Rahim K. Jassim

Application of Exergoeconomic Techniques to the Optimization of a Refrigeration Evaporator Coil With Continuous Fins

An optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator is developed in this paper using the exergy method. The method is based on exergy, economic analysis, and optimization theory. As there are humid air and refrigerant single- and two-phase streams involved in the heat transfer process, then there are irreversibilities or exergy destruction, due to pressure losses IΔP, due to temperature difference IΔT and due to specific humidity gradient IΔω. These principal components of total irreversibility are not independent, and their relative contribution to total irreversibility of a cross-flow refrigeration evaporator is investigated. A change in geometry was obtained by varying the evaporator tube diameter for a selected evaporator capacity, and hence the evaporator tube length and total heat transfer area are calculated for a fixed evaporator face length. In this way, the effect of changes in the geometry on the total number of exergy destruction units of the heat exchange process is investigated. The optimum balance between the three components of irreversibility (IΔP,IΔT, and IΔω) is also determined, thereby giving the optimum solution for the heat exchanger area. The total cost function, which provides a measure of the contribution of the evaporator to the total cost of the refrigeration system, is expressed on the basis of annual capital and electrical energy costs. The total cost function is minimized with respect to the total heat transfer area and the total number of exergy destruction units (NI). The relationship between the operational variables, heat transfer area, refrigerant and air irreversibilities, and the total annual cost for this type of evaporator are developed, presented, and discussed. The pressure, temperature, and specific humidity irreversibilities are found to be 30.34%, 33.78%, and 35.88%, respectively, of the total irreversibility, which is 8.5% of the evaporator capacity.

Energy and exergy analysis of reverse Brayton refrigerator for Gas Turbine power boosting

In this study the use of Reverse Brayton cycle to boost up the power of gas turbine power plants operating in hot humid ambiance is analysed by the energy and exergy methods. The gas turbine inlet temperature is reduced by mixing the chilled air from Brayton refrigeration cycle and the main intake air stream reaching low intake temperatures. In this paper the effect of irreversibilities in the system components have been evaluated along with the exergetic power gain ratio, and the exergetic thermal efficiency change, of the cycle. The energy analysis results indicated that the intake temperature could be lowered below the ISO standard with maximum power increase up to 19.58% and appreciable decrease in the thermal efficiency (5.76% of the site value). Whereas, the exergy analysis approach showed that the power gain could only be 14.66% as a result of the components irreversibilities.

Exergoeconomic optimisation of the geometry of continuous fins on an array of tubes of a refrigeration air cooled condenser

An exergy method for optimising the geometrical parameters of continuous fins on an array of tubes in an air-cooled condenser is developed in this paper. The method is based on exergy, economic analysis and optimisation theory. The condenser is characterised by the single/two phase flow inside the coil and the two components of exergy destruction, IΔP and IΔT, for refrigerant and air sides are calculated for different geometrics. A change in geometry was obtained by varying the condenser tube diameter for a selected condenser capacity, and hence the condenser tube length and total heat transfer area are calculated for a fixed condenser face length. In this way the effect of changes in the geometry on the total number of exergy destruction units is investigated. Also, the optimum balance between the two components of destruction is determined thereby giving the optimum solution for the heat exchanger area. The total cost function is expressed on the basis of annual cost of capital investment and the cost of compensation for the irreversibilities. The results show that the effect of the total irreversibility is 3.885% of the condenser capacity.

Exergy Analysis of Combined Effect of Evaporative Cooling and Steam Injection on Gas Turbines Performance Enhancement in Hot and Humid Climates

In this paper, an exergy analysis has been used to examine for a combination of cooling the compressor intake air and inject steam in the combustion chamber the performance enhancement of gas turbine power plants using by a combination of intake air cooling the compressor intake air and injecting steam in the combustion chamber is studied. The limits of the cooling capability of an evaporative cooler are analyzed and formulated in terms of the characteristic dimensionless groups: the temperature ratio (ξ T ), the power gain ratio (PGR), Thermal efficiency change (TEC) and humidity ratio (RH). Similarly the effect of steam injected is presented as a ratio (y) of total volumetric flow rate. The effect of different pressure ratio (PR) is examined for Saudi Arabia summer weather when the turbine inlet temperature, T 3 , is a predetermined of 1373.15 K. The results for a specific example where the air evaporative cooler drops the temperature to the wet bulb temperature is presented and show the power gain ratio enhancement depends on the ambient temperature, relative humidity, evaporative cooler effectiveness and slightly on the pressure ratio. The results indicate for PR =10, the PGR is enhanced by 9% at 20% relative humidity and drops to 3.37% at 60% relative humidity. The daily performance of the cooling method is examined for the hot humid conditions of Jeddah, Saudi Arabia. The results show that the evaporative cooler increased both the daily power output and the thermal efficiency by 2.52% and 0.112% respectively. In addition, the result shows that the combustion irreversibility is the dominant in the governing the system PGR and TEC with various steam injection ratios for PR=10.

Exergy Analysis of Carryover Leakage Irreversibilities of a Power Plant Regenerative Air Heater

Energy recovery devices can have substantial impact on process efficiency and their relevance to the problem of conservation of energy resources is generally recognised to be beyond dispute. One type of such a device, which is commonly used in fossil fired and air conditioning systems, is the rotary regenerator in which a stream of hot waste gas exchanges heat with fresh atmospheric air through the intermediate agency of a rotating matrix. As there are gas streams involved in the heat transfer and mixing processes, then there are irreversibilities, or exergy destruction, due to chemical reaction, pressure losses IΔP and due to temperature gradients IΔT . These principle components of total process irreversibility are not independent and there is a trade-off between them. Therefore the purpose of this research paper is to demonstrate the importance of the use of exergy analysis in the minimisation of carryover leakage irreversibilities of a symmetric balanced rotary regenerator. The chemical exergy Eo and physical exergy Eph are calculated and the ratio of chemical and physical irreversibilities has been evaluated for a rotary regenerator used for air preheating in a coal-fired power plant. A numerical finite difference technique has been used to calculate the fluid and matrix temperature distributions effect on the regenerator performance. The effects of variation of the principal design parameters on the irreversibilities and on the regenerator effectiveness are examined and recommendations are made for the selection of the most appropriate parameters.Copyright