Mitsubishi Electric QAHV-N560YA-HPB, Service Handbook

Page: 47 / 176

[
II
Restrictions ]
- 40 -
HWE1517A GB
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.
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.
∆T1: Temperature difference between inlet and
outlet
∆T: Temperature difference of high temperature
part (low temperature part)
K: Overall heat transfer coefficient (W/m
2
K)
A: Total heat transfer area (m
2
)
G: Total mass flow rate (kg/s)
C: Specific heat (J/kgK)
NTU 1
ΔT1
ΔT
----------- = NTU 2
K×A
V×C
-------------- =
Effective length
Representative length of 1 channel
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)
×
2
∆P: Pressure loss
A shearing stress of 16 Pa or higher is required to reduce the amount of scale that adheres.
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.
Distance between plates
E
ffective l
ength
Side of heat exchanger
Front of heat exchanger
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