Air Cond: Psychrometric process

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Air Cond: Psychrometric

Moist air can be treated as binary mixture, composed of:

Dry air – essentially mixture of nitrogen, oxygen and traces of argon, carbon dioxide, neon etc.

Water vapour – which exists at low partial pressure. At low low atmospheric pressure and temperature, water vapour can be treated as ideal gas.

Assumptions.

(1) vapour contains no dissolved gases

(2) gaseous phases can be treated as ideal gas

(3) no interaction between components and all components are at the mixture temperature

Psychrometric properties All properties are based on the “dry-air part”. Consider a volume of moist air

Barometric pressure = 101.325 kPa Gibbs phase rule, F = 2 + 2 – 1 = 3 (ie need 3 intensive properties to define the state). Since pressure is fixed, need 2 other properties:

(a) dry bulb temperature, TDB …………… easily measured

(b) moisture content, w

(c) dew point temperature, TDP

(d) percentage saturation,

(e) relative humidity,

(f) specific enthalpy, h

(g) specific volume, v

(h) wet bulb temperature, TWB ……….…….usually measured

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Air Cond: Gas Mixture inside air cond system

(a) Chemical compound – cannot be separated by mechanical means

(b) Mixture – each constituent retains its own identity; can be separated by mechanical means. Constituents in a mixture are called the components. Each component may exist in one or more phases.

Example of mixture:


2 components (air and water)
3 phases (gas, liquid and solid)

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Air Cond system

Air Conditioning (AC) System

AC systems can be classified by the methods of achieving the within-the-space cooling (and heating):

1.1 Unitary unit

It is a compact, self-contained unit located within or next to the air-conditined space. Such a system includes the direct expansion (DX) system.

1.2 Split Unit

It is also a compact, self-contained DX system with the condensing unit (compressor and condenser) located at some distance from the evaporating unit (evaporator and expansion valve). Long refrigerant pipings connect the condensing and evaporating unit.

1.3 Central Systems

Central systems include: all-water system, all-air system and air-water system.

1.3.1 All-water system

In this system, a cooling medium (chilled water or brine – but not refrigerant) is supplied from a remote source and circulated through the terminal unit within the conditioned space.

1.3.2 All-air system

In an all-air system, the final cooling medium (air) is brought into the conditioned space through ducts and distributed within the space through outlets.

1.3.3 Air-water system

Such a system uses chilled water and cold air from a central station. The major part of thermal load is met by chilled water which is circulated through a coil.

1.4 All-air central centralised systems

1.4.1 Constant air volume (CAV) system

In a CAV system, the volume flow rate of air is constant. During part-load operation (eg on rainy days), terminal reheaters can be used to increase the supply air temperature to ensure conditioned space air dry bulb temperature is maintained at comfort level. Without reheat, indoor air temperature cannot be controlled when zone sensible loads are lower than design-day values; resulting in uncomfortable conditions.

1.4.2 Variable air Volume (VAV) system

In a VAV system, the volume flow rate of air is varied to ensure comfortabale indoor air dry bulb temperatures. An indoor air thermostat senses indoor air temperature variation and sends a signal to a VAV box control unit which increases the volume flow rate of supply air when the sensible load increases and vice-versa.

1.4.3 Dual-Duct/Multizone (DDMZ) system

In this system, the supply air temperature is varied to meet changing zone sensible loads. When zone sensible load decreases, zone air temperature will decrease and the indoor air thermostat sends a signal to the mix box damper to mix more bypassed air with less cool air, such that the higher supply air temperature will ensure that the indoor air temperature will rise to the set point value.

In the system, the supply air temperature is variable, but the volume flow rate of supply air is constant.

In dual-duct system, the mixing is done at the terminal unit, while in multizone system the mixing is accomplished at the air handling unit (AHU).

Air cond : Aircrafts Refrigeration

Gas cycle refrigeration

The gas cycle is used exclusively in air- conditioning of aircrafts; military and commercial.

Often reffered to as air cycle refrigeration because air is the working fluid in the cycle. Its COP is lower than that of conventional vapor compression cycle. it can sork as an open or closed cycle. the benefit of an air cycle included:

(i) Small amount of leakages are tolerable since air is the refrigerant.

(ii) The simplest open system requires only one heat exchanger

(iii) Readily available refrigerant

(iv) Cabin pressurization and air-conditioning can be combined into one operation

(v) Initial compression of the air is obtained by the ram effect (conversion of the high kinetic energy of ambient air relative to aircraft into enthalpy and ultimately pressure rise). Refrigerated air capacity ranges from 3 to 70 kg a minute.

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Air cond: Vapor absorption type

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Air cond: Sample Test with solution (Standard and actual cycle)

Air conditioning & Refrigeration sample test 1. This question contain the basics air cond cycle, the differences between actual vapor and isentropic cycle. For the further reading, please click Question and Solution.

Air Cond: Past Year Final Exam Question

Past year Refrigeration & Air Conditioning Final Exam Question. This Question included the following topics:

1. Air Conditioning Basic Cycle
a. T-s & P-h diagram
b. Using R-134a or R-12 table to find value of enthalpy h.
c. Find the work of compressor and refrigerating effect.

2. Sling psychrometer to find the value of the dry bulb and wet bulb with the specific value of
atmospheric pressure. This question want you to evaluate:
a. Partial Pressure
i. Water Vapor
ii. Dry air
b. Relative humidity
c. Humidity ratio
d. Specific enthalpy

3. Room design question. Using Psychrometric Chart to evaluate the value of
a. Outdoor air mass flowrate
b. Supply air
i. Dry bulb temperature
ii. Mass flowrate
c. mix air
i. Dry bulb temperature
ii. relative humudity
d. cooling coil load

4. Ducting design. To approximate tho total pressure drop inside the ducting.

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Air Cond: Type of Refrigerants

International designation as refrigerants of R. There are many type of refrigeration. The type of refrigeration are:
1. Inorganics refrigerants


2. Organics refrigerants

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Air Cond: Multi Pressure System

Air conditioning diagram for the multistage system

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Air Cond: Basics concept of Air cond cycle

The basic concept of the air cond cycle using T-S diagram and P-H diagram.
Air conditioning cycle is reverse carnot cycle.

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Refrigeration & Air Cond: Exercise Psychrometric Chart

These file contents of the exercise for the psychrometric chart. Dry bulb and wet bulb temperature is given with atmospheric Pressure. for the full exercise question please click here!

Air Cond: Psychrometric chart

Sample Plotting of the Psychrometric chart. From the value of the Sensible heat factor (SHF) = 0.7, room design is 24 degree C and Humidity 50%. The outside air temperature is 31.3 degree C and Humidity is 70%. The Supply air, Apparatus Dewpoint, Mix air temperature, Outside air volume, supply air temperature, mix air volume, room air humidity, cooling bypass factor and etc can be evaluated through psychrometric chart.

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