Power Engineering - 2 - Mechanical Engineering MCQ

Reheating is a method of increasing the mean temperature of heat reception. In this the gas is again heated after it has expanded in the gas turbine. This increases the work output of the turbine by keeping the compressor work constant. By reheating the mean temperature of heat rejection is also increased, resulting in a decrease in the thermal efficiency of the plant.

The flame does not leap out during opening of inspection door because there is negative pressure in the combustion area. Negative pressure is achieved in balanced draft boilers by using forced draft fans to force air into boiler (pressuring it) and Induced draft fans to remove air from the boilers to create negative pressure

Boiler rating is the heating capacity of a steam boiler. It is expressed in BTU per hour (BTU/h), horsepower (hp), or steam output in kg/hr at 100°C’ and atmospheric pressure. Each kilogram of steam receives 2257 kJ of heat from the boiler.

The rating indicates the maximum capacity of a boiler. When a boiler operates at its maximum rated capacity, it is referred to as the maximum load. If the load varies from hour to hour, it operates at a varying load. Load and load variations can influence the amount of chemicals required for treatment and the treatment controls required.

It is a multi-tubular vertical fire tube boiler having numbers of horizontal fire tubes. It is the modification of a simple vertical boiler where the heating surface has been increased by means of numbers of fire tubes.

Coke is produced by heating coal for several hours to high temperature (T ≈ 1000‐1100^oC) out of contact of air until all volatile matters are removed. The process is called “carbonization” or Destructive distillation of coal”

There are three main circuits of working medium in ice plant:

Refrigeration circuit: Ammonia as working medium (primary refrigerant) which produces the cold by changing its phase at different location.

Cooling water circuit: Cooling water as working medium to remove the heat of condenser.

Brine circuit: Brine solution as working medium which transfers the cold from ammonia to water filled cans where ice is to be formed.

Gas turbines operate on Brayton cycle/Joule cycle. The Joule cycle consists of four internally reversible processes:

• Isentropic compression (in a compressor)
• Constant-pressure heat addition
• Isentropic expansion (in a turbine)
• Constant-pressure heat rejection

A load curve, also known as load profile, is a chart illustrating the variation of electrical load over a period of time. Generating companies or electrical utilities use this chart to study the pattern of load variation and to obtain information regarding the amount of power to be generated at a specific time.

∴WC=WT−Wnet=10000−4000=6000kW

Brayton cycle is a theoretical cycle for gas turbines. This cycle consists of two reversible adiabatic or isentropic processes and two constant pressure processes. The heat addition and rejection takes place at constant pressure.

R = static enthalpy change in rotor/static enthalpy change in stage

Δh_R = 0.5 Δh_R + 0.5 Δh_S

Δh_R = Δh_S

A deceleration of the air from Mach number 3 at diffuser inlet to Mach 0.3 in combustion chamber causes the pressure ratio to rise to more than 30. Due to shock and other losses inevitable at such velocities of the pressure rise is not available at the required combustion pressure. This principle of ram pressure rise is used in the ramjet engines. Thus compressor does not form a part of ramjet.

Uranium-235, enriched to about 5% from its naturally occurring level of 0.7% is used as primary fuel in the nuclear power plants.

Example of non-positive displacement compressor :

i) Centrifugal compressor

ii) Axial flow compressor

iii) Dynamic compressor

iv) Blade comrpessor

The Benson boiler employs forced circulation and has a unique characteristic of the absence of steam separating drum. The entire process of heating, steam formation and superheating is done in a continuous tube and accordingly it is called as once-through boiler.

Free air delivered is a term used to compare the handling capacity of different compressors. It is volume of air when converted to 1 bar pressure and 15°C.

In impulse turbine pressure drop takes place only in the nozzle. There is no pressure drop either in moving or in fixed blades. The pressure in the moving and fixed blades remains constant.

For turbo jet engines the efficiency is higher for higher speeds. Whereas in turbo prop engines it first increases then decreases.

For minimum work input during compression precess,