8.2.7 Pressure drop in the heat exchanger
Pressure drop calculation is an important task in heat exchanger design. The pressure drops in the tube side as well as shell side are very important and quite a few corelations are available in the literature. One such corelation is given below in the subsequent subsection.
8.2.7.1 Correlation for tube side pressure drop (eq. 8.10)
(8.10)
where,
ΔP_{t,f} = total pressure drop in the bundle of tube
f = friction factor (can be found out from Moody’s chart)
G_{t} = mass velocity of the fluid in the tube
L = tube length
n = no of tube passes
g = gravitational acceleration
ρ_{t} = density of the tube fluid
d_{i} = inside diameter of the tube
m =0.14 for Re > 2100
0.25 for Re < 2100
The above correlation is for the pressure drop in the tubes owing to the frictional losses. However in case of multi pass flow direction of the flow in the tube changes when flow is from 1pass to another pass and the pressure losses due to the change in direction is called returnloss. The returnloss (ΔP_{t,r}) is given by eq.8.11,
(8.11)
n = no of tube pass
v_{t} = velocity of the tube fluid
ρ_{t} = density of the tube fluid
Therefore, the total tube side pressure drop will be,
Δp_{t} = ΔP_{t,f} + ΔP_{t,r}
8.2.7.2 Correlation for shell side pressure drop
The following correlation (eq.8.12) may be used for an unbaffled shell,
(8.12)
The above equation can be modified to the following form (eq.8.13) for a baffled shell,
(8.13)
where
L = shell length
n_{s} = no of shell pass
n_{b} = no of baffles
ρ_{s} = shell side fluid density
G_{s} = shell side mass velocity
D_{h} = hydraulic diameter of the shell
D_{si} = inside diameter of shell
_{fs} = shell side friction factor
The hydraulic diameter (D_{h}) for the shell can be calculated by the following equation (eq. 8.14),
(8.14)
where,
n_{t} = number of tubes in the shell
d_{o} = outer diameter of the tube
The friction factor (f_{s}) can be obtained by the Moody’s chart for the corresponding Reynolds number
8.2.8. Heat transfer effectiveness and number of transfer units (NTU)
The LMTD is required to be calculated for the evaluation of heat exchanger performance. However, the LMTD cannot be directly calculated unless all the four terminal temperatures (T_{c,i}, T_{c,o}, T_{h,i}, T_{h,o}) of both the fluids are known.
Sometimes the estimation of the exchanger performance (q) is required to be calculated on the given inlet conditions, and the outlet temperature are not known until q is determined. Thus the problem depends on the iterative calculations. This type of problem may be taken care of using performance equivalent in terms of heating effectiveness parameter (η), which is defined as the ratio of the actual heat transfer to the maximum possible heat transfer. Thus,
(8.15)
For an infinite transfer area the most heat would be transferred in countercurrent flow and the q_{max} will be dependent on the lower heat capacity fluid as such,
The actual heat transfer
The capacity ratio, which is the relative thermal size of the two fluid streams, is defined as,
On careful analysis, we can say that
U·A: Heat exchange capacities per unit temperature difference.
This thermal sizing (U·A) can be nondimensionalised by dividing it to the storage capacity of one of the fluid streams. Given limits the maximum heat transfers. The nondimensional term obtained is known as the number of transfer units (NTU)
It should be noted that
The actual determination of this function may be done using heat balances for the streams. For a parallel flow exchanger the relation is shown below
The above relation is true for both the condition
Similarly the functional relationship for counter –current exchanger is
(8.16 & 8.17)
The previous relation (eq. 8.16 and 8.17) were for 11 exchanger. The relation for 12 exchanger (counter current) is given by eq. 8.18, 8.19),
(8.18)
(8.19)
When the fluid streams are condensing in a 11 pass exchanger (fig. 8.15) as shown below,
Fig.8.15: Condenser with the temperature nomenclature
the following relation arrives.
Use Code STAYHOME200 and get INR 200 additional OFF

Use Coupon Code 
58 videos70 docs85 tests
