Objectives lecture "Chapter 2 : Refrigerant" leaves: Understand the requirements of refrigerant and coolant, understand the thermodynamic and property of some refrigerants, use the refrigerant chart to display refrigeration cycle. Invite you to reference.
Student can: Student can: - Understand the requirements of refrigerant and coolant coolant - Understand the thermodynamic and property of some refrigerants some refrigerants - Use the refrigerant chart to display refrigeration cycle cycle
[1] Refrigerant and Air Contioning A R Trott [1]. Refrigerant and Air Contioning - A. R. Trott and T. Welch [2]. Danfoss document [2] Danfoss document
REFRIGERANTS
COOLANT COOLANT
REFRIGERANT CHART ANALYST
f i d ill b It will be useful It l 1. Ideal properties for a refrigerant: (page28,[1]) the th l
to remind ourselves of t f requirements for a fluid used as a refrigerant. t A hi h l t ti f i
bl N fl t i
• A high latent heat of vaporization t h • A high density of suction gas • Non-corrosive, non-toxic and non-flammable d i • Critical temperature and triple point outside the
kiworking range
• Compatibility with component materials and
lubricating oil l b i il ti
• Reasonable working pressures (not too high, or
below b l
E k d t ti
• Low cost • Ease of leak detection f l • Environmentally friendly No single fluid has all ll fl N id h i l ti
th d ill h th t t ki h t t l
these properties, and d th meets the new environmental requirements, but this chapter will show the developments that are taking place in influencing the selection and choice of a refrigerant. t f i
2. Ozone depletion potential (ODP):
The ozone layer Th h t l
in our upper atmosphere i provides a filter for ultraviolet radiation, which can be harmful to our health. lth b h l t h f
The Montreal Protocol f th d ti ld b h d h l
in 1987 agreed that the production of these chemicals would be phased out t i by 1995 and alternative fluids developed
iti th t it ill b f i t t i l l
R22 is an HCFC and now regarded as a transitional refrigerant, in that it will be completely t l phased out of production by 2030, as agreed under the Montreal Protocol. l th M t l P t
3. Global warming potential (GWP): - Global warming is the increasing of the world’s ld’ i th i f th i i
Gl b l temperatures, It i i t t h th d b th
d t h k t th’ th f
- It is caused by the release into the atmosphere l of so-called ‘greenhouse’ gases, which form a blanket and reflect heat back to the earth’s surface, bl t b k t fl or hold heat in the atmosphere. t i f Th b h i
500 th h t t i
- The most infamous greenhouse gas is carbon dioxide (CO2), which once released remains in the atmosphere for 500 years, so there is a constant t f build-up as time progresses.
t l b l l i
GWP f 1300
Table 3.3 shows that the newly developed refrigerant gases also have a global warming h f i potential if released into the atmosphere. For example, R134a has a GWP of 1300, which hi h l R134 h F the emission of 1 kg of R134a is
f CO i means that equivalent to 1300 kg of CO2. t t 1300 k l
+ Note :
ll fl d Th ODP id h
f f i dl t t i
4. Ammonia and the hydrocarbons: - These fluids have virtually zero ODP and zero i t GWP when released into the atmosphere and therefore present a very friendly environmental l th picture. Ammonia has long been used as a refrigerant for industrial applications. i d t i t f ti l
t h f i d ll t t i i
li f i - Ammonia cannot be used with copper or copper alloys, so refrigerant piping and components have to be steel or aluminium.
l b ili
- Its normal boiling point is –33 °C. Ammonia has t i i h small 33 °C A in i smell even very
It a characteristic concentrations in air. ti t i i
i d ith i f 13 t t i
- It cannot burn, but it is moderately explosive when mixed with air in a volume percentage of 13 to l h 28%.
- Used in industrial system i d t i U d i t l
carry
“R” f Fluorinated ti
ll h ll f t t i
E t th I i i l
always refrigerants the designation “R” followed by a number, e.g. R22, d i R22 b d b ll R134a, R404A and R407C. The fluorinated refrigerants all have the following features: th f f i - Vapour is smell-free and non-irritant. - Extensively non-poisonous. In the presence of f fluoric acid and
fire the vapour can give off phosgene, which are very poisonous. hi h h i
- Non-corrosive. - Non-flammable and non-explosive. bl N d fl i l
+The most common fluorinated refrigerants are:
b t f th hi h i th
Its 26 1°C It point t i f
di h d ti t t
R134a, which is a substance of the ethane R134 group with the formula CH2FCF3 and has a normal thermodynamic of –26.1°C. boiling i d th b ili for properties make it suitable as a refrigerant medium temperature applications such as domestic ti li refrigerators.
d h ith th f b ili
k it it bl t f f i
R22 i b i diti f i d t i
R22, which is a substance of the methane group with the formula CHF2CI and has a boiling l CHF CI Its thermodynamic properties point of –40.8 °C. make it suitable as a refrigerant for a wide range of f id refrigeration and air applications in commercial conditioning. R22 is being phased out as refrigerant t h in many countries due to its ozone depleting potential. l ti t
HFC t t R32 h d f f i b
t h b t bl fl it
R32 is difluoromethane (methylene fluoride) and it is an HFC type refrigerant. R32 has been used for it i many years as a component of both R407C and R410A. It is flammable on its own, but not when R410A It i mixed with the other components of these blends.
5. Refrigerant blends: - Many of f t lt M f i
for f existing i ti new plants t
t h th d t t t t l i i
the new, alternative refrigerants are th ti ‘blends’, which have two or three components, as and developed l d d l d comparable alternatives to the refrigerants being replaced d l - They are ‘zeotropes’ with varying evaporating or condensing temperatures in the latent heat of f vaporization phase, referred to as the ‘temperature glide’. lid ’
f i ill b d it t
d th t A d t th bl
- To compare the performance between single refrigerants and blends it will be d bl component t necessary to specify the evaporating temperature of the blend to point A on the diagram and the di i th condensing temperature to point B.
i A ith bl d i
concentration t the th f i ti
10%) h ibl th li t l
- A problem associated with blends is that th t t d bl refrigerant leakage results in a change in the refrigerant. of component t f t tests indicate that small changes in however, concentration (say less than 10%) have a negligible ti ( effect on plant performance.
The following recommendations apply to the use
of blends: f bl d
th ti t t t
• The plant must always be charged with liquid refrigerant, or the component concentrations will ill f i shift.
• Since most blends contain at t t bl Si t
least one l t i d into the the entry of air
flammable component, system must be avoided. id d t b
5K h d d t d f th ld fl t
t • Blends which have a large temperature glide, greater than 5K, should not be used for flooded-type t b evaporators.
R404A/R507A (also known as R507), which is
is slightly for R22. than
commercial
a mixture of the refrigerants R125 (CHF2CF3) and R143a (CH3CF3) with a boiling point at (–46.7 °C) which Its lower thermodynamic properties makes it suitable as a low and medium temperature for refrigerant refrigeration (e.g. applications in supermarkets).
R407C, which is a mixture of the refrigerants R32 (CH2F2), R125 (CHF2CF3) and R134a (CH2FCF3) with a boiling point at (–43.6 °C) which is slightly lower than for R22. Its thermodynamic for properties make it suitable as a refrigerant medium and high temperature applications in residential and commercial air conditioning.
R410A, which is a mixture of the refrigerants R32 (CH2F2) and R125 (CHF2CF3) with a boiling point at (–51.4 °C) which is lower than for R22. Its thermodynamic properties make it suitable as a for medium and high temperature refrigerant applications in residential and commercial air conditioning.
Except for R22,
systems with fluorinated lubricated with polyol hydrocarbons are in general ester oils (POE). These oil types are much more sensitive to react chemically with water, the so- called “hydrolysis” reaction. For that reason systems today are kept extremely dry with filter driers.
“primary refrigerants”. As
The refrigerants mentioned above are often an designated transmission from the intermediate link in heat surroundings to the evaporator, the so-called “secondary refrigerants” can be used, e.g. water, brine, atmospheric air etc.
g
p y for and mixture
vapour q regions (
g - The diagram is arranged so that it displays the the liquid, g refrigerant. Liquid is found to the left (with a low energy content) - vapour to the right (with a high energy content
g
p gy - In between you find the mixture region. The y regions are bounded by a curve - called the saturation curve. The fundamental processes of evaporation and condensation are illustrated.
Diagrams are still used as the main tool g for
De-Superheat
Subcool
Superheat
analysis of refrigeration processes.
p g If a refrigerant at
p ,
g corresponding p temperature at a
the same temperature as ambient is allowed to expand through a hose with p an outlet to atmospheric pressure, heat will be taken up from the surrounding air and evaporation will p to occur atmospheric pressure.
If in a certain situation pressure on the outlet side p
p g
it
p (atmospheric pressure) is changed, a different temperature will be obtained since this is analogous is pressure temperature - to the original ) y ( p dependent. ( Open R22 thermodynamic table )
g g p
q p p
g y
When the refrigerant coming from the evaporator is fed to a tank the pressure in the tank will rise until , it equals the pressure in the evaporator. Therefore, refrigerant flow will cease and the temperature p in both tank and evaporator will gradually rise to ambient.
, ,
p
p , In simple terms, p p p p
p To maintain a lower pressure, and, with it a lower temperature it is necessary to remove vapour. y This is done by the compressor, which sucks vapour away from the evaporator. the compressor can be compared to a pump that conveys vapour in the refrigeration circuit.
q
y p
p
the p q p
In a closed circuit a condition of equilibrium will always prevail. To illustrate this, if the compressor sucks vapour away faster than it can be formed in the evaporator the pressure will fall and with it the y, temperature in the evaporator. Conversely, p the if load on the evaporator rises and the refrigerant , evaporates quicker, the pressure and with it temperature in the evaporator will rise.
g Refrigerant
p p
p p y
p
p leaves the evaporator either as saturated or weak superheated vapour and enters the compressor where it becomes compressed. Compression is carried out as in a petrol engine, i.e. by the movement of a piston. The compressor requires energy and carries out work. This work is g transferred to the refrigerant vapour and is called the compression input.
g g
g p
p p y
, The refrigerant gives off heat in the condenser, and this heat is transferred to a medium having a lower temperature. The amount of heat given off is the heat absorbed by the refrigerant in the p evaporator plus the heat created by compression input.
q
p
g, p
g Liquid from the condenser runs to a collecting tank, the receiver. To reduce pressure to the same g p level as the evaporating pressure a device must be inserted to carry out this process, which is called throttling, or expansion. Such a device is therefore known either as a throttling device or an expansion device.
There are many different temperatures involved p y
q g q ,
p p
p g
in the operation of a refrigeration plant since there , are such things as subcooled liquid, saturated liquid, saturated vapour and superheated vapour. There ; y in principle, only two pressures; p , , are however, evaporating pressure and condensing pressure. The p plant then is divided into high pressure and low pressure sides, as shown in the sketch.
High and low pressure sides of the refrigeration g g p
plant
Note :
+ C-C1 : Superheat, + C-C1 : Superheat po=const, tC1>tC + D-E : Desuperheat + A-A1: Subcool l
A A1 S b
Refrigeration process, pressure/enthalpy diagram , p py g p g
this
R134a Survey system refrigeration and display points on and display points on chart
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