System of the built-in heat exchangers

Improvement of the efficiency of power generating units can be achieved in so called enhanced efficiency units (EEUs) by rational reduction of steam extraction from turbine. System of turbine regeneration in the enhanced efficiency units is often bypassed. Bypass of under-heated water is delivered (forwarded) to boiler plant, where it is heated by furnace gases or hot air in the a special heat exchanger, and this reduces the temperature of outgoing gases thus improving the coefficient of efficiency of a boiler plant. System of built-in heat exchangers (SBHE) is installed in the boiler flow system of the 300 MW power generating unit at the Gacko cogeneration plant (Bosnia and Herzegovina).

Heat exchanger consists of 12 packages, and it is operating: in air – in the insert of tubular air heater between its second and third passes; in full-flow condensate – in bypass of the two heaters of low pressure of turbine (PND-3 and PND-4) regeneration. Control valve is installed behind the PND-4 in order to control condensate consumption through SBHE and PND-3, 4 in condensate line. Back-pressure valves, preventing the media from low pressure regeneration system, are installed on the pipeline feeding the condensate to heat exchanger, and on the condensate pipeline behind the PND-4.

Schematic diagram of the system of built-in heat exchangers (SBHE)

In addition to the efficiency, one of the most important purposes of designing the SBHE was provision of such hydro-dynamical resistance that would be sufficient enough to keep the existing group of condensate pumps of the second stage, as well as the existing control assemble of the pressure line KEN-2 st. for this purpose, pipeline of full-flow condensate after the heat exchanger is reinforcingless.

Major specifications of the heat exchanger

Number of packages 12
Surface area of one package, m³ 780
Estimated flow of condensate through a package, t/h 42
Gas consumption, n.cub.m/h 500 000
Type of a beam (staggered / corrid.) staggered
Heat flow direct flow
Length of the ribbed part of a pipe, m. 4,7
Length of the bare part of a pipe, m. 0,2
Outside pipe diameter, mm. 32
Pipe wall thickness, mm. 3,0
Fin length, mm. 13,0
Fin width, mm. 1,0
Fin spacing, mm. 5,0
Intertubular medium. air

Installation of the SBHE at the Gacko cogeneration plant (Bosnia and Herzegovina) resulted in the following:

  • Decrease of temperature (by 30ºC, according to calculations) of gases outgoing from the boiler plant, reaching 200 ºC, when high-pressure heaters were switched off;
  • Collecting much heat in the SBHE (27-29 giga-calories/h);
  • Obtaining extra power reaching 5.8-6.4 MW depending on a type of fuel in the unit at the expense of partial displacement of regeneration scheme;
  • Enhancement of the unit’s efficiency by 0.6%.