transcritical_2007_2

8/7/2019 transcritical_2007_2

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Supercritical or transcriticalPower generation cycle


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Effect of P & T on the Rankine cycle

Effect of boiler P ; cycle ,

heat absorption T ,

moisture content in the turbine exit

turbine , erosion of turbine blade


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Several prototype pulverised combustion-based

supercritical steam cycle plants (using steam pressures

and temperatures of 240 bar and 590C)

due to various technical problems, these units were

found to be unreliable and uneconomic to operate. As aresult, most of these units were withdrawn from service in

the 1980s.

However, experiences with latest design of supercritical

plants are very encouraging.

Efficiencies up to about 44% or approaching 50%

http://www.cormix.info/pdf/Burns&McDonnell2001-3.pdf


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[H.W.] Supercritical Steam Power Plant with Reheat

In an effort to decentralize the national power distribution grid, the following

reheat-cycle, supercritical steam power plant (modeled after the Gavin Power

Plant in Cheshire, Ohio) has been proposed to service about 10,000

households in Athens, Ohio. It is to be placed close to the sewage plant on the

east side of Athens and cooled by water from the Hocking river.


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1. Neatly sketch the complete cycle on the P-h diagram below, indicating clearly all sixstations on the diagram.2. Assuming that both turbines are adiabatic, determine the combined power output of

the two turbines. The steam pipe at the inlet to the low pressure (LP) turbine has adiameter of 20 cm, and that at the outlet of the LP turbine leading to the condenserhas a diameter of 40 cm. Determine and discuss the effect of kinetic energy change onthe performance of the LP turbine.3. Assuming that the feedwater pump is adiabatic, and that the compressed liquidexperiences a change in temperature of 5C while passing through the pump,

determine the power required to drive the pump. Discuss the significance of this smallchange in temperature on the performance of the pump.4. Determine the total heat transfer to the boiler, including the reheat system.5. Assume that all the heat rejected from the condenser is absorbed by cooling waterfrom the Hocking river. To prevent thermal pollution the cooling water is not allowedto experience a temperature rise above 10C. If the steam leaves the condenser as

saturated liquid at 50C, determine the required minimum volumetric flow rate of thecooling water (cubic meters/minute).6. Determine the overall thermal efficiency L of this power plant. (Thermal efficiencyis defined as the net work done divided by the total heat supplied externally to theboiler and reheat system).


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Supercritical or transcriticalrefrigeration cycle


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Natural refrigerant

CFC, HCFC, HFC

J-T effect

Thermodynamic property

Transport propertyHistory


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0 50 100 150 200 250 300 350 400

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10 0oC

80oC

60oC

40oC

20oC

0oC

C:/JKT/Scroll/R134a/FreonTotal/Total.opj/Graph-R134aPh

P-h diagram ofR134a

T=-20oC

Pressure,P[kPa]

Enthalpy, h[kJ/kg]


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-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

-60

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1400 kPa

600 kPa

C:/JKT/Scroll/R134a/FreonTotal/Total.opj/Graph-R134aTs

T-s diagram ofR134a

500

0kP

a

4500

kPa

4000k

Pa

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kPa

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P=100 kPa

Temperature,T[oC]

Entropy, s[kJ/kg K]


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Assessment of Carbon Dioxide as a Refrigerant

In the early part of this century, carbon dioxide (R-744) was widely used as a refrigerant.

Its popularity was based principally on its low toxicity, non-flammability, low cost and

universal availability. Other refrigerants, such as ammonia, sulfur dioxide and methylene

chloride could achieve much higher cycle efficiencies, but had other handicaps that

limited their application. The advent of chlorofluorocarbons (CFCs) in the 1930's

provided refrigerants that had low toxicities as well as high cycle efficiencies, removingmost of the incentives for choosing carbon dioxide its use thus dropped sharply.

Why consider R-744 now?

The ban on the use of CFCs and the phaseout of hydrochlorofluorocarbons (HCFCs)

have necessitated the search for new refrigerants and the reevaluation of old ones. The

reasons for carbon dioxide's original popularity, i.e., low toxicity, non-flammability, low

cost and universal availability, are still strong selling points.

However, two basic features still limit its acceptance: low cycle efficiency and high

operating pressures.


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Cycle Efficiency

R-744 has a low critical temperature of 31.1C (88.0F). If it is applied in a vapor

compression cycle whose heat rejection temperature is less than this, it will havetwo-phase condensation as well as evaporation, as do most other refrigerants.

However, the efficiency of the simple cycle is very low. For example, with an

evaporating temperature of -15C (5F) and a condensing temperature of 30C(86F), the refrigerating cycle coefficient of performance (COP) is only 2.81, ascompared to 4.77 for ammonia, 4.67 for R-22 and 4.41 for R-134a.

For heat rejection temperatures above 31.1C (88.0F), R-744 can still be used,but the high temperature, super-critical vapor does not condense at a single

temperature at a single temperature -- it changes gradually to a dense liquid as its

temperature is reduced. The refrigerating cycle efficiency remains low, however.

Cycle efficiency for R-744 can be improved in at least two ways.


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First, throttling losses can be reduced. As Lorentzen (Ref. 4) has pointed

out, a large part of the cycle inefficiency for R-744 is the energy (work) lost

in the throttling process. By employing an ideal (isentropic) expander torecover work, compressor work is reduced and the cycle COP can be

raised by over 50%. There is a good opportunity for using real (non-ideal)

expanders. Of course, a drawback is the added cost of such devices.

Second, liquid line-suction line heat exchangers can be employed at an

added cost. Pettersen (Ref. 4) used such a device to lower the liquidtemperature before throttling, raising the evaporator capacity and lowering

the throttling losses.

The net cycle COP improvements appear to be less than 5%.


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Operating Pressures

Evaporating pressures for typical air conditioning duty using carbon dioxide are about

3,400 to 4,800 kPa (490 to 700 psia) while high-side pressures are about 8,300 to

13,800 kPa (1,200 to 2,000 psia).

These pressures are about five times higher than with conventional refrigerants. This

presents obvious problems of providing thicker walls for piping, heat exchangers,

receivers and compressor shells. On the other hand, the higher fluid densities lead to

lower velocities and lower pressure drops. The higher densities can also lead to more

compact heat exchangers.

Perhaps the most serious challenge is the design of new compressors for these high

pressures. On the positive side, it has been claimed that pressure ratios for R-744 are

lower than those for other refrigerants and therefore will improve compression efficiency

(Ref. 5, Ref. 7 and Ref. 8).


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In summary,

carbon dioxide (R-744) has several attractive attributes: low toxicity,

nonflammability, low cost and wide availability. However, its inherently low

refrigerating cycle efficiency and high operating pressures will remain

serious challenges to its use in unitary heating and cooling products in the

near term.


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Compressors aren't the only part of a cooling system where

innovation can be found. Improving the heat transfer

characteristics of heat exchangers can also boost a cooling

system's efficiency. A good example is the PFT (parallel flow)

technology developed by Modine Manufacturing Co., Racine,

Wis.

Conventional heat exchangers use round copper tubing with

mechanically bonded aluminum fins. By contrast, PF heat

exchangers are all-aluminum brazed construction, with the

fins metallurgically bonded. More importantly, in a PF heat

exchanger, the tubing has a flattened shape, and its interior is

sectioned into a series of multiple, parallel flow,

microchannels that carry the refrigerant or working fluid.

The microchannels in the heat exchanger provide better heat

transfer on the refrigerant side.

Efficiency gains also found in heat exchangers


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References:

1. ASHRAE Handbook of Fundamentals, 1993, p.167.2. S. Klein and F. Alvarado, Engineering Equation Solver (EES), Version 4.259W,

Fchart Software, Middleton, WI, 1996.3. G. Lorentzen and J. Pettersen, "A New, Efficient and Environmentally Benign Systemfor Car Air Conditioning", International Journal of Refrigeration, 1993, Vol. 16, No.1,pp. 4-11.4. G. Lorentzen, "Revival of Carbon Dioxide as a Refrigerant", H & V Engineer, Vol.66, No. 721, 1994, pp. 9-14.

5. D. Robinson and E. Groll, "Using Carbon Dioxide in a Transcritical VaporCompression Refrigeration Cycle", Sixth International Refrigeration Conference atPurdue, 25 July 1996.6. J. Pettersen, "An Efficient New Automobile Air-Conditioning System Based on CO2Vapor Compression", ASHRAE Transactions, 1994, Vol. 100, Part 2, pp. 657-665.7. G. Lorentzen, "The Use of Natural Refrigerants: A Complete Solution to the

CFC/HCFC Predicament", International Journal ofR

efrigeration, 1995, Vol. 18, No.1,pp. 190-197.8. J. Koehler, M. Sonnekalb, H. Kaiser and W. Koecher, "Carbon Dioxide as aRefrigerant for Vehicle Air-Conditioning with Application to Bus Air- Conditioning",1995 International CFC and Halon Alternatives Conference, October 1995, pp. 376-385.

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