8/11/2019 Rankine_regenerative cycle--good.doc

1/4

MECH 351 Lyes KADEM [Thermodynamics II] 2007

The idea re!enera"i#e $an%ine cyce

The analysis of the Rankine cycle using the second law showed that the largest exergydestruction (major irreversibilities) occurs during the heat-addition process. Therefore any attemptto reduce the exergy destruction should start with this process.

hen we analy!e the Rankine cycle ("igure.#) we can notice that$% considerable percentage of the total energy input is used to heat the high pressure water fromT#to its saturation (point a).

"igure.#. Rankine cycle.

To reduce this energy& the water could be preheated before it enters the boiler by interceptingsome of the stream as it expands in the turbine and mixing it with the water as it exits the first

pump. This will results in$

- % reduction in the energy loss within the condenser& since we condense less stream (onlythe stream that continues expansion from ' to ).

- %n increase in the average temperature at which the heat is supplied * andtherefore .

This configuration of the Rankine cycle is called$$e!enera"i#e $an%ine cyce

&a'or and com(ined 'o)er cyces 3

"igure.+. Regenerative Rankine cycle.

8/11/2019 Rankine_regenerative cycle--good.doc

2/4

MECH 351 Lyes KADEM [Thermodynamics II] 2007

The heat transfer between the intercepted stream and the high-pressure water (called feedwater)occurs in what we call$ feedwater heaters ("*).

% feedwater is basically a heat exchanger where heat is transferred from the high temperaturestream to low temperature feedwater.This can be performed by$

- % direct mixing (open feedwater heater).- ithout mixing (closed feedwater heater).

Rm,$ sometimes& the feedwater heater is called a regenerator. The mass flow between ('-) isdifferent from the mass flow from ('-+)

*+TE, The mass flow rate varies in the regenerative Rankine cycle.

- +'en .eed)a"er hea"er /direc" con"ac"

"igure . pen feedwater heater.

Ad#an"a!es Disad#an"a!es

- /imple - "or each heater& we need a pump- 0nexpensive- *ave good heat transfer characteristics- 1ring feedwater into saturated state

2roblem$ the cold water and the turbine stream must be at the same pressure.

- Cosed .eed)a"er hea"er / no miin!

The two streams now can be at different pressures since they do not mix. The water passesthrough in the tubes and steam surrounds the tubes. The steam is condensated and pumped by acondensate pump into the main feedwater line or it passes through a trap (a device that permitsonly li,uids to pass through).

Ad#an"a!es Disad#an"a!es

- e don3t need a pump for each heater& sincethe pressure is different

- 4ore complex (internal tubing network)

- 4ore expensive- 5ess heat transfer performance& since the twofluids are not in contact.

&a'or and com(ined 'o)er cyces

Cod )a"er in

Tr(ine s"ream

4a"ra"ed iid o"

8/11/2019 Rankine_regenerative cycle--good.doc

3/4

MECH 351 Lyes KADEM [Thermodynamics II] 2007

"igure 6. 7losed feedwater heater.

"igure '. % feedwater heater.

"igure . % steam trap system.

&a'or and com(ined 'o)er cyces 5

8/11/2019 Rankine_regenerative cycle--good.doc

4/4

MECH 351 Lyes KADEM [Thermodynamics II] 2007

Rm,$ usually modern steam power plants use a combination of open and closed feedwaterheaters.- +'"ima nm(er o. .eed)a"er hea"ers

%ll modern steam power plants use feedwater heaters (8 feedwater heaters). The optimumnumber of feedwater heaters is determined from economical considerations$9ou add a feedwater heater only if it saves more fuel than its own cost (: maintenance).ith a large number of heaters& it is possible to approach the 7arnot efficiency but at anunjustifiably high cost.

- A" )hich 'ressre6 "he s"eam ms" (e e"rac"ed .rom "he "r(ine

The pressure at which the steam should be extracted from the turbine must be in such a way thatfor one heater& the steam should be extracted at the point that allows the existing feedwaterheater temperature to be midway between the saturated steam temperature in the boiler and thecondenser temperature. "or several heaters& the temperature difference should be divided ase,ually as possible.

Rm,$ The regeneration cycle increases the thermal efficiency& but it is affected by the moisture atthe outlet of the turbine& therefore& it is not uncommon to combine a reheat cycle and aregeneration cycle.

&a'or and com(ined 'o)er cyces 8

;xample

7onsider a regenerative cycle using steamas the working fluid. /team leaves the boiler and entersthe turbine at 42a and