Throttling

Thermodynamics

Professor Lee Carkner

Lecture 22

PAL #21 Refrigeration

Refrigerator cycle where P = 120 kPa and x = 0.3 before the evaporator and 60 C after the compressor Start at point 4, P4 = 120 kPa, x = 0.3, look up h4 =

h3 = h4 = 86.83, for a saturated liquid this means P3 =

Since P2 = P3 and T2 = 60 , look up h for superheated vapor, h2 =

At point 1, P1 = P4 = 120 kPa, saturated vapor, h4 =

PAL #21 Refrigeration

Mass flow rate if W’in = 0.45 kW W’in = m’(h2-h1)

m’ = (0.45)/(298.87-236.97) =

Find COP from W’in and Q’L Q’L = m’(h1-h4) = (0.000727)(236.97-86.83) =

1.091 kW COP = Q’L/W’in = (1.091)/(0.45) =

Cascade Systems

For larger commercial systems, efficiency becomes more important

e.g., deep freeze

Called cascade cycles

Two-Stage Cascade

Cascade Efficiency

The condenser of cycle B (points 1-4) is connected to the evaporator of cycle A (points 5-8) m’A(h5-h8) = m’B(h2-h3)

COPCascde = m’B(h1-h4)/[m’A(h6-h5)+m’B(h2-h1)]

Multistage Compression

Some fluid is vaporized and is sent back to the high pressure compressor

Can also use just one compressor and multiple throttle valves and evaporators for multiple temperatures

Gas Refrigeration

We can also us a reverse Brayton cycle

Isentropic compression Isentropic expansion in turbine

Reversed Brayton Cycle

Gas Refrigeration Efficiency

wnet,in = wcomp – wturb = (h2-h1)-(h3-h4)

COP = qL/wnet,in = (h1-h4) / [(h2-h1)-(h3-h4)]

Heat Pumps

COPHP = QH/Wnet,in = QH / (QH – QL)COPHP,Carnot = 1 / (1 – TL/TH)

Often designed as dual heat pump/air conditioners

Low COP if the outside temperature is very cold Can also push the heat extraction underground

Reversible Heat Pump

Joule-Thompson Expansion

Can be achieved by a pump circulating fluid through a pipe with an expansion valve in the middle

We know that in this case, hi = hf

What will be the final properties of the fluid?

Isenthalpic Curve

If the apparatus is changed a little, a new Pf and Tf are produced

The curve represents possible beginning and ending points for a throttling process

A series of isenthalpic curves can be produced for a

substance

Inversion Curve Each curve has two regions

Tf >Ti

Tf < Ti

In between, the slope, or Joule-Thompson

coefficient (), is zero:

For a series of isenthalpic curves, a curve connecting =0 points is the inversion curve

Liquefying Gasses In order to cool a gas, its temperature must

start below the maximum inversion temperature

TM.I. is near room temperature for many gasses

Some gasses have to be pre-cooled

Heat Exchanger How is gas liquefied?

Throttled and cooled Cold gas runs back through the heat exchanger

cooling the incoming gas Cycle starts over

Next Time

Read: 12.1-12.3 Homework: Ch 11, P: 35, 56, Ch 12, P: 8,

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