In the old days, when rock wool, polystyrene foam, double glazing and central heating had not yet been invented, it could get so viciously cold in the house in winter that children jug got to bed with a hot water bottle. That was a steel bottle with room for one or one and a half liters of boiling water. The pitcher was put in a sock or bag to prevent the children from getting burned. Sometimes that happened.
The hot water bottle is one of the oldest devices for local body heating. Modern variants can be found in hand warmers such as ‘hot packs’, ‘heat packs’, ‘wheat packs’ and so on. They are not necessary this year, but it can’t hurt to think about what they actually do for you.
Since it was established that an air temperature that is too high or too low in offices and companies does not burden all parts of the human body equally, but some parts more than others, it has been considered whether it would not be better to assist the most affected parts of the body from now on. with some extra cooling or heating. And therefore not adjusting the office temperature yourself. If an office that is too cool only leads to cold feet and cold backs, then you can also only heat those feet and backs a bit. That’s the idea. The logical next step is the office temperature on purpose to lower. That could save energy.
For example, an extensive study has been launched into the parts of the body that, in some cases, are the first to cause complaints. Some of it rests on self-report of subjects who can indicate with astonishing precision which part of the body (a lower arm, an upper back) feels a bit chilly or uncomfortable. When it’s cold, feet, hands and backs quickly feel too cold, when it’s hot, the face can be considered too warm.
Of course, scientists prefer to determine for themselves where the test subject is cold and it was obvious that they would take the skin temperature as a measure. But it turned out that the body parts with the lowest skin temperature are not always labeled as unpleasantly ‘cold’. A cold face doesn’t bother anyone.
The ‘heat flux’, the heat flow through the skin (expressed in watts per m2) seems like a better size for now. According to recently Chinese research it is pre-eminently the parts of the body where the heat flow is strong under the influence of variations in the ambient temperature changes who need help. What the help should then consist of is not entirely clear, because the body is a complex measuring and regulating system. The supply of heat in one place can simply stimulate the dissipation of heat in another place. That’s what makes it so difficult.
Preheated seats
Not everyone has waited for the outcome of the hard work. There are already many cars on the market where the seats and even the steering wheel can be preheated to alleviate the discomfort of a cold start. The elderly are advised without hesitation to put their feet in a tub of cold water if there is a threat of overheating in summer.
This brings us to the hot water bottles and hand warmers. What never became clear is whether these should only remove an uncomfortable feeling or whether they can significantly compensate for occurring physical heat loss. how much heat would you maximum due to a limited body surface, say 150 cm2 (two palms), being able to pump into the body – if you press a warm block of metal of constant temperature against it, for example? That is the question. It cannot be very much because the skin itself often has a temperature of 34 degrees and – that is where the limitation lies – hardly tolerates higher temperatures. The hands just tolerate 45 degrees, the feet only 40. Give it a try.
There is therefore a maximum amount of heat that you can bring into the body per second. For those 150 cm2 is the secure less than 9 watts. This is derived from an experiment in which a large aluminum can filled with water at 42 degrees was placed in a bucket containing water at 34 degrees. The bucket was so large that the value 34 remained practically constant in 45 minutes that the experiment lasted. The water in the bus dropped from 42 to 37 degrees in the first quarter of an hour and then slowly even further. With some graphical hocus pocus it has been estimated that the temperature initial fell at a rate of 0.4 degrees per minute. The 9 watt is derived from that.
The 9 watts for those two palms is anyway little compared to the 110 watts that the average person produces – and loses – in heat, but will in reality never be achieved because ‘skin’ does not conduct nearly as well as aluminum. 9 watts over 150 cm2 means 600 watts per m2. The Chinese research found for the heat fluxes inside out values that only in a single case exceed 80 W/m2 came out. It can be a bit higher from a warm metal from the outside in, but it seems that pitchers and hand warmers mainly take away an unpleasant feeling. The influence on the heat management of the whole body is minimal. It should be noted that things are much more favorable for those warm feet in that tub of cold water.