Refrigerants and Refrigeration Cycle

This is the part 7th of the full HVAC course. You can access the remaining parts from the top menu by clicking on "HVAC Full Course"..

Vapor Compression Refrigeration Cycle (VCC) :

In this Post we will try to explain how the vapor compression refrigeration cycle works and why we use it even though they are inefficient compared to absorption cycles. And while many beginners get too hung-up on these efficiency numbers, don't forget that just because something is efficient doesn't mean its necessarily good. The reality is absorption systems aren't very popular because they are unreliable through regular maintenance and thus require constant monitoring due to frequent failure. On the other hand, the Vapor Compression system is much more reliable and can operate unattended after a short learning period. Even if we accept these shortcomings however, its hard to deny that they are still effective at lowering temperature with far less energy input than absorption systems. In addition, the vapor compression system provides high reliability even under severe environmental stress conditions such as high operating temperatures and pressures.

The vapor compression cycle operates in the range of 1,000 W or higher depending on specific model. This means that if you were to connect it directly to your home's electrical grid, you would likely exceed any maximum power draw of any device connected to the same circuit without exceeding any actual capacity of the circuit. However, we are not connecting it to your home's electrical system; instead, we are using a heat exchanger that allows us to send cool air into your house, essentially putting a load on your furnace (or AC) system. We then use ambient cooling outside of your house in combination with a large compressor/condenser unit to provide adequate cooling. You can calculate the average watts used throughout the setup by multiplying the total area of the units (including fans) and dividing by 60 seconds. This results in 0.0126 Watts per square foot. Multiply that by 4 and you've got 0.0527 Watts of electricity being used by the entire setup (which would generate about $0.08 worth of carbon dioxide emissions).

We can compare the efficiency of the vapor compression cycle to the traditional absorption cycle by looking at the Carnot efficiency of each. Absorption refrigerators run around 80% efficient whereas vapor compression runs closer to 90%. While both are low compared to the 100% theoretical upper limit, the Carnot efficiency of the vapor compression system is actually quite respectable considering its size and complexity.

It is a closed system. It works under four major sub-systems. They are : Compression, which takes place in a compressor. Condensation, which takes place in a condenser. Expansion valve / Throttle valve / pressure reducing valve (PRV), in which the expansion of the refrigerant takes place. Evaporation, which takes place in an Evaporator. Here in this Vapor Compression Refrigeration Cycle, refrigerant is used in the form of liquid or gas.

Vapor compression refrigeration cycle is basically a closed loop system where the heat produced from compression can be transferred into cooling. This cycle is used for air conditioning, heating, industrial processes, etc. The vapor compression refrigeration cycle works through the following steps: Compress gaseous state of a working fluid into liquid state and transfer thermal energy (heat) from high temperature to low temperature; Expand condensed liquid back to evaporate phase and transfer thermal energy (cooling) from low temperature to high temperature and then Return to step 1.

The basic principle of operation involves two major components: condenser and compressor. Only vapor-liquid exchange takes place at the condensing section while no such exchanges occur at the evaporator section. Thus, the latent heat of vaporization is transferred directly to the environment and absorbed by the liquid. In other words, this process allows us to remove heat from the process without having to bring any coolants inside the unit as well. This particular type of refrigeration is commonly known as direct expansion refrigeration due to its ability to operate without using any pressurizing fluids.

Refrigerant :

It is a heat carrying medium or a working substance which takes away the heat and produces cooling or chilling effect during a working VCC cycle. It may be in a liquid or in a gaseous medium also. Ozone layers shields the earth surface from the harm Ultra-violet rays emerging from the sun, which are harmful to human beings and which can cause diseases like cancer, tumors, skin effects etc. Ozone depletion (deduction) takes place because of the many factors that are present in our day-to-day life. One of them is the presence of compounds having chlorine, iodine, bromine atoms etc.

Solution to ozone problem was to use halogenated hydrocarbons which contains no chlorine, but only fluorine atoms, and these materials are known as fluorocarbons.

Desirable Properties of Refrigerants :

It must have a low boiling point. It should have a higher critical temperature. A refrigerant must have a high latent heat of vaporization. It has a low specific heat of liquid. It has a low specific volume of vapor. It should have non-corrosive affinity towards materials. It should be Non-flammable / In-flammable and non explosive. It should have a low economical cost. It should be easily possible to liquify to moderate pressure and temperature.

Classification of Refrigerants :

* CFC Refrigerants :

It contains Chloro-fluorocarbons in CFC compounds which has ozone depletion (reduction) effect due to the presence of fluorines. for example, R-12 , R-502 (Which are under banned section and are restricted to use ).

* HCFC Refrigerants :

It contains "Hydro chloro fluoro carbons" in it, which contains less chlorine or slighter chlorine and has a minimum / less effect on ozone depletion layer as compared to CFC's. But in UK, Europe, USA, its use was banned in 2011. For example, R-22 (Chlorine di fluoro methane ), R - 408a, R - 409a.

* HFC Refrigerants :

Hydro fluoro carbon type of compound, in which there are no chlorine atoms and it is very much user friendly to ozone layer or atmospheric condition of air. First Fluoro carbon / Hydro fluoro carbon which was made commercially available was R - 134a , which is used as a replacement of R - 12. For example, R - 134a, R - 404, R - 410a, R - 507.

R - 134a : It is also known as "Tetra fluoro ethane" which is from the family of Hydro fluoro carbon family of refrigerants.

* Properties of R - 134a Refrigerant :

Boiling Point : -14.9 °F / 26 °C

Auto Ignition Temperature : 1418 °F / 770 °C

Ozone Depletion effect : 0

Solubility in water : 0.11% by wt at 77 °F / 25 °C

Critical Temperature : 250°F / 122 °C

Color code : Light - Blue

Global Warming Potential (GWP ) : 1200

* Fluorocarbons (F-C) :

In the mid 1990's, scientists discovered that some F-C gases were powerful greenhouse gases. By 2007, they were determined to have a global warming potential 300 times greater than carbon dioxide. Some research has indicated that plants are much more sensitive to fluorinated compounds than other organic contaminants. This is because they can cause cellular damage and lead to changes in gene expression. They may also interfere with the function of hormones and enzymes, and could potentially alter the biochemical activity of cells.

* Hydrofluorocarbons (HFCs) :

Hydrofluorocarbons are chemicals that are similar in structure to CFCs but do not contain chlorine or bromine. They are commonly used as refrigerants in air conditioners and heat pumps, although many countries around the world have phased them out due to their adverse effects on the environment.

* - R134a (Freon) :

R134a is commonly used as a refrigerant gas in residential air conditioning and heat pumps. In commercial applications, it is utilized primarily in small room and window AC units. The majority of these systems are installed in newly constructed homes. As a result, R134a has become the most widely used refrigerant gas in U.S. domestic heating/cooling equipment since 1979. R134a was first registered as an industrial refrigerant under the name R12 in 1929.

* R404A (HFO-1234yf) :

As a replacement for ozone depleting halogenated hydrocarbons (HCFCs), R404a belongs to group of fluorinated hydrocarbon compounds known as hydrofluorocarbons (HFCs). These chemicals have been proposed as replacements for HCFCs due to their high stability, low toxicity, non-interference with existing refrigeration systems, and their ability to pass through the stratospheric ozone layer. R404a has a lower global warming potential than both carbon dioxide and methane.

* R290 (Isopropane) :

Isopropane (isobutane) is a colorless, flammable light hydrocarbon that can be used as a refrigerant or propellant in aerosol cans. Isopropane is also used in many other industries including medicine, industry, agriculture, and chemical manufacturing. Because it is inexpensive and relatively easy to use, isopropane remains one of the most common refrigerants today.

Well, Here completes the 7th part of the full HVAC course. Till then keep learning.. Keep Growing..

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