MOD ST ELow OCI ESSE N.B. All pressures are absolute, l. A boiler uses coal at t
ID: 1842173 • Letter: M
Question
MOD ST ELow OCI ESSE N.B. All pressures are absolute, l. A boiler uses coal at the rate of 3 000 kgh in producing steam with a specific enthalpy of water with a specific enthalpy of 280 kJkg. The combustion of 1 of coal which 80% is useful in producing steam. Calculate the rate at which steam is produced. (27 800 kg/h) 2. Fluid with a specific enthalpy of 2 280 ku/kg enters a condenser at the rate of 4 500 kg/h. and leaves with a specific enthalpy of 163 kl/kg If the enthalpy of the cooling water circulating through the condenser tubes increases at the rate of 148 000 kJ/min, determine the rate at which heat energy nows the condenser to the atmosphere. 3. Steam flows through a turbine stage at the rate of 4 500 kg/h. The steam velocities at inlet and outlet are 15 m/s and 180 m/s respectively. The rate of heat energy flow from the turbine casing to the surroundings is 23 kJ/kg of steam flowing. the specific enthalpy of the steam decreases by 420 kJ/kg in passing through the turbine stage. calculate the power developed. (476 kW) 4. An air compressor requires 11.5 kW to deliver air at the rate of 2.3 kg/min. The compressor is cooled by water whose specific enthalpy rise during the process is not to exceed 35 kJ/kg. Assuming that the change in the kinetic energy of the air is negligible and that the heat energy loss to the atmosphere is negligible, determine the flow of cooling water required. The following data relate to the state of the air at inlet to and outlet from the compressor: Specific volume Specific internal Pressure 10 237 0,185 (208 kg h) 5. A nozzle is supplied with steam having a specific enthalpy of 2780 k./kg at the rate of 9.1 kg/min. An outlet from the nozzle the velocity of the steam is 1070 m/s. Assuming that the inlet velocity of the steam is negligible and that the process is adiabatic, (a) The specific enthalpy of the steam at the nozzle exit is 18.75 ms/kg (b) outlet area required if the final specific volume of the steam (2 208 kJ/kg; 2660 mm2) 6. Fluid at 10.35 bar having a specific volume of 0.18 m /kg is throttled to a pressure of 1 bar. If the specific volume of the fluid after throttling is 0.107 m3/kg, calculate the change in specific internal energy during the processExplanation / Answer
Answer to the question no 1 -
We have,
Heat generated by burning of 1 kg coal, Qtotal = 28000 kJ
Out of which 80% is useful, Quseful = 28000 x 0.8 kJ
.: Quseful = 22400 kJ______________ (equation 1)
Now, it is also given that the boiler uses 3000 kg/h of coal
.: Heat energy supplied in one hour is (from equation 1), Q1hr = 22400 x 3000 kJ
.: Q1hr = 67,200,000 kJ/h ____________ (equation 2)
Further, it is given that
Specific enthalpy of steam, hsteam = 2700 kJ/kg,
and specific enthalpy of water hwater = 280 kJ/kg
As water possesses the given amount of energy already, it needs to be subtracted from the energy of the steam.
.: Heat required to generate 1 kg of steam, Qsteam = 2700-280 = 2420 kJ/kg_______ (equation 3)
And finally, from equations 2 & 3, we get the amount of steam generated in one hour.
.: mass of steam, msteam = 67,200,000 / 2420
.: msteam = 27768.6 kg/h
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