Calculating heat transfer coefficient
Calculating heat transfer coefficient
(OP)
Hello.
I am attempting to design a system that involves running an external half-pipe cooling jacket around a firepot assembly. This jacket is limited in size to about a 1.5" radius and will be using ammonium hydroxide as the cooling agent. The NH4OH will only be flowing @ .5gpm. The firepot is made of cast iron. I attempted calculating the Reynolds number, Prandtl number and then the Nusselt number in order to get to the heat transfer coefficient but I'm not sure if I used the equations applicable to my piping and flow conditions. For example in calculating the Reynolds number for flow in a half pipe what value would be used for the Diameter? Any help would be much appreciated.
density NH4OH .9g/cm3
thermal conductivity cast iron 29BTU/ft hr F
viscosity NH4OH 5cP
specific heat capacity of solution 1.12BTU/lb F
equations used:
Pr= kinematic visc/thermal diffusivity
kin. viscosity= viscosity/density
thermal diffusivity = therm conductivity/ (density*specific heat cap)
Nusselt= .023(Re^.8)*(Pr^.4)
Re=speed*density*diameter/viscosity
heat transfer coefficient= Nu* thermal conductivity/ L
(is L the thickness of boundary layer or length of piping?)
I am attempting to design a system that involves running an external half-pipe cooling jacket around a firepot assembly. This jacket is limited in size to about a 1.5" radius and will be using ammonium hydroxide as the cooling agent. The NH4OH will only be flowing @ .5gpm. The firepot is made of cast iron. I attempted calculating the Reynolds number, Prandtl number and then the Nusselt number in order to get to the heat transfer coefficient but I'm not sure if I used the equations applicable to my piping and flow conditions. For example in calculating the Reynolds number for flow in a half pipe what value would be used for the Diameter? Any help would be much appreciated.
density NH4OH .9g/cm3
thermal conductivity cast iron 29BTU/ft hr F
viscosity NH4OH 5cP
specific heat capacity of solution 1.12BTU/lb F
equations used:
Pr= kinematic visc/thermal diffusivity
kin. viscosity= viscosity/density
thermal diffusivity = therm conductivity/ (density*specific heat cap)
Nusselt= .023(Re^.8)*(Pr^.4)
Re=speed*density*diameter/viscosity
heat transfer coefficient= Nu* thermal conductivity/ L
(is L the thickness of boundary layer or length of piping?)





RE: Calculating heat transfer coefficient
RE: Calculating heat transfer coefficient
RE: Calculating heat transfer coefficient
RE: Calculating heat transfer coefficient
Please note that the hydraulic radius is not half the hydraulic diameter, not even for a circular cross-section !
You should use the hydraulic diameter in your calculations of Re, Nu, and the friction factor.
In this particular case, if I'm not mistaken,
Do you already have an idea of the number of windings around
the firepot ? Are you limited on the pressure drop ?
BTW, L in your approximate heat transfer formula is again Dh. Please note that the proposed empirical (Ditttus-Boelter) heat transfer expression formula is for fully developed turbulent flow.
Are you sure the flow régime is indeed fully turbulent with Re>10,000? Pls note that for Re<2,100, Nu = ƒ(Re,Pr,L/Dh) where L is the coil length.
I prefer the 0.33 to 0.4 as exponent of the Pr number, following Sieder and Tate classical measurements.
Good luck.
RE: Calculating heat transfer coefficient
1.I start with Vol. flow:
.5gpm*(3.785liter/gall)*(.8gm/liter)=1.51 gm/min mass flow.
2. Then I solve for velocity: v=M/(rho*(xArea)/2)
(1.51gm/min)/(.8gm/liter)(1/2)(pi/4)(3in^2) then
*61.02in^3/liter to get the units right= 32.6in/min
3. Then for the Re # =rho*velocity*Dh/mu
(.8gm/liter)(32.6in/min)(1.833in)/(.05Poise)(2.54cm/in)(61.02in^3/liter)(60sec/min)
and I get the ridiculously low .1 for my answer.
Thanks for the help.
RE: Calculating heat transfer coefficient
RE: Calculating heat transfer coefficient
RE: Calculating heat transfer coefficient
The estimated Re is low. It is, in fact, in the laminar régime. Stonecold is right. I suspect the viscosity is too high; what is the ammonia % and the temperature ?
Anyway, given:
Linear velocity = 0.0138 m/s
Density: 900 kg/m3 (as originally stated)
Hyd. diameter = 0.0466 m
Abs. visc. = 5 cP = 0.005 kg/(m.s)
Now, assuming we are (conservatively) speaking of a straight pipe the values of Nu*Pr-0.33 = (hD/k)(Cpμ/k)-0.33, where h is the heat transfer coefficient, and reading from a small (old) graph, would be:
for L/D=25, ~3; for L/D=100, ~2; for L/D=500, it drops to ~1.