What is the latent heat of steam

Latent warmth

Lexicon> letter L> latent heat

Definition: heat that is absorbed or given off by an object without changing its temperature

More general term: heat

Counter-term: sensible warmth

English: latent heat

Categories: energy storage, physical principles, heat and cold

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 03/09/2010; last change: 03/14/2020

URL: https://www.energie-lexikon.info/latente_waerme.html

Latent warmth is heat that is absorbed or given off by an object without changing its temperature. This situation occurs when heat is consumed or released during phase transitions.

Figure 1 shows the temperature profile over time when 1 kg of water ice (initially at −30 ° C) is heated evenly with an output of 100 W. The constant supply of heat initially leads to a constant rise in temperature to 0 ° C. The temperature then remains at 0 ° C for some time; the heat supplied is completely used up to melt the ice (as Heat of fusion) instead of increasing the temperature. In this respect, the heat supply remains hidden (latent), although the enthalpy is constantly increasing. Only when all the ice has melted does the temperature of the water continue to rise. At 100 ° C the water begins to evaporate; only when the Heat of evaporation for all the water has been applied, the temperature of the steam can continue to rise. Since the heat of evaporation is much greater than the heat of fusion, the evaporation takes correspondingly longer.

The opposite is true for continuous heat extraction, e.g. B. starting with liquid water. Once freezing (solidifying) begins, the temperature will stay at 0 ° C until all of the water is frozen. The entire heat of fusion is released again as solidification heat and prevents the temperature drop during this time. Similarly, when water vapor condenses, the Heat of condensation free again.

There are others Phase change materials (PCM = phase change materials) that can store latent heat. For example, certain waxes (paraffins) have phase change temperatures close to room temperature and are therefore well suited to greatly increase the heat storage capacity of building materials. For slightly higher phase change temperatures, certain salt hydrates are suitable, for example Glauber's salt and sodium acetate. The latter can be liquefied by supplying heat at 58 ° C, which is not a matter of melting, but of dissolving the salt in its own crystal water. The stored heat is released again during later crystallization. A special feature is that the crystallization does not inevitably occur when the temperature falls below 58 ° C, but can still not occur in the absence of crystallization nuclei even at freezing temperatures. If the crystallization z. B. triggered by the introduction of a germ, the temperature rises rapidly to 58 ° C. This effect is z. B. exploited in rechargeable heat pads.

Latent heat in energy technology

The phenomenon of latent heat is relevant in various ways in energy technology:

  • The heat of evaporation and condensation play an important role in the functioning of steam turbines.
  • The condensation heat is also used in condensing boilers, which means that the overall heat yield is increased accordingly.
  • Air with high humidity has a higher enthalpy (per cubic meter) than equally warm dry air. The evaporation of water requires heat, which is then found in the air as latent heat.
  • The heat storage capacity of a house can be increased by Latent heat storage to be built in. These contain phase change materials, e.g. B. Salts or small wax spheres that melt at a certain temperature and absorb heat, which they give off again when they solidify (see above).
  • Mobile heat accumulators have also been developed in which a latent heat accumulator (e.g. based on sodium acetate) with waste heat z. B. is charged by a power station, driven by truck to a consumer and discharged there. This can be economical if the transport distance is short enough, but a local heating pipe would be too expensive.
  • Small latent heat accumulators are occasionally used in cars to preheat the engine block before starting. While driving, they are heated up again by the engine's cooling water.

Small heat pads made from sodium acetate are used to keep hands warm on icy winter days. There are other applications in special functional textiles, including washing machines and dishwashers to recover heat from wastewater.

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See also: latent heat storage, heat, condensing boiler, temperature, enthalpy, heat of evaporation and heat of condensation, heat of fusion and heat of solidification
as well as other articles in the categories of energy storage, physical principles, heat and cold