If you want to know the answer for what are the general comfort conditions in an air-conditioning system, then the answer to this question is quite tricky to pin down exactly what makes for comfortable conditions in an air-conditioning system.
You might be thinking that temperature is the key factor, but that’s not necessarily the case. Other things like humidity, air purity, and airflow speed all play a role in determining how comfortable people feel.
So, it’s not as simple as just measuring one thing to determine whether the conditions are comfortable or not.
What are the general comfort conditions in an air-conditioning system?
Extensive tests have been conducted on the impact of temperature, humidity, and airflow speed. But, the results haven’t fully agreed on what makes for comfortable conditions.
Another challenge is finding a way to measure comfort using just a single parameter which can take into account air temperature, humidity, airflow speed, and air purity to get a complete picture of comfort.
Usually, a single factor called the effective temperature is used to assess comfort levels.
What are the general comfort conditions recommended for summer air conditioning?
For summer air conditioning, the general comfort conditions recommended are:
an effective temperature(ET) of 21.7°C,
What are the general comfort conditions recommended for winter air conditioning?
During winter, the body gets acclimatized to withstand lower temperatures. So, the recommended general comfort conditions for winter air conditioning are:
an effective temperature below 21.7°C,
a dry bulb temperature of 21°C,
relative humidity of 50%, and
air velocity of 0.15-0.2m/s.
What is meant by Effective Temperature “ET” of comfort air conditioning?
Effective temperature (ET) is the temperature of saturated air that would make you feel just as comfortable as you do in the actual unsaturated environment. You can see this line in Figure, which shows the ET at 21.7°C.
When it’s less humid, the dry bulb temperatures (DBTs) of the air can be higher and still feel comfortable because the body loses more heat through sweat evaporation.
Increasing the airflow speed can also make up for an increase in temperature. For instance, if the DBT goes up by 2 to 3°C, increasing the air velocity from 0.1 to 0.3 m/s can compensate for it.
Importance of ventilation in comfort air conditioning
Besides keeping the temperature, humidity, and airflow speed in check, it’s crucial to ensure the air in the room stays clean.
Even if there aren’t any sources of pollution in the room, the carbon dioxide level can go up just because people are breathing. That’s why it’s essential to let fresh air or ventilation air in.
When some people smoke in the room, the need for ventilation is even higher. Auditoriums, in particular, need a lot of ventilation air due to their large occupancy. So, smoking should be banned in assembly halls and auditoriums to make sure everyone breathes clean air.
The salient features governing human comfort
Our comfort as humans depends on how quickly our bodies produce heat and how well we can release that heat into the environment. There are three main factors that affect this: our metabolic rate, the way our body loses heat, and a mathematical model of how heat is exchanged between us and our surroundings.
The Metabolic Rate
Did you know that the rate at which our bodies produce heat is called the metabolic rate? When we’re asleep, a healthy person produces about 60 watts of heat, which is called the basal metabolic rate. However, when someone is working hard, that rate can increase up to ten times!
Our body temperature stays pretty consistent at around 36.9°C (98.4°F) on the surface and 37.2°C in our deep tissues. Interestingly, our body temperature is usually about 0.5°C cooler in the morning compared to the afternoon.
If our body temperature reaches 40.5°C (104.9°F), that’s considered serious, and anything over 43.5°C (110°F) is definitely fatal.
Mechanism of Body Heat Loss
The body loses heat through convection, radiation, and evaporation of moisture. The total heat loss is given by Q=(C+R) + E, where “C+R” forms the sensible heat component “Qs”, and “E” forms the latent heat component “QL”.
The sensible heat component depends on the temperature difference between the body and the surroundings. Similarly, the latent heat component depends on the difference in water vapour pressures.
In summer, the temperature difference between the body and surroundings is less, which reduces convective and radiative heat losses. To keep cool, the body starts sweating to increase evaporative loss. On the other hand, in winter, the sensible heat transfer is increased, reducing evaporative losses.
Mathematical Model of Heat Exchange between Man and Environment
The exchange of heat between a person and their surroundings can be expressed with the equation:
M-W=Q+S. “M” represents metabolic rate, “W” is the work done by the body, “Q” is the rate of heat loss through convection, radiation, and evaporation, and “S” is the rate of heat storage.
During summer, the body temperature has a tendency to increase because of positive stored energy. The blood flow rate through the extremities increases, and the body starts sweating, which is called vasodilation.
In contrast, during winter, the temperature tends to decrease, and the stored energy may be negative, leading to vasoconstriction resulting in shivering.
To feel comfortable, the body needs to maintain thermal neutrality, which means no stored energy and no change in body temperature.
If there is any variation in body temperature, it acts as a stress signal to the brain, resulting in perspiration or shivering.
The net heat release rate of the body due to oxidation is H=(M-W)=M(1-η), where “η” is the thermal efficiency of the body heat engine.
The values of heat liberated depend on the activity level of the body, such as during outdoor games, where it is around 20 per cent. The thermal efficiency in most cases is zero, except in cases of high activity.
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References
Refrigeration and Air Conditioning by C P Arora