HVAC SYSTEMS - HFT Stuttgart

Page - 25 -
CHAPTER 02
to ensure as low collector supply temperatures as possible. For an ideal heat
exchanger the temperature of the cold fluid could be heated up to the inlet
temperature of the cold fluid. The heat exchanger efficiency describes how
close a heat exchanger comes to the ideal case and is therefore defined as the
ratio between the maximum possible heat transfer and the actual heat transfer
as shown in Equation 2.2.1-24.
m&
1c
1
=
m&
c
1
( T1,
in −T1,
aus )
( T −T
)
1
1,
in
2,
in
m&
2c
2
=
m&
c
1
( T 2,
aus −T
2,
in )
( T −T
)
φ (2.2.1-24)
1
1,
in
The heat exchanger effectiveness of the most common heat exchangers
(counter flow, concurrent flow, cross flow) are functionally dependent on the
ratio between the heat transfer power UA and the capacity flow C& = m&
⋅ c . This
ratio is called as number of transfer units NTU.
UA
NTU =
C&
(2.2.1-25)
The heat exchanger efficiency for counter flow heat exchangers is according to
(Bosnjakovic, 1951) for heat capacity flows C&
1 < C&
2 as follows:
φ =
1 − e
C 1 1 − e
C&
⎛ C ⎞ 1 & −⎜1
⎟
⎜
− ⋅
C&
⎟
2
2
⎛ C&
⎞ 1 UA
−⎜1
⎟
⎜
− ⋅
C&
⎟
2 C&
⎝ ⎠ 1
⎝
& UA
C&
⎠ 1
2,
in
(2.2.1-26)
In INSEL a block called ‘HXS’ is already available based on the equations
shown above. This block is used for the simulation of the counter flow heat
exchangers of the solar systems analysed in Caper 3, 4 and 5.