Mon, 30 May 2016
Agitator equipped vessels with half pipe coil jackets are widely used in variety of process applications. This article shows how to calculate heat transfer in an agitated vessel provided with an external half pipe coil jacket.
Overall heat transfer coefficient, U is defined as
1/U = 1/hi + ffi + x/k + ffo + 1/ho
hi : film coefficient process side
ho : film coefficient coil side
ffi : fouling factor process side
ffo : fouling factor coil side
x : vessel wall thickness
k : vessel wall thermal conductivity
Process side film coefficient, hi depends upon type of impeller, Reynold's number (Re) and Prandtl number (Pr).
Re = D².N.ρ / μ
Pr = Cp.μ/k
where, D is impellor diameter, N is impellor rpm, ρ is fluid density, μ is fluid viscosity, Cp is fluid specific heat and k is fluid thermal conductivity at bulk fluid temperature. hi is defined as following :
Nui = C.Rea. Prb. (μ/ μw)c. Gc
hi.DT/k = Nui
where constants C, a, b & c are available in literature for different type of impellors. DT is vessel diameter. Gc is geometric correction factor for non-standard geometries. μ/ μw is viscosity correction factor due to difference in viscosities at bulk fluid and wall temperatures. These constants are available in references mentioned below.
Pipe coils are made with a 180° central angle or a 120° central angle. Equivalent diameter (De) and Flow area (Ax) is defined as following.
De = (π/2).dci
Ax = (π/8).dci²
De = 0.708 dci
Ax = 0.154 dci²
where, dci is inner diameter of pipe.
Reynold's and Prandtl number are calculated based on jacket fluid properties and velocities.
Re = De.v.ρ / μ
Pr = Cp.μ / k
where, ρ is coil fluid density, μ is coil fluid viscosity and k is coil fluid thermal conductivity. v is fluid velocity in coil.
For Re > 10000
Nuc = 0.027 Re0.8 Pr0.33 (μ/ μw)0.14 (1 +3.5 De/Dc)
where Dc is the mean or centerline diameter of the coil. Coil outer diameter (Do) is determined as following.
180° Coil : Do = DT + 2(dci/2) + 2.x
120° Coil : Do = DT + 2(dci/4) + 2.x
Dc = ( Do + DT) / 2
For Re < 2100
Nuc = 1.86 [ Re.Pr.De/L ]0.33 (μ/ μw)0.14
where, L is coil length along the vessel.
For 2100 < Re < 10000
Calculate NuLaminar based on Re = 2100 and NuTurbulent based on Re = 10,000.
NuTransient = NuLaminar + (NuTurbulent - NuLaminar).(Re - 2100)/(10000 - 2100)
ho is determined as following
ho.De/k = Nuc
ho and hi thus calculated are used to get value of overall heat transfer coefficient U.