Perspiration Cooling of Body

For 600 gm/day perspiration, cooling rate Q/t = 17 watts. 对于每天蒸发600克的汗水，散热速率Q/t = 17瓦特。 Why is the heat of vaporization greater at body temperature? 为什么在体温时汽化热更大？

When the ambient temperature is above body temperature, then radiation, conduction and convection all transfer heat into the body rather than out. Since there must be a net outward heat transfer, the only mechanisms left under those conditions are the evaporation of perspiration from the skin and the evaporative cooling from exhaled moisture. Even when one is unaware of perspiration, physiology texts quote an amount of about 600 grams per day of "insensate loss" of moisture from the skin. 当环境温度高于体温时，辐射、传导和对流都会将热量传入体内而非传出。由于必须有净的热量向外转移，此时剩下的传热机制只能是皮肤表面汗液的蒸发以及呼出的湿气的蒸发散热。即使人们并未意识到出汗，生理学教材仍引用每天约600克的‘无意识失水’从皮肤流失的数据。 The cooling effect of perspiration evaporation makes use of the very large heat of vaporization of water. This heat of vaporization is 540 calories/gm at the boiling point, but is even larger, 580 cal/gm, at the normal skin temperature. 汗水蒸发的冷却效应利用了水非常大的汽化热。这种汽化热在沸点时为540卡/克，但在正常皮肤温度时更大，达到580卡/克。

As part of the physiological regulation of body temperature, the skin will begin to sweat almost precisely at 37°C and the perspiration will increase rapidly with increasing skin temperature. Guyton reports that a normal maximum perspiration rate is about 1.5 liters/hour, but that after 4 to 6 weeks of acclimatization in a tropical climate, it can reach 3.5 liters/hr! You would have to just sit around drinking constantly, just to keep from getting dehydrated! That maximum rate corresponds to a maximum cooling power of almost 2.4 kilowatts!

Heat Transfer by Vaporization

If part of a liquid evaporates, it cools the liquid remaining behind because it must extract the necessary heat of vaporization from that liquid in order to make the phase change to the gaseous state. It is therefore an important means of heat transfer in certain circumstances, such as the cooling of the human body when it is subjected to ambient temperatures above the normal body temperature.

Cooling by Evaporation

Because of the large heat of vaporization of water, the evaporation from a liquid surface is a very effective cooling mechanism. The human body makes use of evaporative cooling by perspiration to give off energy even when surrounded by a temperature higher than body temperature. The cooling process is an example of the approach to thermal equilibrium. The evaporative cooling rate is given by

The cooling rate for a liquid (below boiling) is more complicated since the heat of vaporization changes with temperature, and the rate of evaporation depends upon ambient temperature and relative humidity.