Poster for Aircon - Lecture notes 1 PDF

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Poster for Aircon...

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1. Basics of Psychrometrics • The study of dry air and water vapour mixtures is called Hygrometrics, or Psychrometrics. • For engineers, cooling towers and airconditioning units are direct applications of psychrometrics. • Our other topics assume dry air. • Air by volume is roughly: 21% Oxygen, 79% Nitrogen (plus other trace gases). By mass, air has 23.2% of oxygen gas, and 76.8% of nitrogen gas. • Two important definitions:

3. Airconditioning Unit: General Hardware Diagram

4. Cooling Tower Hardware Diagram

Saturated Air is a mixture of dry air and saturated water vapour. Unsaturated air is a mixture of dry air and superheater water vapour. (The air around us is usually unsaturated). 2. Dalton’s Law, and Gibb’s Law • If a container of a fixed volume contains two or more gases, the total pressure of the container is the sum of the partial pressures of the constituent gases. • Partial pressure is defined as that pressure that an individual gas would exert if it occupied the entire volume of the container. Dalton’s Law states: 𝒑𝒕𝒐𝒕𝒂𝒍 =𝒑𝒘𝒂𝒕𝒆𝒓 𝒗𝒂𝒑𝒐𝒖𝒓 +𝒑𝒅𝒓𝒚 𝒂𝒊𝒓

1 and 7: Range of inlet and outlet conditions are specified by the “client”. Calculations must satisfy the whole range. 2: Fan is placed somewhere in the cycle, sometimes just before the outlet. 3: Heating is required if inlet enthalpy is too low to evaporate spray in section 4a. 3 and 6: Can be a part of the same heat pump, (condenser and the evaporator respectively).

4a-5a: Cooling/Heating by spraying and collection of condensate. Depending on the spray temperature, this can increase or decrease ω. 5b-5b: If it is necessary to reduce specific moisture content at the outlet: cooling by refrigerated coils and collection of condensate. 6: Heating to reduce relative humidity, while specific moisture content stays fixed. Note: You can decrease ω by spraying water!

• Used when no useful cold sink source (such as a river) exists, for steam power stations. • Some cooling towers are based on natural draught (hot air rising due to density difference), and have no fans. • Note that some water will leave this cycle and some make up water is required to keep the mass flow constant.

• Gibbs’ Law states: 7. Dry Bulb, Wet Bulb, and Dew Point Temperature Definitions

𝒉𝒕𝒐𝒕𝒂𝒍 =𝒉𝒘𝒂𝒕𝒆𝒓 𝒗𝒂𝒑𝒐𝒖𝒓 +𝒉𝒅𝒓𝒚 𝒂𝒊𝒓 𝒔𝒕𝒐𝒕𝒂𝒍 =𝒔𝒘𝒂𝒕𝒆𝒓 𝒗𝒂𝒑𝒐𝒖𝒓 +𝒔𝒅𝒓𝒚 𝒂𝒊𝒓 5. Specific Moisture Content or Humidity Ratio • Mass of water vapour in the mixture divided by the mass of dry air in the mixture (of air and water vapour): 𝜔 =𝑚𝑣 𝑚𝑎 =0.622∗ 𝑝𝑣 𝑝𝑎

=0.622∗

𝑝𝑣 𝑝−𝑝𝑣

• Different forms of the equation originate from Dalton’s Law and the ideal gas relationships: 𝑝𝑎 =𝑚𝑎 𝑅𝑎 𝑇

𝑎𝑛𝑑

𝑝𝑣 =𝑚𝑣 𝑅𝑣 𝑇

• Note V and T are same for either gas as per Dalton’s Law. • 0.622 is the ratio of the molar masses of water and air. • Units of moisture content are kg /kg (moisture/dry air). 6. Relative Humidity or Percentage Saturation • Actual partial pressure of the vapour in the mixture divided by the saturation pressure of the vapour at the same temperature as the mixture, or mass of vapour in the air divided by the mass of vapour in saturated air at the same temperature: ∅=𝑝𝑣 𝑝𝑔 = 𝑚𝑣 𝑚𝑔

thus

• Dry bulb temperature is the temperature as measured by a thermocouple (no radiation) = Tdb. • Wet bulb temperature is the temperature of adiabatic saturation = Twb. Imagine a thermocouple surrounded by a wet cloth. The water in the cloth will evaporate, thus causing cooling of the cloth. The amount of cooling depends on the relative humidity of the surrounding air. If it is dry, then more evaporation can take place, thus dropping the wet bulb temperature more.

𝜔 = 0.622∗∅𝑝𝑔 𝑝𝑎

• Humidity ratio of 1 implies a particular temperature (Dew Point Temperature), below which condensation occurs. • A low humidity ratio implies more evaporative cooling can take place (thus making us feel more dry and colder

• Dew point temperature is the temperature at which condensation begins to occur = Tdp. At Tdp the air is fully saturated and cannot have more water vapour present in it. At this point relative humidity is 1, and all three temperatures are equal (Tdb = Twb = Tdp). Below Tdp condensate begins to form. • The relationship between dry bulb and wet bulb temperatures gives us the relative humidity. (See the table below and section 9).

Dry Bulb °C 25 20 15

Depression of Wet Bulb And Relative Humidity (%) 0.5 5 9 96% 63% 38% 96% 59% 30% 95% 52% 20%

8. Measuring Partial Pressure of Water Vapour

• Cool down a shiny metal container by adding ice inside it (mixed in with water). When condensate begins to form on the surface of the container, measure the dew point temperature. In this process, moisture content is constant and Pv/Pa must be constant. Because the total (ambient) pressure is constant, Pa + Pv is also constant (Dalton’s Law). This means both Pa and Pv are constant individually. Thus Pv can be measured as the saturated vapour pressure at the dew point temperature. Adding heat increases Pg and reduces relative humidity. Adding moisture content with constant dry bulb temperature increases relative humidity and the dew point temperature (changes Pv too). • It is simply a question of the equilibrium point, change moisture content or dry bulb temperature to change the characteristics of the air (see section 9 also): 𝑝𝑣 𝑝𝑣 =𝑐𝑜𝑛𝑠𝑡 = 𝑝𝑎 𝑐𝑜𝑛𝑠𝑡−𝑝𝑣

Pv and Pa must be const. individually.

9. Psychrometric Chart

• All values are on the basis of per kg of dry air, because this makes calculations easier. • Careful with constant property lines (there are many). • Note you only need to know two properties to fix the thermodynamic state of the system! • Typical airconditioning unit processes drawn in red on this chart. Relate to section 3 hardware diagram (refrigerated coils version). • Study this chart a lot until you get all the relationships embedded in it! For example, as explained in section 8, it is really easy to realize from this graph that the dew point temperature is only a function of the moisture content, ω. Aircon and Cooling Towers Summary...