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