Exergy analysis of two novel all-welded plate air preheaters

ABSTRACT

With the stricter requirements for energy saving and emission reduction, air preheaters have always been the focus of scientific research, especially the development of plate air preheaters with high compactness, good heat transfer performance and reduced media flow pressure has a sharp upward trend. At present, there are two kinds of plate type air preheaters on the market, one is popular in the market for welding support points between plates, or the Stubs-inserted Plate Air preheater (SIPA); the other is the Corrugated Plate Air preheater (CPA). In order to evaluate the thermodynamic performance of the two series of air preheaters, this paper analyzes the exergy loss of the CPA and SIPA through experimental research. Experiments were conducted where environmental air was heated by high-temperature flue gas produced from the burning of natural gas. Variations in the effectiveness and dimensionless exergy losses of the air preheaters according to gas flow rates are compared. The temperatures of the inlet air and flue gas were kept constant at 30°C and 300°C, respectively. The Reynolds numbers varied from 2.95 × 10³ to 5.47 × 10³ for air and 2.01 × 10³ to 3.14 × 10³ for flue gas. The experimental results show that the number of heat transfer units and the effectiveness of the CPA were much higher than those of the SIPA under the same conditions. The maximum and minimum values for the CPA were 0.564 and 0.438, respectively, and were 0.337 and 0.232 for the SIPA. Simultaneously, the exergy losses of both air preheaters increased with increases in gas flow. The CPA had a higher value than the SIPA but smaller dimensionless exergy losses under the same conditions, which indicates that the SIPA had greater irreversibility than the CPA at the same heat transfer rate. In consideration of its thermodynamic characteristics, the heat transfer performance of the CPA is superior to that of the SIPA.

KEYWORDS:

• All-welded plate air preheater
• number of heat transfer units
• dimensionless exergy loss
• thermodynamic characteristics

Nomenclature

$A$	=	heat transfer area (m2)
$C_p$	=	specific heat (KJ/Kg K)
$C_h$	=	heat capacity of hot fluid (W/K)
$C_c$	=	heat capacity of cold fluid (W/K)
$E$	=	exergy loss rate (W)
$e1,e2$	=	dimensionless exergy loss
$\stackrel{\bullet }{m}$	=	mass flow rate (Kg/s)
$C_{max}$	=	maximum heat capacity (W/K)
$C_{min}$	=	minimum heat capacity (W/K)
$NTU$	=	number of heat transfer units
$\stackrel{\bullet }{Q}$	=	heat transfer rate (W)
${\stackrel{\bullet }{S}}_{gen}$	=	entropy generation rate (W/K)
$s$	=	specific entropy (J/Kg K)
$T$	=	temperature of fluid flow (ºC)
$T_e$	=	environmental temperature (K)
$U$	=	overall heat transfer coefficient (W/m2 K)
$\mathrm{\Delta }{T}_{LMTD}$	=	logarithmic mean temperature difference (ºC)
$\mathrm{\Delta }T$	=	temperature difference (ºC)
$\mathrm{\Delta }P$	=	pressure drop (Pa)
$\epsilon$	=	effectiveness
Subscripts	=
$c$	=	cold fluid
$h$	=	hot fluid
$in$	=	inlet
$max$	=	maximum
$min$	=	minimum
$out$	=	outlet

Acknowledgments

This study is funded by the National Natural Science Foundation of China (51574179)

Additional information

Funding

This study is funded by the National Natural Science Foundation of China (51574179).