TS-135
Thermal-hydraulic Prediction for Wet Region of Economizer Using Spirally Finned Tubes
Masahiro OSAKABE* and Tugue ITOH*
In order to improve the boiler or thermal plant etfficiency, latent heat recovery from the flue gas is very important concept. Condensation heat transfer on horizontal spirally finned tubes of fin pitch 5 and 10 mm was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiments were conducted at different steam mass concentrations of the flue gas and a wide range of tube wall temperature. The mass concentration was controlled with a steam injection into the flue gas. Fin efficiency at the condensation region was significantly lower than that at the dry region. The empirical correlation developed for a single-phase fluid was extrapolated to the condensation heat transfer region. The fin efficiency was evaluated with an equivalent heat transfer coefficient used as a first approximation. The heat and mass transfer behaviors on the spirally finned tube were well predicted with the analogy correlation based on the empirical correlation. The obtained correlation was incorporated into the prediction code and verified with the economizer experiments. The experimental results for the temperature distributions of water and flue gas agreed well with the prediction.
Keywords: Boiler, Actual flue gas, Heat and mass transfer, Condensation
1. INTRODUCTION
The most part of energy losses in boiler or thermal plant is due to the heat released by the exhaust flue gas to atmosphere. The released heat consists of sensible and latent one. Recently, for a biological and environmental safety, a clean fuel such as a natural gas is widely used in the boiler or power plant. As the clean fuel includes a lot of hydrogen instead of carbon, the exhaust flue gas includes a lot of steam accompanying with the latent heat. So the latent heat recovery from the flue gas is very important to improve the system efficiency.
For the latent heat recovery from a boiler, condensation heat transfer experiments [1-3] have been conducted by using steam-air mixtures. These experiments yield different opinions on the analogy for heat and mass transfer. Furthermore, condensation in actual flue gas is more complex and least understood. So it was difficult to predict the accurate thermo-fluid behavior of an actual condensing heat exchanger using an actual flue gas. Condensation heat transfer on horizontal spirally finned tubes of fin pitch 5 and 10 mm was investigated experimentally by using the actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios and steam mass concentrations of the flue gas in a wide range of tube wall temperature.
Thermal-hydraulic behavior of economizer for the latent heat recovery was investigated experimentally by using an actual flue gas from a propane gas fuel boiler. Two kinds of countercurrent cross-flow heat exchangers, which consist of spirally finned tubes of fin pitch 5 and 10 mm, were designed and used for the experiment. Based on the above basic studies, a prediction method for the economizer was proposed, In the prediction, the flue gas was treated as a mixture of CO2, CO, O2, N2 and H2O, and the one-dimensional heat and mass balance calculation along the flow direction of flue gas was conducted. The heat and mass transfer on tubes was evaluated with the simple analogy correlation. The fin efficiency at the condensing region was calculated with a semi-empirical correlation obtained in the basic study.
2. NOMENCLATURE
A: heat transfer area [m2]
C: mass concentration per fluid of a unit volume [kg/m3]
Cp: specific heat [kJ/kg]
d: outer diameter of base tube [m]
di: inner diameter of base tube [m]
D: mass diffusivity (m2/s)
hV: heat transfer coefficient [kW/(m2K)]
hC: mass transfer coefficient [m/s]
LW: latent heat [kJ/kg]
LF: fin height [m]
mS: injected steam flow rate [kg/s]
Nu: Nusselt number(= hVd / λ)
P: pressure [Pa]
q: heat flux [kW/m2]
Pr: Prandtl number (= v/K)
Re: Reynolds number (= ud/v)
R: relative humidity of air
SF: fin space [m]
Sh: Sherwood number (= hCd/D)
Sc: Schmidt number (= v/D)
T: temperature [｡?
tF: thickness of fin plate [m]
u: velocity at minimum flow area [m/s]
V: volumetric flow rate [mN3/s]
w: mass concentration per fluid of an unit mass [kg/kg]
n: fin efficiency
K: thermal diffusivity (= λ/(ρCP))
λ: heat conductivit1y [W/(mK)]
μ: air ratio
v: kinematic viscosity [m2/s]
ρ: density [kg/m3]
