Steam heating

The steam heating is a heating system , the water vapor uses as an energy source. The heat of condensation and, to a lesser extent, the sensible heat of the steam and the condensate is used for heating purposes.

description

Steel jacket pipe laid exposed for the transport of steam

Steam heating is mainly used for heating industrial systems (hall heating) that generate process steam or have waste heat steam boilers. In addition, steam is used as a heat carrier in many older district heating networks, with the steam condensing at a heat exchanger in the district heating transfer stations and the heat being transferred to a secondary hot water network. Such facilities are indirectly heated hot water systems.

Steam as a heating medium

The enthalpy of evaporation of water under atmospheric conditions is 2258 kJ / kg or 0.627 kWh / kg. In colder places (surface temperature less than saturated steam temperature ) the water vapor condenses and this energy and, in the case of further hypothermia, the latent heat is released. By removing the condensate, the water vapor flows in.

Steam heaters are used to heat large rooms (halls, workshops, production plants), preferably in companies that generate process steam. For space heating, steam at low pressures (approx.> 0.5 bar overpressure), which can be generated via steam reducing stations or by turbine taps, can be used. With combined heat and power generation , electricity is generated and the waste heat is used. The exhaust gas can be used in a waste heat boiler to generate steam.

From around 1870 to the 1990s, passenger coaches were often heated by means of steam, which was provided either by the locomotive or by separate heating cars .

The steam heating consists of the steam line, the heat exchanger ( condenser ), the condensate drain and the condensate return line. A thermostatically controlled control valve in the steam supply line or a condensate drain control valve ("condensate") is used to regulate the heating. When laying longer pipelines, there are greater pressure losses in the return and can lead to undesired re-evaporation in the condensate line. In these cases, a condensate collecting tank is installed, from which the condensate is pumped back to the feed water tank of the steam boiler system. Alternatively, a condensate lifting station can be used, which consists of a pressure vessel into which the condensate flows when the ventilation valve is open. When the upper level is reached, the valve is closed to the environment and steam is superimposed, when the valve to the condensate return line opens, condensate is returned. Some systems, for example the district heating network of New York City , generally do without a return, instead the condensate is fed into the sewer system. Hence the typical image of the steaming manhole cover. As a disadvantage, new water always has to be treated , as drinking water is not suitable for steam boilers.

A distinction is made depending on the steam pressure used

Low pressure steam heating (assured operating pressures up to 1 bar overpressure ),
High pressure steam heating (secured operating pressures above 1 bar overpressure),
Vacuum steam heating (operation in negative pressure )

Differences between steam and hot water heating

The advantage of steam heating compared to conventional hot water heating is the high energy density of the steam, so that the circulating mass of the heat transfer medium is significantly lower than that of hot water heating. Due to the higher flow temperature compared to hot water heating, smaller heat transfer surfaces can be used. Pipelines with a very small cross-section can be used for the condensate return.

In a hot water heating system, the enthalpy difference between the flow ( T ₁ ) and return temperature ( T ₂ ) is used as a heat flow. The required mass flow to be circulated results from:

{\ displaystyle {\ dot {m}} = {\ frac {\ Delta Q} {c _ {\ mathrm {p}} \ cdot (T_ {1} -T_ {2})}}}

In the case of steam heating, the enthalpy of condensation represents the greatest amount of heat that can be used. In addition, the steam temperature is usually higher than the flow temperature of hot water heating systems. However, it must be taken into account that the heat capacity of water vapor is only about half as great as that of water. The usable heat results from the use of saturated steam:

{\ displaystyle {\ dot {m}} = {\ frac {\ Delta Q} {h_ {D} (p1) -c _ {\ mathrm {p}} \ cdot T_ {2}}} \!}

example

Heat flow ${\ displaystyle \ Delta Q = 10 \ mathrm {\ kW}}$

Parameter warm water heating: T ₁ = 100 ° C and return: T ₂ = 70 ° C; specific heat capacity of water c _p = 4.18 kJ / (kg K)

Steam parameters: saturated steam with p = 10 bar _a and T = 180 ° C; specific enthalpy h _D = 2777 kJ / kg, condensate return: T ₂ = 70 ° C

Hot water heating:

{\ displaystyle {\ dot {m}} = \ mathrm {\ frac {10 \ kW} {4 {,} 18 \ mathrm {\ frac {kJ} {kg \ cdot K}} \ cdot (100 \, ^ { \ circ} C-70 \, ^ {\ circ} C)}} = 0 {,} 081 \ mathrm {\ {\ frac {kg} {s}}} = 294 \ mathrm {\ {\ frac {kg} {H}}} }

For steam heating results

{\ displaystyle {\ dot {m}} = \ mathrm {\ frac {10 \ kW} {2777 {\ frac {kJ} {kg}} - 4.18 {\ frac {kJ} {kg \ cdot K}} \ cdot 70 \, ^ {\ circ} \ mathrm {C}}} = 0 {,} 00402 \ mathrm {\ frac {kg} {s}} = 14 \ mathrm {\ frac {kg} {h}}}

The example shows that the circulating mass flow in steam heating is many times lower than in hot water heating. Therefore, when using steam heating, no or only very little electrical energy has to be used to operate pumps. The heat transfer in the installation rooms can take place through thermal radiation or through fans that are thermostatically controlled.

The disadvantage of steam heating is the high temperatures at the heat exchangers, so that these must always be set up so that they are protected from direct contact. Furthermore, the potential danger from the medium steam compared to the practically harmless hot water must be taken into account. Thus the use of steam heating in residential buildings is practically impossible nowadays. Another disadvantage is the steam hammer, which can occur in the case of condensate accumulation in the steam supply line during shutdowns. The steam lines must therefore be automatically drained at low points. Cold steam lines with negative pressure lead to air penetration and corrosion in the steam network and can in turn lead to malfunction of the condensate drain. Therefore, steam heating systems are much more maintenance-intensive than heating networks with closed water circuits. Steam heating is usually less energetic than hot water heating systems, as these can be operated with lower flow temperatures that slide with the outside temperature. The hot water generator can mainly be operated with lower flow temperatures and thus lower flue gas temperatures compared to a steam generator, so that higher firing efficiencies are possible with the hot water generator.

Heat generator

Large-capacity water boilers , high-speed steam generators , electrically heated steam generators and waste heat boilers from CHP systems are used today to generate steam . Which steam generator is used often depends on the operating conditions, the technological processes and the consumers.

Individual evidence

↑ Heinrich Krohn: … on the rails. The history of passenger coaches and freight cars. Anniversary band of the Talbot wagon factory in Aachen on the occasion of the 150th anniversary, Prestel, Munich 1988, p. 71 f.

Web links

Wiktionary: Steam heating - explanations of meanings, word origins, synonyms, translations

Low pressure steam heating

[1] Heinrich Krohn: … on the rails. The history of passenger coaches and freight cars. Anniversary band of the Talbot wagon factory in Aachen on the occasion of the 150th anniversary, Prestel, Munich 1988, p. 71 f.