour within the ME part of the process thereby improving the GOR and reducing external enthalpy requirements. Please note that ME process efficiency and system cost tends to increase when Top Brine Temperature (TBT) is increased since more effects can be accomodated within the overall operational temperature difference. Conversely this condition leads to increased overall pressure difference reducing the effectiveness of adding a thermocompressor. Thus when ME and thermocompressors are merged the resulting mix of temperature and pressure driven systems leads to complex design optimization. D/ For multi-stage flash (MSF) systems the GOR is indirectly related to the number of flash stages. Typically for cross tube MSF the number of stages will be approximately 2.5 to 3 times the GOR value. For long tube this is typically 3.5 to 4.0 times the GOR value. This is because the incremental cost of adding a stage to the long tube configuration (during the design phase) is lower than for the cross tube design. E/ When more than one steam pressure will be utilized, or when the desalination system will be integrated with another thermal cycle, it may be more practical to consider how much water is produced per unit of energy

consumed. This is often referred to as Economy or Performance Ratio; see below ECONOMY or Performance Ratio The most common version of Performance Ratio was developed from the US Customary Units version of GOR; lb of water per lb of steam. It is not uncommon to assume each pound of steam has an average enthalpy of 1000 British Thermal Units (BTU), hence;

Performance Ratio = lb of water produced per 1000 BTU of heat consumed. The metric version has been adopted by industry to be Performance Ratio = kg of water produced per 2326 kJ of heat consumed. It is still important for all parties, client, vendor and engineer, to agree on which water and steam streams are considered in the calculation. It is often most expedient to ignore steam consumed for vacuum systems. Care must be taken to ensure condensate which is not returned is properly accounted for in heat and mass balances.