Hot water system

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Flow diagram of a hot water system with pump pressure maintenance

As a hot water system closed heating systems which are referred to in, the boiler water temperature of 110 ° C or more is achieved and in which the pressurized water is used as heat carrier.

Hot water systems are used in district heating systems for cities or are created for the transport of heat in industrial systems.

The water must not evaporate in the hot water boiler. The secured flow temperature of the hot water boiler must therefore be lower than the saturated steam pressure of the water associated with the safety temperature .

A hot water system consists of the hot water boiler or hot water generator, the circulation pumps, the pressure maintenance, the pipes and the heat consumer. A circulation pump ensures that the heat transfer medium circulates. A distinction is made between internal and external pressure maintenance. With internal pressure maintenance , the hot water in the boiler or in a high expansion vessel is overlaid with steam, so that saturated steam conditions exist in the boiler. Uncontrolled evaporation is physically not possible here. When maintaining external pressure , the maximum flow temperature, the maximum and the minimum system pressure must be safeguarded with limiters. The circulation must be monitored with a flow monitor if water in the steam boiler can inadmissibly evaporate in the event of a pump failure.

The water used as a heat transfer medium must meet relatively high quality requirements; conditioning agents are also added. It is routed over long distances to the consumers through a heat-insulated pipeline network, consisting of flow and return.

The heat consumers can be connected directly or indirectly via a secondary network. In district heating networks, consumers are usually connected indirectly in order to limit the temperature in the secondary network and to exclude possible contamination of the primary network. The consumers must have suitable converters and heat exchangers . In the converter, the flow temperature of the hot water system is converted (reduced) to the temperature required for the application of the consumer.

Pressure maintenance

Working and control range of a hot water system: The pressure min-limiter is set to a pressure that is above the saturated steam pressure of the secured flow temperature. The set pressure of the max pressure limiter is below the response pressure of the safety valve .

In a hot water system, the heat transfer medium must not evaporate; ie at no point may the pressure drop below the saturated steam pressure . The permissible operating temperature of the hot water generator must therefore be below the saturated steam temperature of the heat transfer medium at the secured operating pressure of the hot water generator. If the consumers are located higher than the boiler, then the static pressure in the water system at the highest point must be added to the operating pressure . When the pressure drops below the saturated steam pressure, the heat transfer medium begins to evaporate . This results in steam hammer, which has an explosive effect and which particularly affects the pipelines . The suspensions can tear off and this can lead to the pipelines being displaced. As a result, flange connections can tear open and this leads to a further drop in pressure, which can spread the damage.

The following forms of pressure maintenance are used:

  • Internal steam pad. The boiler is operated like a steam generator with a steam cushion, but here the hot water is drawn off via a dip tube. This assumes that the consumers are located lower than the boiler.
  • Elevated expansion vessel with steam cushion,
  • Nitrogen blanketed expansion vessel,
  • Diaphragm expansion vessel with intermediate vessel,
  • Pump pressure maintenance with pressure-free expansion vessel and overflow valves .

Since the evaporation of the water leads to steam hammer when the pressure drops , which can also endanger people, the minimum pressure and the flow temperature must be safeguarded with limiters, which are usually designed twice.

advantages

  • The heat is transported at a high temperature level.
  • In contrast to a steam network, there is no condensate, so that there is no contamination of the condensate, particularly in the case of extensive networks.
  • Provided that the pressure in the heat generation center is reliably maintained, no pressure surges can occur in the network.
  • In the stratified storage tank, larger amounts of heat can be temporarily stored in the form of hot water.

disadvantage

  • When the system pressure drops, steam hammer occurs .
  • Only the sensible heat ( heat capacity • temperature difference) at the consumer can be used, since no phase transition occurs.
  • Due to the high flow temperature , the cost of insulation is particularly high.

maintenance

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