P. K. Nag

Second law analysis of a waste heat recovery steam generator

In recent years a great deal of attention is focussed on the efficient utilization of energy resources with minimum heat loss. There is a growing interest on second law analysis to minimize the entropy generation in various thermal units and thereby to improve and optimize the design and performance. In the present work, a waste heat recovery steam generator is considered, which consists of an economizer, an evaporator and a super heater. The unit produces superheated steam by absorbing heat from the hot flue gases. A general equation for the entropy generation has been proposed, which incorporates all the irreversibilities associated with the process. By using suitable non-dimensional operating parameters, an equation for entropy generation number is derived. The effect of various non-dimensional operating parameters, on the entropy generation number are investigated. The role of gas specific heat, non-dimensional inlet gas temperature difference ratio (τ), heat exchanger unit sizes (NTUB, NTUS, NTUE) on entropy generation number are also reported. The results will help to understand the influence of different non-dimensional operating parameters on entropy generation number, which in turn will be useful to optimize the performance of the unit.

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.

Performance simulation of heat recovery steam generators in a cogeneration system

The performance simulation of heat recovery steam generators in a cogeneration system has been investigated to see the effect of pinch and approach points on steam generation and also on temperature profiles across the heat recovery steam generator. The effects of operating conditions on steam production and also on exit gas temperature from the heat recovery steam generator are discussed. Low pinch point results in improved heat recovery steam generator performance due to reduced irreversibilites. The supplementary firing enhances the steam production.

Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross-section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright

A mathematical model for the prediction of heat transfer from finned surfaces in a circulating fluidized bed

Abstract A mathematical model has been developed to predict heat transfer coefficients on projected finned surfaces in a circulating fluidized bed (CFB). To validate the model, experiments were conducted in a 100 mm i.d., 5.15 m high CFB unit, in which heat transfer coefficients were measured for fins having rectangular and pin shapes. Experiments covered a range of superficial velocity from 5.6 to 11.4 m s−1, bed temperature from 66.5 to 91.5°C and for 310 μm sand particles. Heat transfer coefficients predicted from the model have been compared with those experimentally obtained and a good agreement is observed.

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s−1. Local sand of mean size (dp) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed.

Pressure drop and collection efficiency of cyclone and impact separators in a CFB

Experimental results of pressure drop and gas–solid separation of impact separators with three rows of impact blades with included angles (60°, 90° or 120°) in a cold CFB set-up of 102 mm×102 mm cross section are presented for two different samples of sand and they are compared with the same of a conventional reverse flow cyclone. The pressure drop increases with higher superficial velocity for all the solid separators including cyclone at a constant solid circulation rate. ‘Overall collection efficiency’ (ηoc) remains practically unaltered for the cyclone within the range of superficial velocity of the experiment while it decreases with higher superficial velocity for the impact separators. ‘Fractional collection efficiency’ (ηfc) of cyclone increases monotonically with grain size while that of the impact separator becomes minimum for an intermediate grain size. The collection efficiency of impact separator is comparable to that of cyclone for either very small or very large particles. Copyright

Effect of lateral and extended fins on heat transfer in a circulating fluidized bed

Abstract Experimental investigations are made in a hot circulating fluidized bed unit to study the effect of lateral, and lateral and extended fins together on heat transfer coefficient and on the rate of heat transfer. The circulating fluidized bed (CFB) unit consists of a riser column of 102 × 102 mm in bed cross-section (square), 5.25 m in height with the return leg of the same dimensions. Both liquid petroleum gas and coal are used as the fuel. A mathematical model is also proposed to predict the heat transfer coefficient to a water-wall test section with lateral and extended fins. The model results are compared with the present experimental data and also with those of published literature and a fair agreement has been observed.

An experimental investigation into the effect of fins on heat transfer in circulating fluidized beds

Abstract An experimental investigation into the effect of projected fins on bed to wall heat transfer in circulating fluidized beds at room temperature is reported. Experiments are conducted in two different test rigs at two locations, one at Indian Institute of Technology. Kharagpur, India and the other at Technical University of Nova Scotia, Halifax, Canada, under different operating conditions. Experimental conditions include three sizes of sand (130, 280 and 310 μm), suspension densities in the range of 8–80 kg m −3 and velocities of 5–11 m s −1 . The overall heat transfer rate is found to increase in all cases within the above range of operating and design conditions due to the use of fins.

Effect of operating parameters on heat transfer in the lean phase region of a CFB riser column

The radial variation of the heat transfer coefficient across the bed width, including the effect of fins and fin shapes (surface roughness), on the heat transfer characteristics in the lean phase region of a circulating fluidized bed riser column are investigated. Three test sections (bare horizontal tube, V-fin tube and square fin tube) are employed for the investigation. The experimental unit consists of a riser column of 102×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions, and both made up of plexiglas columns. For the same operating conditions, the provision of fins results in a drop in heat transfer coefficient, but increases total heat transfer owing to increased surface area. The present experimental data are compared with published literature and good agreement has been observed. The experimental data also corresponds to the two-zone hydrodynamics (i.e. the core–annulus structure) of a circulating fluidized bed.