Heat transfer between flue gas and feed water?

Oct 2018
I need to determine the temperature of flue gas in a fire tube boiler.

Tube inlet gas temperature, T1 = 1045 deg.C
Saturation water temperature, tsat = 186 deg.C
Flue gas temperature, t2 = ?

2 equations to be used here:
Q = U.A.LMTD -------------(1)
Qg = Mg.Cg.(T1 - T2) ------(2)

Mg = mass of flue gas = 2415 kg/hr
Cg = specific heat of gas = 1.92 kJ/kg.K
U = overall heat transfer coefficient = 25.37 W/m2.K
A = heating surface of tube = 26.85 m2
LMTD = log mean temperature difference bet. the 2 medium

Equating both equation 1 & 2, and solve for T2, I get 674 deg.C

Problem is that inside this boiler tube has spiral tube, which will increase the heat transfer and will bring down the temperature of T2.

I just don't know how to go about this spiral tube. How to solve this?
Oct 2018
I think I didn't make it very clear without any image. The spiral tube/mixer I was talking about is something like in the link. Please have a look at it.

The flue gas flow through the tube with the mixer will have being in a turbulence state, which will increase the overall heat transfer coefficient, which in turn the heat transfer, which in turn will bring down the temperature of the flue gas at outlet. I calculate it at 674 deg C, but in actual is around 250 deg C

Problem is how to determine the heat transfer coefficient of the flue gas in mixer?

Any input is welcome.