Transcript Bild 1 - lsta.lt

Sizing of district heating substations
and optimum maintenance of
domestic hot water circuits in Sweden
Janusz Wollerstrand
Lund Institute of Technology
Department of Energy Sciences
Sweden
Topics
• Balancing of DHW circulation circuits
• Dynamic sizing of control valves in
domestic hot water (DHW) heaters
• Heat exchanger operation at overload
condition
• Practical experiences
A district heating substation and secondary
circuits in a residential building
Dhw
taps
Domestic Hot
Water circuit
Space heating
circuit
Dhw
post-heater
R
DH
supply
Dhw circulation
circuit
V
2.54 m3/h
00123 kWh
DH
return
Heat energy
meter
Dhw
pre-heater
A district heating substation and secondary
circuits in a residential building
Dhw
Tindoor
21-22ºC
Space heating
circuit
taps
Domestic Hot
Water circuit
55ºC
Dhw
post-heater
R
DH
supply
Dhw circulation
circuit
V
2.54 m3/h
00123 kWh
DH
return
Heat energy
meter
Dhw
pre-heater
A district heating substation and secondary
circuits in a residential building
Dhw
Tindoor
21-22ºC
Space heating
circuit
taps
Domestic Hot
Water circuit
55ºC
Dhw
post-heater
50ºC
R
DH
supply
Dhw circulation
circuit
V
2.54 m3/h
00123 kWh
DH
return
Heat energy
meter
Dhw
pre-heater
50ºC
Connecting scheme of the domestic hot water
circulating system in a large university building.
Temperatures at end-points and some short-cuts resulting in
low temperature in one of branches shown.
50,7
54,1
(short-cut)
47,7
too low
temperature
45,3
too low
temperature
51,1
55,6
51,0
N
DH-substation
and DHWC-pump
51,2
51,3
53,5
(short-cut)
S
Thermostatic balancing valve installed at
the end point of DHW circuit
Thermostatic
balancing valve
Temperature
registration
Connecting scheme of the domestic hot water
circulating system in a large university building.
Thermostatic balancing valves installed resulting in equalized
temperature level in the circuit.
50,4
50,1
50,4
48,6
still too low
temperature
51,1
56,4
N
S
50,8
DH-substation
and DHWC-pump
50,7
50,9
51,3
52,1
Strongly reduced valve size as a consequence
of dynamic sizing being employed.
Valve
actuator
Replaced
valve
Heat
exchanger
New valve
Source: C. Forslund, Gävle Energy AB, Gävle
Reasons for oversizing of control
valves in practice:
• overestimated design load values for DH
substation
• overestimated operating conditions of the
substation
• round up of the valve size in case of
discrepancy between the calculated size
and the available size (almost always)
• dynamics of the system not taken to
consideration
Dynamic sizing of control valves in domestic hot
water heaters – field measurements
Position changes of control
valve in a hot water heater with
varying valve size
(field measurements performed
by Gävle Energy AB)
DH water flow rates in a hot tap
water heater with different
control valve sizes. The peak
flow rate increases by
increased valve size but at
small loads the flow rate
remains mainly unchanged
1,0
120
0,9
DH water flow, l/s
Valve position, %
100
80
60
40
kvs=0,63
m 3/h
kvs=1,0
m 3/h
20
0
2000-08-15
kvs=1,6
3
m /h
2000-08-22
2000-08-29
2000-09-05
0,8
kvs=1,6
0,7
m 3 /h
0,6
kvs=1,0
0,5
kvs=0,63
0,4
m 3/h
m 3 /h
0,3
0,2
0,1
0,0
2000-08-15
2000-08-22
2000-08-29
2000-09-05
Dynamic sizing of control valves in domestic hot
water heaters as employed in Gävle, Sweden
Size of the
building
Valve size
kvs, m3/h
Heat exchanger size,
kW
10-60 flats
kvs=0,63 m3/h
80 kW
61-125 flats
kvs=1,0 m3/h
80 kW
126-200 flats
kvs=1,6 m3/h
140 kW
Special case 1: Secondary distribution system –
next higher valve size
Special case 2: Floor heating or towel dryers supplied by
domestic hot water circuit –
next higher valve size
Source: C. Forslund, Gävle Energy AB, Gävle
Outgoing hot water temperature and primary
return temperature from a heat exchanger vs.
hot water flow when the primary flow is limited.
If mixing of the hot
and the cold DHW
at the tap is taken
to account, 20%
overload at 45ºC
DHW temperature
is possible
Domestic hot water (DHW) temperature
measured at the outlet of the heater while short
overload condition occurs
Valve
position
DHW
temperature
DHW circul.
temperature
Domestic hot water temperature measured at
the tap during morning hours in a hotel
Theoretical flow characteristic of a control valve of
logarithmic type. Control ratio: 1:100, kvs=10 m3/h.
10
kvs 10
Logarithmic valve with
kvs=10 m3/h size and
the control ratio 1:100,
with the o
8
Adaptive
limiting of
capacity of
existing
control
valve
instead of
replacing
the valve by
a smaller
one −
a promising
solution
kv , m3/h
kvs 6.3
6
pening ratio
limited to 50%, acts as
kvs 4.0
a nearly logarithmic
valve with kvs=1 m3/h
and the control ratio
kvs 2.5
1:10
4
kvs 1.6
kvs 1.0
2
0
0
20
40
60
Valve position, %
80
100
Number of flats with size of control valve employed
in tap water heaters for a large group of residential
buildings in Gävle, Sweden
200
180
No
of
flats
160
poor <-circulation-> good
140
120
100
poor <-circulation-> good
80
kvs 2.5
60
40
20
kvs 1.0
kvs 0.63
kvs 1.6
0
1
11
21
31
41
51
61
Buildings
71
81
91
Short tappings often do not coincide when
hot water circulation is working well (the left
picture) but are likely to coincide otherwise
(the right picture).
0,2
taping, l/s
0,2
0,15
0,15
0,1
0,1
0,05
0,05
taping, l/s
Time, min
0
00:00
02:00
04:00
06:00
Time, min
0
00:00
02:00
04:00
06:00
Conclusions
• Dynamic sizing of control valves works
well in practice
• The choice of the size of control valve
strongly depends on proper
functioning of DHW circulation
• Adaptive adjustment of control valve
capacity − optimum solution
• Do not relay on DHW circulation return
temperature as a minimum temperature
of the circuit