week 5

Question 1

practicability of carnot cycle:
processess 1-2 and 3-4
(2)

Answer 1

isothermal heat transfer to or from a 2 phase system.

limiting heat transfer processes to 2 phase systems, limits the max temp that can be used in the cycle

Question 2

practicability of carnot cycle:
process 4-1
(3)

Answer 2

involves compression of liquid-vapor mixture to a saturated liquid

not easy to control state 4

not practical design compressor to handle 2 phases

Question 3

what is a rankine cycle

Answer 3

a model used to predict the performance of steam turbine systems

Question 4

what is a carnot cycle

Answer 4

theoretical cycle with the highest possible efficiency of all other thermodynamic cycles

Question 5

2 disadvantages of rankine cycle

Answer 5

trubine exhaust is very wet

max operating temp limted by critical point, limiting maximum efficiency

Question 6

way to improve efficiency

Answer 6

by changing boiler/condenser pressures and temps

Question 7

what are the limiting factors of efficiency (2)

Answer 7

maximum working temperature/stress of materials

temperature of cooling fluid

Question 8

explain a superheated rankine cycle

Answer 8

Steam entering turbine is superheated. This keeps turbine exit steam in superheated region.

Question 9

explain a Reheated Rankine cycle

Answer 9

Multiple turbines are used with re-heating in between.

Question 10

explain a Supercritical Rankine cycle

Answer 10

Steam is superheated avoiding wet region. It passes through supercritical region.

Question 11

explain Regenerated Rankine cycle

Answer 11

Heat is recovered from steam to pre-heat water entering boiler

Question 12

what does the work input require for a rankine cycle

Answer 12

required to compress the liquid
water is much less than the work output of the turbine. It is also much less than the work input required to compress a gas

Question 13

what pressure are experienced on the boiler and compressor (high or low), on a rankine cycle

Answer 13

• Operating pressures are very high in the boiler and very low in the condenser. This requires high structural integrity from the enormous lengths of pipework involved. In the boiler the pipes are subjected to high temperatures and corrosive flue gases. The pressure ratio of the cycle is
generally very high.

Question 14

what is the optimal way of increasing boiler pressure but not increasing moisture content in the exiting vapour

Answer 14

to reheat the vapor after it exits
from a first-stage turbine and redirect this
reheated vapor into a second turbine