Mitsubishi Electric QAHV Série Manuel D'installation & D'instructions page 19

Step 3 Determination of specifications of the heat exchanger
Determine the model of heat exchanger and number of plates in consultation with the heat exchanger
manufacturer based on the above requirements.
* To determine the number of plates, calculate the number of plates while referring to the example below.
Values to use when determining the number of plates:
1 Overall heat transfer coefficient of corresponding heat exchanger
2 Heat transfer area per plate
Calculation method
A Obtain the data of 1 and 2 from the heat exchanger manufacturer.
B Estimate the number of plates of the heat exchanger.
C Check that the number of transfer units for the corresponding number of plates matches between
NTU1 and NTU2 (NTU1=NTU2).
If they are matched, select a heat exchanger having the corresponding number of plates. If they are
not matched, change the number of plates and then return to B to perform the calculation again.
T1
NTU1 = ‐‐‐‐‐‐‐‐‐‐‐
NTU2
T
Step 4 Calculation of the shearing stress
Calculate the shearing stress using the following method.
Values required for calculation
• Relationship between flow rate and pressure loss of corresponding heat exchanger (Obtain the data
from the heat exchanger manufacturer.)
Calculation method
Calculate the shearing stress using the following formula.
Representative length of 1 channel
Effective length
Effective length:
Length between water inlet and water outlet
(refer to the figure on the right)
Representative length of 1 channel:
Distance between plates
(refer to the figure on the right)
∆P: Pressure loss
A shearing stress of 16 Pa
If the shearing stress is low:
• Select a vertically short shape.
• Change the shape of the plates.
Reselect a heat exchanger that will increase the shearing stress by following methods described above.
∆T1: Temperature difference between inlet and
outlet

K A
=
---------------------------------
∆T: Temperature difference of high temperature


V C 3600
part (low temperature part)
K: Overall heat transfer coefficient (BTU/ft
×
2
(0.01 ftAq)
or higher is required to reduce the amount of scale that adheres.
GB-19
A: Total heat transfer area (ft
V: Total mass flow rate (lb/s)
C: Specific heat (BTU/lb
2
•°F•
h)
Side of heat exchanger
Front of heat exchanger
Distance between plates
2
)
•°F
)