Lecture 6

Question 1

throttle valve is

Answer 1

a restriction in the flow

Question 2

there is a what across a throttle valve

Answer 2

pressure drop

Question 3

Throttle valvue SFEE

Answer 3

h1 = h2 everything else cancels enthalpy before = enthalpy after

Question 4

isenthalpic

Answer 4

enthalpy does not change

Question 5

for ideal gas what is the only thing that matters

Answer 5

p1v1 = p2v2

as temperature cannot change for an ideal

Question 6

throttle valve fluid near liquid vapout boundary

Answer 6

fluid undergoing expansion is either completely or partially vapourised
energy needed to do this causes reduced temp

Question 7

refrigeration cycle

Answer 7

saturated liquid goes through valve it vapourises and cools

Question 8

less effective a nozzle is the more like a throttle it becomes as

Answer 8

flow is not accelerated as much as it could be
sudden expansion prevent pressure recovery
friction and turbulence increase entropy
friction dissipates the flow without recovering any of the pressure

Question 9

a water impulse turbine is made out of

Answer 9

nozzle and a rotor

Question 10

ideal impulse turbine converts

Answer 10

enthalpy into work with no effect on the entropy of the fluid

Question 11

most efficient turbine will

Answer 11

convert all energy into work least efficient extracts no work at all therefore acts as a throttle

Question 12

most efficient turbine will

Answer 12

convert all energy into work least efficient extracts no work at all therefore acts as a throttle

Question 13

main factor that reduces efficiency of a turbine

Answer 13

velocity of flow is reduced with providing work
friction in flow
exiting from the turbine still possessing KE

Question 14

heat exchanger

Answer 14

takes energy from one stream of fluid and transfers it to another

Question 15

heat is always transferred from

Answer 15

hot to cold steams

Question 16

parallel flow diagram

Answer 16

flow goes in the same direction

Question 17

counter flow

Answer 17

the two flows go in opposite directions

Question 18

cross flow

Answer 18

two flows cross at right angles (no corners)

Question 19

evaporators and condensers

Answer 19

special cases of heat exchanges instead heat transfer changing temp of fluid it changes the quality

Question 20

heat exchangers SFEE

Answer 20

Q=m(h2-h1) idealised heat exchanger

Question 21

steam heat exchangers

Answer 21

find h1 from tables

calculate h2 use tables to find out exit temperature at that pressure

Question 22

ideal gas heat exchangers

Answer 22

Q = m *cp (T2-T1)

Question 23

if both liquid and water stream

Answer 23

equate Qs
-macpadTa = mwcpwdTw
negative as one is increasing other is decreasing

Question 24

assumed simplifcations in heat transfer

Answer 24

always pressure drop in real heat exchanger
mostly due to turbulent
losses in the fluid
and pressure loss when flow enters or leaves the device
must be of infinite length so all heat is transferred

Question 25

evaporator

Answer 25

heat goes temp doesnt increase but enthalpy does

Question 26

condenser

Answer 26

temp remains constant enthalpy decreases

Question 27

superheater

Answer 27

once fluid enthalpy has been raised to a saturated vapour additional heat added raise temp into superheated region

Question 28

change in entropy for a heat exchanger

Answer 28

change in entropy total = heat cold/T cold - heat hot/T hot

change in entropy see powerpoint

Question 29

total entropy always increases in a heat exchanger

Answer 29

therefore heat transfer across a finite temperature is inherently irreversible

Question 30

total entropy always increases in a heat exchanger

Answer 30

therefore heat transfer across a finite temperature is inherently irreversible