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Heat Pumps > Vapour > Flashcards

Vapour Flashcards

1
Q

Define systems and surroundings

A
  • Surroundings are the whole universe
  • The system is part of the universe which contains a definite quantity of matter and is confined within some closed surfaces
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2
Q

Define Controll volume

A

space into which, or from which, a substance flows

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3
Q

Define an intensive property

A

does not depend on mass

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4
Q

Define an extensive property

A

depend on mass of system

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5
Q

Define a thermodynamic property

A
  • qualities that serve to describe a system.

- have a unique value when a system is in a particular state and do not depend on the previous state

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6
Q

Define a thermo dynamic system

A

quaintly if mater contained within some closed surface

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7
Q

Define a phase

A

quantity of matter that has the same chemical composition throughout

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8
Q

Define a state

A

condition as described by giving values to its properties at particular instants

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9
Q

What is a process?

A

path of successive states through which a system passes

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10
Q

How do you work out the change in property?

A

integral of d phi between phi 2 and phi 1 = phi 2 - phi 1

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11
Q

What is the quality of a liquid-vapour system

A

ratio of mass of saturated vapour to total mass in the system

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12
Q

What is the equation for quality

A

x = mg / m

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13
Q

What does first law state?

A

during any cycle that a system undergoes, the cyclic integral of the heat quals to the cyclic integral of the work

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14
Q

Draw a TV graph and state its regions

A
  • R4
  • superheated
  • quality
  • compressibility
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15
Q

What is the alternative form of first law?

A 
Q(1-2) = E2 - E1 + W(1-2)
dE = delta Q - delta W
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16
Q

What does the total heat transfer equal?

A

Q = u2- u1 +m(v2^2 - v1^2)/2 + mg (z2-z1) + w2

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17
Q

What does enthalpy equal for steam?

A

h = (1-x)hf + xhg

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18
Q

What does enthalpy equal for other gases?

A

h = cpT

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19
Q

What does the volume of steam equal?

A

V = m (xvc + (1-x)vf)

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20
Q

For constant volume and specific heat, what does the change in internal energy equal?

A

u2 - u1 = integral of Cv dt between T2 nd T 1

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21
Q

For constant pressure and specific heat, what does the change in enthalpy equal?

A

h2 - h1 = integral of cp dt between T2 nd T 1

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22
Q

From first law what does h1 equal?

A

H1 = H2

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23
Q

What does the heat transfer out equal?

A

Q out = hin - hout

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24
Q

What are the relations for a isobaric process?

A 
P1 = P2 = V1/T1 = V2/T2
Q = mCv(T2 - T1)
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25
Q

What are the relations for an isothermal process?

A

T1 = T2 = P1 V1 = P2 V2
Q = mCp(T2 - T1)
Q - W = delta U

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26
Q

What are the relations for an adiabatic process?

A

P1V1 ^ y = P2V2 ^ y
change in U = (P2V2 - P1V1)/(1 - k)
T2/T1 = (V1/V2)^k-1
T2/T1 = (P2/P1) ^k-1/k

27
Q

What is an adiabatic process?

A

occurs without transfer of heat or matter

28
Q

What are the relations for a poly tropic process?

A

P1V1^n = P2V2^n

29
Q

What does the mass flow rate equal?

A

row 1 A 1 V 1 = m1 = m2 for a steady state

30
Q

What are pumps?

A
  • devices that transfer energy to a gas flowing through them
  • increase pressure
31
Q

What are compressors and blowers?

A
  • transfer energy to a gas flowing through them

- increase pressure

32
Q

What are turbines?

A
  • work is down by a fluid on a set of rotating blades

- pressure decreases

33
Q

What is a nozzle?

A

-increase velocity for a flowing fluid by decreasing pressure

34
Q

What is a diffuser?

A

-increase pressure by decreasing velocity

35
Q

What is the equation for net work on a compressor/turbine/pump?

A

W = m (h2 - h1)

36
Q

What is a heat exchanger?

A

devices that transfer heat from hotter body to colder body or to surroundings

37
Q

What are the features of heat exchangers?

A
  • velocity doesn’t change
  • pressure drop is small
  • protenial energy change is negligible
  • no work occurs
38
Q

What is the heat transfer equation for heat exchangers?

A

Q = m (h2 - h1)

39
Q

What does the second law state?

A

Processes proceed in certain directions

40
Q

What is a heat engine?

A

device that operates in a thermodynamic cycle and does a certain amount of net positive work through the transfer of heat from a high temperature body to a low temperature body

41
Q

What is a heat pump?

A

device that operates in a cycle that requires work and that accomplishes the transfer of heat from a low T body to a high T body

42
Q

What is the thermal efficiency of a heat engine?

A

n = W/Qh = 1 - Ql/Qh

43
Q

What is the efficiency of a heat pump?

A

B = Ql/ W = Ql/(Qh-Ql)

44
Q

What is the kelvin - plank statement?

A

It is impossible to construct a heat engine that operates in a cycle that receives a given amount of heat and does an equal amount of work

45
Q

What is clausis statement?

A

It is impossible to construct a refrigerator that operates without input of work. This implies that coefficient of performance b is less than infinity

46
Q

What is a reversible process?

A

a process that once having occurred can reverse and in doing so leave in changes in either system of surroundings

47
Q

Why is a carnot cycle the most efficient?

A

every process is reversible

48
Q

What are the four stages in a carnot cycle?

A

1) revisable isothermal expansion
2) reversible adiabatic expansion
3) reversible isothermal compression
4) reversible adiabatic compassion

49
Q

For all revisable heat engines what is Q equal to?

A

integral of delta Q/ T = 0

50
Q

For all irreversible heat engines what is Q equal to?

A

integral of delt Q/T less than 0

51
Q

What is entropy?

A

unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.

52
Q

What is the change in entropy?

A

s2 - s1 = integral of delta Q / T

53
Q

In a saturated region, what is the entropy?

A

s = sf + xsfg

54
Q

What is the change in entropy of ideal gasses?

A

s2 - s1 = cv ln(T2/T1) + k ln(v2/v1)

55
Q

What does T ds equal?

A

T ds = dH - Vdp

= du + Pdv

56
Q

What is the efficiency of a turbine?

A

Wa/Ws

57
Q

What is the efficiency of a compressor?

A

Ws/Wa

58
Q

What are the 4 processes in a ranking cycle?

A

1-2 isentropic compression (pump)
2-3 constant pressure heat addition (boiler)
3-4 isentropic Expansion (turbine
4-1 contrant pressure heat extraction

59
Q

What is the efficiency of a rankine cycle?

A

n = Wt/Qb

60
Q

How can you improve a rankine cycle?

A
  • increase output pressure P2 or boiler outlet temp T3
  • increase boiler pressure while maintain T mad and min P
  • however this does lead to low quality of steam and the droplets cause damage to turbines
61
Q

What is the rankine reheat cycle

A
  • steam passing through turbine is reheated which raises T to state 5
  • steam then passes through low P section of the turbine and enters condenser at state 6
  • this then reduces moisture in the turbine
62
Q

What is the coefficient of performance for the vapour refrigeration cycle

A

COP = B = Q in / W in

63
Q

Why isn’t an actual vapour refrigeration cycle ideal

A
  • pressure drops due to friction in pipes
  • heat transfer through pipes
  • pressure drops and heat transfer in condenser
  • fictional effects

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