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

T indoor 21-22ºC 55ºC

A district heating substation and secondary circuits in a residential building 50ºC 55ºC T indoor 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.

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.

Strongly reduced valve size as a consequence of dynamic sizing being employed. Source: C. Forslund, Gävle Energy AB, Gävle New valve Replaced valve Valve actuator 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

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 the building Valve size k vs, m 3 /h Heat exchanger size, kW flatsk vs =0,63 m 3 /h80 kW flatsk vs =1,0 m 3 /h80 kW flatsk vs =1,6 m 3 /h140 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, k vs =10 m 3 /h. Logarithmic valve with k vs =10 m 3 /h size and the control ratio 1:100, with the o pening ratio limited to 50%, acts as a nearly logarithmic valve with k vs =1 m 3 /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

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