Heat Exchanger Design

I would suggest applying the 1st law of Thermo for a control volume.

[ h + 1/2v +gh ]inlet (for each fluid) + Heat(Q) = [ h + 1/2v + gh ]exit (for each fluid) + Work

Also, you must make a few assumptions: - No work(incompressible liquid), No change in velocity, no change in potential. Also if you take your system to be around both pipes and assume no heatloss then Q=0. Therefore you are left with,

m1*[h1]i + m2*[h2]i = m1*[h1]e + m2*[h2]e; now if you have some sort of table, then use that. If not, /\ (change) h = m*cp/\(Temp):

Solve for T2 of water.

Use the following formula to calculate the chilled water outlet temperature Toutlet in degree Celcius:

M1 x Cp1 x ((273 + 40) - (273 + 20)) = M2 x Cp2 x ((273 + Toutlet) - (273 + 5))

M1 = Product mass flow rate in kg/hr

Cp1 = Product Heat Capacity, Kcal/kg.degree Kelvin

Make sure that you have the correct units.

Since the K°=°C it is not necessary to refer to the absolute zero. The equation can be solved in °C only

if both of the flow rate were known the answer is straight forward by using equation:

Energy balance: Q=m_hCp_h(T_hin-T_hout)=m_cCp_c(T_cout-T_cin)

But for this type of problem, usually only one of the flow rates is known. Therefore iteration is required.

Calculation requires tables and charts, but basically there are 2 equations that u should use:

Q=m_hCp_h(T_hin-T_hout)=m_cCp_c(T_cout-T_cin) and Q=UA F(LMTD);

F is the thermal correction factor and LMTD is the log mean temperature difference.

correction factor F will determine the type of S&T HE to be used (i.e single pass or multiple pass).

first u should determine/assume the unknown flow rate. Using the 1st equation u will obtain Temp. outlet of the product.

Temp outlet from the first equation is used to obtain Q from the 2nd equation This is where the table and charts come into play, which is to determine U,A and F.

From this point on u should iterate until the Q from the 1st equation is approximately equal to the Q from the 2nd equation. Tout questioned is the Tout that makes this condition possible.

good luck.