Sizing of district heating substations

1

• 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

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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

3

• A district heating substation and secondary
  circuits in a residential building

4

• A district heating substation and secondary
  circuits in a residential building

Tindoor 21-22ºC
55ºC
5

• A district heating substation and secondary
  circuits in a residential building

50ºC
Tindoor 21-22ºC
55ºC
50ºC
6
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.
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Thermostatic balancing valve installed at the end
point of DHW circuit
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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.
9
Strongly reduced valve size as a consequence of
dynamic sizing being employed.
Valve actuator
Replaced valve
Heat exchanger
Source C. Forslund, Gävle Energy AB, Gävle
New valve
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• 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

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Dynamic sizing of control valves in domestic hot
water heaters field measurements
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
Position changes of control valve in a hot water
heater with varying valve size (field
measurements performed by Gävle Energy AB)
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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 kvs0,63 m3/h 80 kW
61-125 flats kvs1,0 m3/h 80 kW
126-200 flats kvs1,6 m3/h 140 kW
Special case 1 Secondary distribution system      next higher valve size 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 Floor heating or towel dryers supplied by domestic hot water circuit   next higher valve size
Source C. Forslund, Gävle Energy AB, Gävle
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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
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Domestic hot water (DHW) temperature measured at
the outlet of the heater while short overload
condition occurs
Valve position
DHW temperature
DHW circul. temperature


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Domestic hot water temperature measured at the
tap during morning hours in a hotel
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Theoretical flow characteristic of a control
valve of logarithmic type. Control ratio 1100,
kvs10 m3/h.
Logarithmic valve with kvs10 m3/h size and the
control ratio 1100, with the o pening ratio
limited to 50, acts as a nearly logarithmic
valve with kvs1 m3/h and the control ratio 110
Adaptive limiting of capacity of existing control
valve instead of replacing the valve by a smaller
one - a promising solution
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Number of flats with size of control valve
employed in tap water heaters for a large group
of residential buildings in Gävle, Sweden
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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).
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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