19592504
Description
Flashcards by Alice Kimpton, updated more than 1 year ago
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Created by Alice Kimpton
about 5 years ago
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| Question | Answer |
| What is air conditioning | The control of temperature, humidity, purity, and motion of air in an enclosed space, independent of outside conditions. |
| What is comfort air conditioning? | It provides a comfortable and healthy indoor environment (20 - 26 degrees C, 30-60% relative humidity). |
| What is process air conditioning? | It provides needed indoor environmental control for manufacturing, product storage, or other research and development processes. |
| What is psychometry? What does it include and where is it used? | - Psychometry is the study if thermodynamic properties of moist air. - Includes a set of assumptions, equations and charts. - Used extensively to illustrate and analyse the characteristics of various air conditioning processes and cycles. |
| What is moist air? | Mixture of dry air and water vapour. Dry air is comparatively stable. |
| How does water vapour content vary? | Water vapour content varies: 0.05% - 3% by mass at the ambient temperature between -17.8-37.8 degrees C. |
| How would you work out mass for water vapour or dry air? | m = PV / RT |
| What is Dalton's law (how to work out partial pressure of air)? | Pa = Pda + Pv |
| Total enthalpy (of moist air)? | Ha = mda.hda +mv.hv |
| Specific enthalpy of moist air? | ha = Ha / mda (h = H/m) |
| Working out specific humidity or humidity ratio or moisture content? | |
| What is Tdp equal to? And what is that equal to? | |
| What will happen to the moist air temperature? | Temperature will reduce e.g. 20 to 11.4. This is called saturation, water vapour saturated by liquid. |
| What is the Dew Point? | The Dew Point is the temperature at which water vapour starts to condense out of the moist air at constant barometric pressure, the temperature at which air becomes completely saturated. |
| What is the pressure of moist air equal o to? | |
| What happens when you add more water vapour to dry air and water vapour? | After adding water vapour the dry air pressure drops, and the water vapour pressure rises. |
| At this state of saturation, what happens? | At the state of saturation, the moist air has the maximum level of moisture, and the partial pressure of the water vapour equals the saturation pressure of water at the temperature of air. |
| Equation for relative humidity? | |
| If Pv < Pv,sat, how else could this be written? | i.e. saturation pressure at the Dew Point is less than at the saturated moist air temperature. (Where the pressure of the water vapour is equal to the saturation pressure at the Dew Point, and the pressure of saturated water vapour is equal to the saturated moist air temperature) |
| How is percentage saturation/degree of saturation defined? How is this expressed | Defined as the ratio of the moisture content of a moist air to the moisture content of the saturated moist air at the same temperature and pressure. |
| How is moist air saturated? | Moist air is saturated by cooling, adding water, or compressing. |
| How would you measure the dry-bulb temperature? | If you measure the temperature of a moist air with a conventional thermometer, such as mercury in glass tube, you would measure the dry-bulb temperature. |
| Why would we use the term dry-bulb temperature? | The term dry-bulb temperature is used to distinguish between the temperature that is measured using a conventional thermometer and that is taken using a wet-bulb thermometer. |
| How would you measure the wet-bulbs temperature? | Mercury in glass thermometer can easily be converted to a wet-bulb thermometer by fitting an absorbent muslin sock over the mercury bulb and immersing the sock into small reservoirs of distilled water. The muslin sock acts as a wick and draws water over the bulb causing it to be wetted: |
| What happens when the 2 thermometers are exposed to a flow of air? | When the 2 thermometers are exposed to a flow of air, the dry-bulb records the ‘true’ air temperature e.g. 25 C db. The wet-bulb will record a temperature that is usually less than the dry-bulb as it is affected by the evaporation of water from saturated wick. |
| What must happen for evaporation to take place? | In order for evaporation to take place the water must first absorb heat from the mercury bulb causing its temperature to fall below the ambient temperature. When the water and the mercury bulb are in thermal balance, their temperature is less than that of the ambient air. |
| How is the rate of evaporation controlled? | The rate of evaporation is controlled by the difference between the moisture content of the air stream and that of the saturated air film at the surface of the wet-bulb. I.e. the closer the ambient air is to the saturated condition, the difference will be smaller between dry and wet-bulb temperatures. |
| What does the difference between the wet and dry bulb temperature give? | Therefore, the difference between the wet and the dry-bulb temperatures gives a measure of the moisture content of the air relative to the maximum quantity it will hold at a given dry-bulb temperature. |
| How to work out temperature of adiabatic saturation / thermodynamic wet-bulbs temperature? | See notes for example. |
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