Chemical Engineering in Daily Life – Question 14 with Standard Answers Provided

Teachers have quite a lot of insights into life, which are worth learning from*. In fact, this is two processes working together: mass transfer (of water) and heat transfer (of heat). In winter, when moving from the outdoors to indoors, the human body is cold while the ambient temperature is high; the environment transfers heat to the body. At the same time, the evaporation of water from the body accelerates, and evaporation is an endothermic process. As a result, warming and cooling occur simultaneously, which is why it takes some time to feel warm. Take a hot bath before going outside; the body is warm, and when the cold air outside comes into contact with the body, it condenses and releases heat

Agree with the previous view. But there are different views later on. Taking a hot bath means that one doesn’t feel cold when going outside, because the body’s temperature is **higher than the ambient temperature. The heat emitted by the body condenses upon encountering cold air, which results in heat loss; this slows down the rate at which heat is lost from the body and reduces the temperature difference.

Clothes worn in winter are thicker; from a chemical engineering perspective, this is equivalent to having a thick insulation layer. Moreover, such an insulation layer has a low thermal conductivity. When entering a warm room from the outside, due to the thickness of the clothes and their excellent insulating properties, heat transfer is slow, so it takes some time for the body to warm up. Winters are very cold in the mornings, but taking a thorough and satisfying hot bath (or even a tub bath), with hot water coming into direct contact with the skin, allows for rapid heat transfer; as a result, the body’s surface temperature rises quickly, giving a feeling of warmth. Wearing clothes before going outside is like adding an insulating layer to the body’s surface, which reduces the rate at which heat is lost. So it doesn’t feel cold anymore.

Mammals’ skin serves as a insulation mechanism for the tissues beneath it; however, due to human evolution, we have less hair on our bodies. In winter, we cannot rely on fur to keep warm like other animals, so we need to wear cotton clothing to help us withstand the cold. When we move from the cold outside into a warm indoor environment, our clothes are cold and our skin temperature is not high. Heat must pass from the indoor air to our bodies through the thick layers of clothing; once the clothes warm up, the heat is then transferred to the skin. The heat transfer coefficient of gases is low to begin with, and coupled with the thermal resistance of all those layers of clothing, it takes a long time before the skin feels warm ； When taking a hot bath indoors, the heat transfer coefficient of hot water is high; as a result, the skin is quickly warmed by the hot water. After that, one puts on thick cotton clothes before going outside, where the temperature is low. The body then begins to lose heat, but winter clothing has good insulating properties, allowing it to maintain a balance between the heat lost by the body and the heat generated by the body, so one does not feel cold for a long time.

Standard answer: When you stay in a very cold environment (such as outdoors in the winter in the north) for a long time, many parts of your body become cold, such as your buttocks, legs, arms, and lower back. Your clothes truly carry a chill in the air as well. The clothes on your skin and the air inside them are cold, and part of the area beneath the skin is also cold. Your heart is warm, and the heating in the room (the fire) is warm as well; there’s pressure from both inside and outside. Yet your sensors for heat and cold are located not far beneath the skin. Its temperature rise is delayed. When you have just taken a hot bath, for example, every part of your body is warm (at a temperature of 35-38 degrees), the viscosity of the blood is low, which facilitates good blood circulation. The clothes worn also warmed up from the body’s heat. Comparing such a warm person to the previous cold one, the overall deltaH, calculated using mCT, is quite significant. This person went outside where it’s very cold; it will take some time for their body to cool down through their clothes, and that time could be around half an hour – anything is possible.