Sizing of district heating substations and optimum maintenance of
domestic hot water circuits in Sweden
Janusz Wollerstrand
Lund Institute of TechnologyDepartment 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
R V
Heat energy meter
Dhw circulation circuit
2.54 m3/h00123 kWh
Dhw pre-heater
Dhw post-heater
DH supply
DH return
Dhw taps
Space heating circuit
Domestic HotWater circuit
A district heating substation and secondary circuits in a residential building
R V
Heat energy meter
Dhw circulation circuit
2.54 m3/h00123 kWh
Dhw pre-heater
Dhw post-heater
DH supply
DH return
Dhw taps
Space heating circuit
Domestic HotWater circuit
Tindoor
21-22ºC
55ºC
A district heating substation and secondary circuits in a residential building
R V
Heat energy meter
Dhw circulation circuit
2.54 m3/h00123 kWh
Dhw pre-heater
Dhw post-heater
DH supply
DH return
Dhw taps
Space heating circuit
Domestic HotWater circuit
50ºC
50ºC
55ºC
Tindoor
21-22ºC
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.45,3too lowtemperature
50,7
54,1 (short-cut)
47,7too lowtemperature
51,0
51,2
53,5(short-cut)
51,3
DH-substationand DHWC-pump
SN55,6
51,1
Thermostatic balancing valve
Temperature registration
Thermostatic balancing valve installed at the end point of DHW circuit
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
56,4
50,1
50,4
48,6still too lowtemperature
50,8
50,7
51,3
50,9
51,1
SN
52,1
DH-substationand DHWC-pump
Strongly reduced valve size as a consequence
of dynamic sizing being employed.
Source: C. Forslund, Gävle Energy AB, GävleNew valve
Replaced valve
Valveactuator
Heat exchanger
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
0
20
40
60
80
100
120
2000-08-15 2000-08-22 2000-08-29 2000-09-05
Val
ve p
ositi
on, %
kvs=0,63
m3/h kvs=1,0
m3/hkvs=1,6
m3/h
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
2000-08-15 2000-08-22 2000-08-29 2000-09-05
DH
wat
er f
low
, l/s
kvs=1,6
m3/h
kvs=1,0
m3/hkvs=0,63
m3/h
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
Dynamic sizing of control valves in domestic hot water heaters – field measurements
Dynamic sizing of control valves in domestic hot water heaters as employed in Gävle, Sweden
Size of thebuilding
Valve sizekvs, 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 bydomestic 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
Valveposition
DHW temperature
DHW circul. temperature
Domestic hot water temperature measured at the tap during morning hours in a hotel
0
2
4
6
8
10
0 20 40 60 80 100Valve position, %
kv, m
3/h
kvs 10
kvs 6.3
kvs 4.0
kvs 2.5
kvs 1.6
kvs 1.0
Theoretical flow characteristic of a control valve of logarithmic type. Control ratio: 1:100, kvs=10 m3/h.
Logarithmic valve with kvs=10 m3/h size and
the control ratio 1:100, with the o
pening ratio limited to 50%, acts as a nearly logarithmic valve with kvs=1 m3/h
and the control ratio 1:10
Adaptive limiting of capacity of existing control valve instead of replacing the valve by a smaller one − a promising solution
Number of flats with size of control valve employed in tap water heaters for a large group of residential
buildings in Gävle, Sweden
0
20
40
60
80
100
120
140
160
180
200
1 11 21 31 41 51 61 71 81 91
kvs 0.63 kvs 1.0kvs 1.6
kvs 2.5
No offlats
Buildings
poor <-circulation-> good
poor <-circulation-> good
0
0,05
0,1
0,15
0,2
00:00 02:00 04:00 06:00
taping, l/s
Time, min
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).
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