DIMENSIONING
EXPANSION VESSELS
HEAT EXCHANGER
Werner Weiss
AEE - Institute for Sustainable Technologies (AEE INTEC)
A-8200 Gleisdorf, Feldgasse 19
AUSTRIA
Monitoring Results – Collector Temperatures
T-Solar VL
System
T_Solar RL
T-Prozess
140
120
100
80
T
Temperatur[°
°C]
60
40
20
0
20.9.06 0:00
21.9.06 0:00
22.9.06 0:00
23.9.06 0:00
24.9.06 0:00
25.9.06 0:00
26.9.06 0:00
27.9.06 0:00
Stagnation
g
behaviour – Test lab
Test- und Versuchssystem mit eingezeichneten Messstellen
Test
Geplanter Aufbau des Versuchs- und Testsystems
Stagnation behaviour – Test lab
Test system setup and positions of the sensors
Stagnation behaviour – Test lab
Test system: collectors
Stagnation behaviour – Test lab
Test system: Overall system
Stagnation behaviour – Test lab
Test system: Heat Exchangers and Expansion Vessels
Stagnation behaviour – In situ Monitoring
Stagnation behaviour – In situ Monitoring
Meßanlage Bauer
T 12
T 14
T 13
T 15
T8
T 10
Vorlauf 10,23 m
T7
T 11
T6
T 16
Dachboden 9,01 m
T5
T9
3, 2 m
T4
3, 2 m
T3
T2
T1
Rückla f 5,91
Rücklauf
5 91 m
22 m² Brutto - West
22 m² Brutto - Ost
Kellerdecke 2,44 m
T 21
4,8 m³
Energiespeicher
T 20
p-Vor
dpd
Vor
Druckmessung 2,27 m
p-Rück
T 18
1,45 m
T 17
1,05 m
dp-Rück
p-AG
T 19
T 23
Ausdehnungsgefäß
T 22
Druckmessung 0,29 m
Bezugsniveau
M 1:100
M 1:50
Stagnation behaviour – In situ Monitoring
Meßanlage Bauer, 10. 09. 1999 - 13:30
Globalstrahlung: 858 W/m²
138°C
138
C
138°C
138
C
138°C
138
C
Vorlauf 10
10,23
23 m
138°C
138°C
151°C
137°C
137°C
Dachboden 9,01 m
150°C
137°C
149°C
3 2m
3,
EN12975
137°C
3 2m
3,
139°C
138°C
0= 0.8
a1aa=2.4 W/(m² K)
a2a=0.015 W/(m² K²)
22 m² Brutto - Ost
2,39 bar
98°C
4,8 m³
Energiespeicher
139°C
139°C
Rücklauf 5,91 m
22 m²² Brutto - West
Kellerdecke 2,44 m
2,40 bar
Druckmessung 2,27 m
121°C
39°C
1,45 m
38°C
1,05 m
Ausdehnungs34°C gefäß
2,58 bar
44°C
45°C
Druckmessung 0,29 m
Bezugsniveau
M 1:100
M 1:50
Stagnation behaviour
Temperaturen
160
T1 Sa
Sam. 1
T2 Sa
Sam. 2
T3
3 Sa
Sam. 3
T4 Sa
Sam. 4
T6 Sam. 6
T7 Sam. 7
T8 DB Vorl G1
Pumpe
150
T5
5 Sa
Sam. 5
Phase 5
120
110
100
90
Phase 3
Phase 4
80
70
60
50
Phas
se 2
Temperatur [° C]
T
130
Phase 1
140
40
11 24
11:24
12 36
12:36
13 48
13:48
15 00
15:00
Zeit
16 12
16:12
Stagnation behaviour
P Ausdehnungsgefäß
P has e 2
P Vorlauf
Pumpe
Phase 5
P has e 1
D ruc k [ba r]
2,5
2
P Rücklauf
Druck
3
15
1,5
Phase 4
Phase 3
1
0,5
11:24
12:36
13:48
15:00
Zeit
16:12
Collector with bad emptying behaviour
Kollektorverschaltung mit ungünstigem Entleerungsverhalten:
Kollektor im
Normalbetrieb
EN12975
Stagnationszustand
Dampfbildung im Kollektor
0= 0.8
a1aa=2.4 W/(m² K)
a2a=0.015 W/(m² K²)
Collector with good emptying behaviour
Stagnationszustand
Dampfbildung im Kollektor
Kollektor im
Normalbetrieb
EN12975
0= 0.8
a1aa=2.4 W/(m² K)
a2a=0.015 W/(m² K²)
HYDRAULIC
U C SC
SCHEME O
OF A SO
SOLAR HOT
O WATER S
SYSTEM
S
6
1 circulation pump
collector area
2 gravity brake
3 lock valve
4 thermometer and pressure gauge
5 pressure relief valve
3bar
°C/bar
°C
5
4
6 escape valve
hot water
7 thermometer
8 expansion tank
7
°C
3
1
storage
tank
2
8
10
cold water
9
fill and empty valve
EXPANSION VESSEL
EXPANSION VESSEL
correct
wrong
EXPANSION VESSEL
Mode of operation
In order to keep the increase in pressure in all cases of
operation at least 20% below the responding
pressure of the security valve the expansion vessel
h tto contain
has
t i
1. the expansion volume of the heat transfer fluid
2. the overall vapour volume (VD) at the state of
stagnation
General - MEV
Adjustment of the Nitrogen-pre-pressure prior to the installation
Annual check of the pressure
Hanging installation with not insulted copper pipe
Installation before the pump and after the non-return valve
Membrane has to be resistant against glycol (anti-freeze fluid)
EXPANSION VESSEL
1) Delivery state
2) Normal working condition
3) Max.
M
pressure (3 – 6 b
bar))
Terms for dimensioning
Efficiency (Nutzeffekt) N
Capacity of the MEV, which can be used
without damage (over expansion) of the
membrane
b
Primary pressure in the vessel P0
Static height of the system plus 0.5 - 1 bar
over pressure at the highest point of the
system
t
Primary fluid (Gefäßvorlage) VV
Reserve of fluid in case of a slight loss of
pressure during air release
max. temperature
at the membrane:
90°C
Dimensioning
Difference pressure Pdiff [bar]
Hdiff = HMEV – HSV [m]
ρ Density of the heat transfer fluid [kg/m³]
MEV Efficiency N [-]
Pe = Pressure of the system [bar]
P0 = Primary pressure in the MEV [bar]
Coefficient of expansion n [1]
MEV Nominal volume VN [l]
VD = max.
max volume of vapour [l]
VG = Total volume of the heat transfer fuid [l]
VV = Primary fluid (Gefäßvorlage) [l]
Pdiff 
N
 H diff  kalt  9,81
100.000
Pe  Pdiff  1 
Pe  Pdiff
(P0  1)
0,9
1
 kalt
n
 1  0,09
 warm
VN 
VG  n  VV  VD
N
Calculation of the Primary Fluid
TK  Tmax
VV  VK 
Tmax  TV
VV
VK
TK
TV
Tmax
Primary fluid [l]
Volume of the collector [l]
Temperature of the collector fluid at the expansion vessel [°C]
Origin temperature of the primary fluid in the MEV [°C]
[ C]
Max. permissible temperature in the MEV [°C] z.B. 90 °
Result: The volume of the primary fluid should
correspond to the volume of the collector.
Example of calculation
Conditions:
·
10 m² collector area
·
flow pipe VL: 15 m Cu pipe 18x1
·
ret rn pipe VL: 15 m Cu
return
C pipe 18x1
18 1
·
safety valve: 6 bar
·
pressure of the system: 2.5 bar
·
primary pressure in the expansion vessel: 2.0
2 0 bar
1st step : volume of the expansion vessel
VG  n  VV  VD
VN 
N
MEV
VD
VG
VV
nominal volume VN
maximum vapour volume
total volume of the heat transfer fluid
primary fluid
N
MEV efficiency
n
coefficient of expansion of the heat transfer fluid
litre
litre
litre
litre
2nd step: Calculation of the MEV efficiency
( P0  1)
Pe  Pdiff  1 
0
.
9
N
Pe  Pdiff  1
N
Pe
P0
MEV efficiency
nominal p
pressure of safety
y valve
primary pressure
bar
bar
3rd step: Calculation of the MEV efficiency
Pdiff 
r
 H diff   cold  9.81
100,000
Pdiff
pressure difference
Hdiff
HMEV–HSV
density of the heat transfer fluid kg/m³
bar
m
~1051 kg/m³
3rd step: Calculation of the MEV efficiency
Pdiff
r
0.5 1051 9.81

 0.052bar
100,000
Pdiff
pressure difference
Hdiff
HMEV–HSV
density of the heat transfer fluid kg/m³
bar
m
~1051 kg/m³
3rd step: Calculation of the MEV efficiency
( P0  1)
(2.0  1)
Pe  Pdiff  1 
4.8  0.052  1 
0.9 
0.9  0.43
N
Pe  Pdiff
4.8  0.052  1
d ff  1
Pe = nominal pressure of safety valve – tolerance of respond (20 %)
Pe = 6 bar – 20 % = 4.8
4 8 bar
Pressure difference Pdiff = 0.052 bar
Nominal pressure of safety valve Pe = 4.8
4 8 bar
Po = 2.0 bar
4th step: Calculation of the heat transfer fluid
VG = Vpipe+ Vcoll + Vheat exchanger
Factor of expansion
lit
litre
n
 coldld
n
 1  0.99
 hot
0.45 l/ m²
0.16  
VG 
 300  4.5  2.0  12.5
4
2
5th step: Calculation of the primary fluid VV
The volume of the primary fluid is more or
less equivalent to the volume of the collector.
->VV = Vcoll = 4.5 litre
6th step: Calculation of the volume of vapour VD
VD = Vcoll + Vpipe-vapor
litre
Vcollll = 4.5
4 5 litre
Calculation of the volume of vapour in the pipes
Maximum vapour power = 10 m² * 50 W/m² = 500 W
Vapour Power:
Good draining collectors: 50 W/m²
Bad draining collector: 120 W/m²
6th step: Calculation of the volume of vapour VD
Calculation of the reach of the vapour in the solar
pipes Vpipe-vapor (calculation through the thermal power
loss of the pipes):
Meßanlage Bauer, 10. 09. 1999 - 13:30
Globalstrahlung: 858 W/m²
138°C
138°C
138°C
Vorlauf 10,23 m
138°C
138°C
151°C
137°C
137°C
137
C
Dachboden 9,01 m
150°C
137°C
149°C
137°C
3, 2 m
3, 2 m
139°C
138°C
139°C
139°C
Rücklauf 5,91 m
22 m² Brutto - Ost
22 m² Brutto - West
2,39
, bar
98°C
4,8 m³
Energiespeicher
Kellerdecke 2,44 m
2,40
, 0 ba
bar
Druckmessung 2,27 m
121°C
39°C
thermal power loss of the pipe: 25 W/m
1,45 m
38°C
1,05 m
Ausdehnungs34°C gefäß
2,58 bar
44°C
(reach of vapour in the pipe) =
(max. vapour power)/(thermal power loss of the pipe
per meter of pipe)
(reach of vapour in the pipe) = 500/25 = 20 meter
16 C
16er
Cu pipe
i
45°C
Druckmessung 0,29 m
Bezugsniveau
M 1:100
M 1:50
6th step: Calculation of the volume of vapour VD
0.16  

 200  4.0
4
2
V pipevapour
VD = 4.5 + 4.0 = 8.5 litre
6th step: Volume of the expansion vessel
VG  n  VV  VD 12.5  0.09  4.5  8.5
VN 

 32.8 litre
N
0.43
Rule of Thumb
Nominal volume of expansion vessel [ltr.] = collector area x 3.5
VN = acollll x 3
3.5
5
VN = 10 x 3.5 = 35 litre
Heat Exchanger
Flat plate heat exchanger
Possibility of high pressures at operation
Principle of counter current
The high grades of turbulence leads to a self cleaning effect and
subsequently to a minimization of costs and a longer lifetime
One p
plate heat exchanger
g can be used to load more than one
heat storage tank
Disadvantages:
Like at all other external heat exchangers an additional pump is
necessary on the secondary side of the heat exchanger
Flat plate heat exchanger
They are very compact compared with ordinary coil heat
exchangers. They save about 85 to 90 % in volume and weight.
Maximum exploitation of the material: the capacity is 25 % higher as
the capacity of screwed plate heat exchangers.
exchangers The capacity is 10
times higher as the capacity of coil heat exchangers.
Less use of energy because of a better heat transfer coefficient and
subsequently a better temperature difference
Heat transfer still at a temperature difference of 1 K
Design of Flat plate heat exchanger
Temperature difference of a heat exchanger shown in a -T-diagram
T diagram
T1
m1
T1
T4
T2
T2
T3
m2
Q
Design of Flat plate heat exchanger
 m
 1  cp1  T1  T2   m
 2  cp 2  T4  T3 
Q
  U  A  T
Q
log
Tlog
T1  T2

T1
ln
T2
Design of Flat plate heat exchanger

Q
transferred power
W
 1, m
2
m
mass flow per second
kg/s
cp1,cp2 specific
ifi heat
h t capacity
it
kJ/k K
kJ/kgK
Ti
temperature of the media
°C
U
heat transmission coefficient of the heat exchanger
W/m²K
A
heat transfer area
m²
DTlog
logarithmic temperature difference
K
Design of Flat plate heat exchanger
Plate heat exchangers reach u-values
u values between 1000 and 2000
W/m²K (high power, low volume). Table 1 gives the necessary
values in order to design a heat exchanger.
Data for design
g of heat exchangers
g
Media
Primary circuit
Secondary circuit
propylenglycol/water
water
Concentration of the fluid
%
40
0
Temperature at entrance
°C
65
25
Temperature at exit
°C
32
60
Mass flow
kg/s
0.25
–*
*results from assumed data
Design of Flat plate heat exchanger
Design of plate heat exchanger
Design of coil heat exchangers
Smooth pipe heat exchanger:
approx. 0.2 m² heat exchanger surface per m² collector area
Finned tubes heat exchanger:
approx. 0.3 – 0.4 m² heat exchanger surface per m² collector area
Design of coil heat exchangers
General rules of thumb
Collector Area
1 m² collector area per person (bed, shower)
Storage Capacity
70 litre storage capacity / m² collector area
Heat Exchanger
0 2 m²² / m²² collector
0.2
ll t area ffor smooth
th ttube
b HX
0.3 – 0.4 m² / m² collector area for finned tube HX
Expansion Vessel
VN = collector area x 3.5