Write a short note on mechanical cooling
The simplest example of mechanical cooling is the domestic refrigerator, shown diagrammatically in figure 3.4. A suitable gas, the ‘refrigerant’, is circulated in a closed circuit by a compressor.
This is most often a gas called Freon (CF₂Cl₂); in small installations in large plants, such as cold storage buildings, ammonia (NH3) or carbon dioxide (CO₂) is often used: the former is toxic, therefore any leakage may be troublesome; the latter requires very high pressure.
The circuit consists of two coils:
a) the warm coil or condenser, and
b)the cold coil or evaporator.
The two coils are connected on one side through a compressor and on the other side through a pressure release valve. The warm coil is thus kept under high pressure and the cold coil is under negative pressure.
The refrigerant is in a liquid state under compression and in a gaseous state under low pressure. Without changing the heat content, compression increases the temperature; expansion decreases it.
When liquefying, the refrigerant releases its latent heat of evaporation, and when evaporating, it absorbs a similar amount of heat.
The cycle can be described as follows:
(i) increases pressure
(ii) no change in heat content
(iii) temperature from, say, 0°C to 30°C
(i) no change in pressure
(ii) in condensation latent heat is released and dissipated to the environment
(iii) temperature from 30 to, say, 26°C
c. pressure valve
(i) admits liquid only above a set pressure, thus guarantees a low pressure in the evaporator
(ii) no change in heat content
(iii) temperature from 26 to, say, -4°C
(i) no change in pressure
(ii) in evaporation latent heat absorbed
(iii) heat is taken from the environment
(iv) temperature from -4 to, say, 0°C
If the evaporator coil is placed into an air supply duct (instead of into a refrigerator cabinet) the air blown across it will be cooled.
What are the various types of mechanical systems available for ventilation? Discuss in Detail.
In mechanical ventilation, the air is moved by motor-driven fans, which can be.
a. propeller type or axial flow fans
b. impeller type, centrifugal or tangential flow fans
These can be local, e.g. built into a window or a wall, or maybe central in which case ducts will be necessary to deliver and distribute the air to where it is required.
The installation can take the following forms:
1. an exhaust system-removing the used air and letting fresh air find its way in through grilles and opening (room under reduced pressure)
2. a plenum system – supplying air into the space and forcing out used air through grilles, etc. (slight overpressure in the room)
3. a balanced system both supplying and removing air. The most dependable, but most expensive, system is used when combined with warm air heating, as it permits partial recirculation.
With a plenum or balanced system the air will normally be filtered at the point of intake, by one of the following means:
a. dry filters, fibrous or porous materials (paper, cloth or glass fibers)usually disposable.
b. wet filters, metal turning or some loose material with a large specific surface, where all surfaces are coated with oil, normally by dipping. These can be cleaned and reused.
C. washing, by a curtain of water flowing down the face of a metal or porcelain. grille, or a spray through which the air is drawn.
d. electrostatic filters, in which the suspended dust particles are ionized by a high static electrical charge and stick to the face of electrode plates
Explain the body’s heat production factor for thermal comfort in detail.
Heat is continuously produced by the body. Most of the biochemical processes involved in tissue-building, energy conversion, and muscular work are exothermal, i.e. heat producing.
Thermal Comfort in Buildings Explained – HVACR Design
All energy and material requirements of the body are supplied from the consumption and digestion of food.
The processes involved in converting foodstuff into living matter and a useful form of energy are known as metabolism.
The total metabolic heat production can be divided into basal metabolism, i.e. the heat production of vegetative; automatic processes which are continuous, and the muscular metabolism, i. e. the heat production of muscles whilst carrying out consciously controlled work.
Of all the energy produced in the body, only about 20% is utilized, the remaining 80% is ‘surplus’ heat and must be dissipated to the environment.
This excess heat production varies with the overall metabolic rate and depends on the activity. The following table indicates the rate of excess heat output of the ad body in various activities.
Define the following tomes
a. Effective Temperature (ET)
b. Corrected Effective Temperature (LET)
c. Equivalent Warmth (EW)
d. Operative Temperature (OT)
a) Effective Temperature (ET): The first such scale was produced by Houghton.
and Yaglou in 1923, working at the American Society of Heating and Ventilation Engineers. Their findings were plotted on a psychometric chart, producing’ equal comfort lines’.
They named the new scale as effective temperature and it can be defined as the temperature of a still saturated atmosphere, which would, in the absence of radiation, produce the same effect as the atmosphere in question.
In 1947 Yaglou slightly revised the scale, but other modifications also become generally accepted.
b) Corrected Effective Temperature (LET) Whilst the ET scale integrates the effects of three variables originally of temperature and humidity but a later form included air movement- the corrected effective temperature scale also includes radiation effects. This is at present the most widely used one.
c) Equivalent Warmth (EW) Experiments were carried out by Bedford in England among over 2000 factory workers. The subjects were engaged in light work, under varying indoor conditions.
Air temperature, humidity and mean radiant temperature were measured and recorded together with the subjective response of the workers. The surface temperature of skin and clothing were also measured and recorded.
After correlating the findings, using statistical analysis methods, the equivalent warmth scale was constructed and defined by a nomogram.
It is now thought to be reliable within the comfort zone up to 35 °C with low RH and up to 30°C with high RH, but it underestimates the cooling effect of air movement with high humidities.
d) Operative Temperature (OT): Another scale was developed in the USA, by Winslow, Herrington, and Gagge, in principle very similar to the scale of equivalent warmth.
It combined the effect of radiation and air temperature Studies were carried out for a specific region with cool conditions, where the effects of humidity were small and the rate of air movement was also negligible.
Explain the following index related to thermal comfort Indices.
a. Equatorial Comfort Index
b. Heat Stress Index
c. Index of thermal stress
a) Equatorial Comfort Index: This was developed by C G Webb in Singapore during 1960.
Subjective responses, not acclimatized subjects were recorded together with measurements of air temperature, humidity, and air movement the experimentally found relationships were organized into a formula and shown on a graph, very similar to the ET nomogram.
b) Heat Stress Index: On the basis of theoretical considerations similar to the above, a further scale was developed in the USA.
Several Physiological assumptions were made and calculation methods evolved to find an indication of heat stress on the basis of Environmental measurements.
Metabolic heat production of subjects doing various kinds of work was measured and taken as an indication of heat stress.
It is thought to be reliable for still air between 27 and 35 °C, 30 and 80% RH, and for lower humidities, if temperatures are higher but unsuitable for the comfort zone.
c) Index of thermal stress: After reviewing and checking the validity and reliability of many previous thermal indices Givoni set out to establish a new index from the first principles.
The index of thermal stress developed by him is the calculated cooling rate produced by sweating, which would maintain thermal balance under the given conditions.
The calculation is based on a refined biophysical model of the man environment thermal system. The index takes into account all the subjective and objective thermal factors.
Its usefulness extends from comfortable to overheated conditions as far as the physiological adjustments are able to maintain thermal balance.
Due to the complex calculations involved, its use is probably restricted to research workers and it is not used by practitioners.
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