Heat Exchanger problem

Thats because heat transfer is more of an art than a science. When you calculate the Nu number if you are even with 30% error it is considered "very good" results.

We did a lab where we found the Nu number to be off by more than 50%.

 

What are the flowing media?

Did you do an energy balance between the hot and cold streams to see if they are relatively equal?

If you don't have a properly sized orifice, it could be off on flow rate by over 25%. I would start with the inclined manometer and use Bernoulli to solve for the velocity and thus volumteric flow rate.

 

i would go with the manometer, it is more accurate.
There is an error in the formula u r given, no wonder ur off target. the Nusselts number is DIRECTLY proportional to Reynolds number, so u have to lose the -8, actually the equation is: Nu= 0.023Re^0.8 x Pr^n. where n is 0.4 for heating of the fluid, and 0.3 for cooling. This formula is usually correct for fully turbulent flows with Pr between 0.6 and 100. A correction to this formula would be 1. Nu=0.0214(Re^0.8 - 100)Pr^0.4 for (Pr between 0.5-1.5, and Re between 10^4---5x10^6) or 2. Nu=0.012(Re^0.87 - 280)Pr^0.4 (for Pr bet.1.5-500) and Re bet. (3000---10^6). Remember all those equations go for SMOOTH tubes, so if u want to account for mismatches, try to find a correction for unsmooth tubes. i have in front of me now the correction formula used to account for friction losses, if u want it just let me know, and anyway u can find it in any heat transfer book.

 

ok follow this up, i hope ur familiar with the Staunton number: here goes
Eq.1: St*Pr=f/, f is the friction coefficient defined by:
Eq.2: Delta P (pressure difference)=f (L/d)(Rho)(Um^2/2g)
Um is the mean velocity
f=1.325/[ln(epsilon/3.7d)+5.74/Re^0.9]^2 for tubes..
Another approximation u can use is the following:
f*Re/4 = 16000...in this approximation if the tube is under constant heat flux then Nu =4.364, if constant wall temperature Nu=3.657
Check those up, i hope they help