(a) Explain the functions of three types of superheaters used in power boilers. Sketch the heat addition process in them on a T-s…

(a) Superheaters and desuperheater. In a power boiler the superheater raises saturated steam to a higher temperature at essentially constant pressure, increasing cycle efficiency, reducing moisture at turbine inlet and increasing the available enthalpy drop. A radiant superheater is mounted in the furnace or near the flame; it receives mainly radiant heat, gives high initial superheat and tends to give higher outlet temperature at low load, so it is often used for the first stage of superheating. A convective superheater is placed in the flue-gas pass; its heat transfer rises with gas velocity and load, so outlet temperature increases with load and it is useful for final superheat and load following. A combined radiant-convective superheater uses both heat-transfer modes to obtain a stable outlet temperature over the load range. On a T-s chart the boiler heat addition is drawn at constant pressure: saturated liquid to saturated vapour is the horizontal evaporation line; from the saturated-vapour point the radiant section is the first upward superheat branch, the convective section is the next branch at higher entropy, and the combined unit is a single continuous branch joining the two. A desuperheater sprays water or low-pressure steam into the superheated steam to reduce its temperature, protecting turbine blades, controlling turbine-inlet temperature and maintaining efficiency during load changes or startup.

(b) Cogeneration. Cogeneration, or combined heat and power, produces electricity and useful process heat simultaneously from one fuel input. The schematic is: boiler → high-pressure turbine → generator; a steam extraction line or back-pressure exhaust from the turbine is taken to a process heater or industrial load; the remaining steam may expand in a low-pressure stage to a condenser, and condensate is returned to the boiler. In extraction type, part of the steam is bled off at intermediate pressure; in back-pressure type, the turbine exhaust itself is used for process heat and no condenser is required. The high-grade heat is first converted into work in the turbine; the residual heat, which would otherwise be rejected to a condenser, is recovered at a lower temperature for heating, drying, absorption chilling or domestic use. This energy cascade raises overall plant efficiency, expressible as (W_e + Q_useful)/Q_fuel, because the same fuel energy serves both electrical and thermal demands, and it reduces fuel consumption and emissions compared with separate power and heat generation. In India such arrangements are common in industrial plants, bagasse-fired sugar-mill units and steam cogeneration where process steam is available.

(c) Refrigerator calculation. At 101.3 kPa the table gives T_L = -26.3°C, h1 = hg = 382.16 kJ/kg and s1 = 1.7453 kJ/kg-K. At 1200 kPa, interpolating between 45°C and 50°C gives T_H ≈ 46.3°C and hf ≈ 266.1 kJ/kg. The throttling valve is isenthalpic, so h4 = hf = 266.1 kJ/kg. The question supplies the reversible-compressor outlet enthalpy as h2 = 430 kJ/kg; this is just above the saturated-vapour enthalpy at 1200 kPa and represents the small superheat at the compressor exit. The refrigeration effect is q_L = h1 - h4 = 382.16 - 266.1 = 116.1 kJ/kg. The compressor work is w_c = h2 - h1 = 430 - 382.16 = 47.84 kJ/kg. Hence cycle COP = q_L/w_c = 116.1/47.84 ≈ 2.43. For a 500 W refrigeration load, electrical power = 500/2.43 ≈ 206 W. The corresponding refrigerant mass flow is 500/116100 ≈ 0.0043 kg/s. The specimen at -20°C is warmer than the evaporating refrigerant at -26.3°C, so heat transfer from specimen to evaporator is possible; the specimen temperature is not the cycle T_L and is not used in the COP. As a check, Carnot COP between 246.8 K and 319.4 K is about 3.4, so the calculated COP is physically consistent; it is below Carnot because the expansion valve is irreversible.

(d) UHC emissions. SI engines emit higher unburnt hydrocarbon emissions than CI engines. In SI engines the charge is premixed and combustion is initiated by a spark; local flame quenching near cylinder walls, in valve crevices and in the piston-ring crevice allows fresh mixture to escape oxidation. Cold start, misfire, fuel-rich pockets, poor atomization of liquid fuel and absorption of fuel in the oil film also leave HC. CI engines operate overall lean with high compression and diffusion combustion, so more fuel is burned; nevertheless HC is produced by fuel trapped in crevices, oil-film absorption, incomplete or over-mixing in the diffusion flame, and quenching near walls. Thus the dominant difference is that SI engines have larger quench and crevice losses relative to their premixed charge, while CI engines have lower UHC but not zero.

(e) Air washer. An air washer is a chamber in which air is brought into contact with sprayed water. A fan draws air through the chamber; nozzles atomize water from a recirculating sump; a pump returns water to the nozzles; eliminators remove entrained droplets; and a cooler or heater controls the spray-water temperature. On a psychrometric chart the possible processes are selected by the water temperature relative to the air dry-bulb and dew-point temperatures. If the water is below the dew point, the air is cooled and dehumidified, moving down and left, because moisture condenses and latent heat is removed. If the water is between the dry-bulb and dew-point temperatures, the air is cooled and humidified, moving left and upward, because evaporation cools the air while increasing moisture content. If the water is at the wet-bulb temperature, the washer operates by adiabatic saturation, with no net heat exchange with the surroundings and the air approaching the wet-bulb state. If the water is above the dry-bulb temperature, the air is heated and humidified, moving up and right, by sensible heat transfer and evaporation. Thus the air washer is a single heat-and-mass-transfer device whose conditioning process is selected by the spray-water temperature.