In air conditioning technology, it is necessary to define thermodynamic processes and the properties of moist air.
This may be achieved by a good knowledge of physics, with theoretical calculations using complicated formulae and tables. The procedure can be time consuming.
By presenting the interrelated factors on a psychrometric chart, an immediate decision can be made regarding the feasibility of controlling an air
conditioning system and the means required to carry this out.
For a given air sample nine different parameters are shown on the psychrometric chart.
A position on the chart can be established at the intersection of two ordinates for known conditions and the others obtained.
Since the properties and behaviour of moist air depend on barometric pressure, a psychrometric chart can only be drawn for a specific barometric pressure. Allowances may be made for changes in barometric pressure by using correction factors.
Note that the chart indicates a condition of 21 °C Dry bulb temperature and 48% Relative humidity. These are typical values to provide comfort in an office.
Fig. 27.10
Example: Find the missing values for the following
case. | ||
1 |
Dry bulb temperature |
tsic |
2 |
Absolute humidity |
X |
3 |
Partial water vapour pressure pD | |
4 |
Saturation pressure |
Psat |
5 |
Saturation temperature |
tsat |
(dewpoint) | ||
6 |
Relative humidity |
9 |
7 |
Enthalpy |
h |
8 |
Wet bulb temperature |
thyg |
9 |
Density |
P |
Example: Find the missing values for the following
The point on the chart is defined by the temperature and the relative humidity given above.
Solution:
The point of intersection P between the 20°C isotherm from the dry bulb temperature (1) and the line of 50% constant relative humidity (6) clearly defines the position of the required condition.
The absolute humidity (2) is found by drawing a horizontal line through the point P and extending it until it meets the ordinate on the right.
If this horizontal is extended to the left it will intersect the scale for partial water vapour pressure pD (3).
To obtain the saturation pressure (4) the isotherm from P must be extended until it intersects the 100% relative humidity line. At this point the air is saturated, i.e. it cannot absorb any further moisture without a dense mist forming. An extension of the horizontal line through this point of intersection to the left intersects the partial pressure scale at point (4). The pressure of the saturated air can now be read.
Where the horizontal line from (P) intersects the saturation curve a similar condition occurs, whereby the air cannot absorb any additional moisture (5). The dewpoint or saturation temperature can now be read on the saturation curve (and also on the dry bulb temperature scale).
By following the isenthalp (line of constant enthalpy) which passes through the condition (P) we can determine the enthalpy at the points of intersection (7) with the enthalpy scale. If an adiabatic line is drawn through
Fig. 27.12 Typical displays.
(a) Pull down menus for selecting functions relating to any schematic in the building layout.
(b) Pictorial graph of control operations with shaded bands where limit values have been exceeded.
(c) Data evaluation and display. Electricity consumption in various zones for one month is given in this example
91919
91919
30 35
30 35
91944
91944
30 35
30 35
Fig. 27.14
the point (P) towards the saturation curve, the two intersect at point (8) to give the wet bulb temperature. This is lower than the starting temperature because the absorption of moisture has caused sensible heat to be converted into latent heat.
The density is determined from the nearest broken lines of constant density (9).
The required values are:
Refrigeration systems and energy-saving applications
In order to appreciate the engineering diagram examples relating to refrigeration practice, we have included an explanation of a typical cycle of operations.
When you pour liquid ether on to the back of your hand, after a few seconds you feel your hand turn ice-cold. The liquid evaporates very quickly, but in order to do so it requires heat—so-called heat of evaporation. This heat is drawn in from all around the ether— including from your hand—and it is this which causes a sensation of cold.
If you could catch this evaporated ether and liquefy it again by compressing and cooling, the heat absorbed during evaporation would be released back into the surroundings.
This is precisely the principle on which the refrigeration cycle works. A special refrigeration agent, which is even more suitable for this purpose than ether, is evaporated close to the medium to be refrigerated. The heat necessary for this process is drawn in from all around, thereby cooling the air or water.
The most widely known refrigerating agent used to be ammonia but this has now been almost entirely superseded by halogen refrigerants, the best known of which are R12, R22 and R502.
The refrigerant cycle
The refrigerant circulates in a closed system. To produce this circulation, a very powerful pump is required—
Low pressure
High pressure
Air to be cooled _
Evaporator
Expansion valve Liquid
'(low pressure)
Condenser
Liquid
(high pressure)
Medium for cooling and condensing the refrigerant the compressor. This draws in the expanded refrigerant in vapour form and compresses it. On being compressed, the temperature of the vapour rises. It moves on to the condenser and is cooled by means of cold water. The cooling is so great that the condensation temperature is reached and the condensation heat thus produced is given up to the water in the condenser.
The refrigerant is then pumped on in liquid form into a receiver and from there on to the evaporator. But just before this, it flows through an expansion valve. This valve reduces the pressure on the liquid so much that it evaporates, drawing in the required heat from its surroundings. This is precisely as intended for the air or water to be cooled is led past the group of pipes in the evaporator. After leaving the evaporator, the refrigerant is once again drawn into the compressor.
To summarize: In one half of the cycle, the heat is removed by evaporation (i.e. cooled where cooling is required) and in the other half, heat has been released by condensing. Thus energy (heat) is moved from where it is not wanted to a (different) place where it is tolerated or, in fact, required.
When the theoretical cycle of operations is applied in practice it is necessary to include controls and safety devices. In a domestic system the motor and compressor are manufactured in a sealed housing.
The heat extracted from the inside of the refrigerator, where the evaporator is positioned, passes to the condenser, generally at the back of the cabinet, where natural convection currents release the heat into the surroundings.
In a large industrial installation it may be economically viable to recover heat from a condenser and use it for another process.
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