Air con Flashcards
What is air conditioning?
The process of altering the properties of air:
- Temp
- Humidity
- Pressure
- Composites
…to more comfortable conditions. Further the air is to keep air properties const. or change them to given program.
Dry-bulb temp
Measured temp when thermometer is exposed to the air. True thermodynamic temp. Shielded from radiation and moister.
Wet-bulb temp
Is the temperature if the air was cooled to saturation (100% relative humidity. )It is the temperature felt when the skin is wet and exposed to moving air.
Dew-point temp
The temperature when water vapour in the air condenses at constant pressure into liquid, at the same rate as it evaporates.
The temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%
Humidity
Amount of water vapour in the air. Higher humidity reduces the effectiveness of sweating.
Absolute humidity
Total amount of water vapour in a given volume.
Relative humidity
Ratio of partial pressure of water vapour to the saturation vapour pressure, at the same temperature.
Tells us how much water vapor is in the air, compared to how much it could hold at that temperature.
Central air con system, medium type: (different types)
Central air con system
Air:
1. Conventional: Both with and without zone.
2. High pressure unit: One and two ducts
Air + freon:
- Split: 1 unit
- Multisplit: 5-6 units
- VRV: 12-24 units
Air + water:
- Radiant cooling panel: Natural flow
- Chilled beam:
- Active: Forced flow
- Passive: Natural flow - Fan coil: Forced flow
Describe heat gain (figure)
External: Wall, roof, window, floor
Internal: Lights, people, electrical equipment
Infiltration: Air leak, moister
Total fenestration heat gain (figure + eqv)
Q = Qdir + Qdiff + Qconductive
- Reflection on outer panel
- Absorption in outer panel
- Reflection on the inner panel
- Absorption in the inner panel
- Directly transmitted solar energy
Ad- and disadvantages of fixed-plate cross flow heat exchangers
Advantages:
- Relatively high sensible effectiveness
- Little cross leakage between air streams
- Easy to clean
Disadvantages:
- Transfers only sensible energy
- Requires adjacent airstreams
- Relatively high frost threshold
- Heavy
- High first cost in large applications
Ad- and disadvantages of rotary heat exchangers
Advantages:
- Total energy wheels transfer both sensible heat and moister (latent heat)
- High effectiveness
- Fits inside an air-handler
- Self cleaning in terms of dry particles
Disadvantages:
- Transfers only sensible heat
- May permit cross-leakage between air streams
- Requires periodic maintenance
Ad- and disadvantages of coil loop heat recovery tech
Advantages:
- Transfers heat between air streams that are separated by distance
- No cross leakage
- Flexible design
- Easily turned off
- Easy to clean
- Fits in air handler
Disadvantages:
- Transfers only sensible heat
- Requires pump, piping, expansion tank and mixing valve
- Maintenance of the above
Ad- and disadvantages of heat pipe heat tech
Advantages:
- Little cross-leakage between air streams
- Low maintenance
- Fits inside an air-handler
Disadvantages:
- Contains refrigerant
- Transfers only sensible heat
- Requires external dampers to prevent unwanted heat transfer
Definition of olf and decipol (figures)
Olf - pollution load:
- Olf is the sensory load on the air from an average sitting adult in thermal neutrality
Decipol:
- 1 decipol is the air quality in a indoor space with the pollution source strength of 1 olf, ventilated by 10 l/s clean air
Calculation of ventilated rate for comfort
Qc = 10Gc/(Cc,i - Cc,o)1/ev
Qc - Ventilated rate [l/s] Gc - Sensory pollution load [olf] Cc,i - Indoor air quality [decipol] Cc,o - Outdoor air quality [decipol] ev - ventilated effectiveness
Calculation of ventilated rate for health
Qh = Gc/(Ch,i - Ch,o)*1/ev
Qh - Ventilated rate [l/s]
Gh - Pollution load of a chemical [ug/s]
Ch,i - Guideline value of a chemical [ug/l]
Ch,o - Outdoor concentration of a chemical [ug/l]
ev - ventilated effectiveness
Calculation of ventilation effectiveness
ev = (Ce - Cs)/(Ci - Cs)
ev - ventilation effectiveness
Ce - Pollution concentration in exhaust air
Cs - Pollution concentration in the supply air
Ci - Pollution concentration in the breathing air
Describe PMV-PPD model and the way it is used for thermal comfort prediction
- The human response to the thermal environment is expressed by the Predicted mean rate (PMV).
- The predicted % of dissatisfied (PPD) indices(index) the % of occupants feeling to warm or too cold.
- PPD index predicts the % of large group of people likely to feel thermally dissatisfied. PPD depends on PMV.
Resultant heat load in summer
Sum(Qsummer) = Qtransient +- Qtechnical + Qpeople + Qgain + Qlight
Qtransient - Heat load in summer, depends on time, eg sunlight
Qtechnical - Heat load from tech processes. Can be both + & -. eg positive, a machine creating heat in a factory
Qpeople - Head load from people
Qgain - Internal heat gain, like heater inside
Qlight - heat load from lightning
Resultant heat load in winter
Sum(Qwinter) = -Qtransmission +- Qtechnical + Qpeople + Qgain + Qlight [W]
Qtransmission - Transmission heat loss between the occupied zone and environment
Qtechnical - Heat load from tech processes. Can be both + & -. eg positive, a machine creating heat in a factory
Qpeople - Head load from people
Qgain - Internal heat gain, like heater inside
Qlight - heat load from lightning
Air distribution system. Main types
Two types of ADS:
- Displacement ventilation - Based on lift force
- Mixing ventilation - Based on higher air velocities
Working principle of Displacement ventilation
This system is based on the lift force and the air that is introduced to the ventilated space has a low velocity. The lift force comes from temperature diffs between supply air and indoor air.
Working principle of Tangential ADS
The supplied air jet touches some surface const. Coanda effect. Applied in classrooms, hospitals. The air always creeps along the a surface.
Archimedes number in ventilation
Ar = g/(Tsup§Cp,mix)q/(H^2)1/(h^3)
g - Gravitation [m/s^2] Tsup - Supply air temperature [K] § - Density [kg/m^3] Cp - specific heat [KJ/KgK] q - Specific heat load [w/m^2] H - Internal height [m] h- Air exchange number h = Vsup/V [1/hrs]
Calculation of turbulence intensity of the air flow
Tu = 100*(Vrms/Vm)
Calculate DR number and explain quantities + Draught comfort categories
DR = (34-tm)(Vm-0,05)^0,62(0,37Tu*Vm+3,17)
- 20 < tm < 26 celcius
- 0,05 < Vm < 0,4 m/s
- 0 < Tu < 70% (Allt är i <= )
Categories:
A: DR <= 15%
B: 15 < DR <= 20%
C: 20 < DR <= 25%
