Mechanical Engineering

Brayton and Otto cycles generate mechanical energy out of thermal energy. This article discusses in detail on the topic Otto cycle vs Brayton cycle.

Brayton cycle is used in jet engines whereas Otto cycle is used in SI engine vehicles. Lets find out what other differences and similarities exist between these cycles.

Major working parts used in Brayton cycle

A set of machines work together to make Brayton cycle possible.

The different working parts used in Brayton cycle are compressor, mixing chamber and turbine. Compressor compresses the air, fuel is added in mixing chamber where the compressed air and fuel interact. Finally, thermal energy is converted to mechanical energy by turbine.

Working of Brayton cycle

Air is used as working fluid in Brayton cycle. Minimum three processes are required to complete this cycle (Three processes for open cycle and four processes for closed cycle).

Following processes combine to make up Brayton cycle-

• Isentropic compression- Process 1-2 represents isentropic compression in which air is compressed without changing its entropy.
• Isobaric heat addition- Process 2-3 represents isobaric heat addition in which heat is added to the mixing chamber; heat combined with compressed air produces high thermal energy.
• Isentropic expansion- Process 3-4 represents isentropic expansion in which the thermal energy is converted to mechanical energy. Rotation of turbine shaft represents mechanical energy.
• Isobaric heat rejection- Process 4-1 represents isobaric heat rejection where the heat is removed from the working fluid and is sent further to get compressed for next cycle.

Major parts used in Otto cycle

The parts used in Otto cycle are much smaller than those used in Brayton cycle.

The parts used in Otto cycle are-

• Piston- Piston performs up-and-down reciprocating motion that compresses the working fluid inside the cylinder.
• Cylinder- Cylinder is the foundation of Otto cycle. Cylinder is the place where all the energy conversion takes place.  
• Valves- The suction and delivery valves are used for intake of working fluid and exit of exhaust gases respectively.

Working of Otto Cycle

Otto cycle uses steam as its working fluid.

Following processes take place in Otto cycle-

• Isentropic compression- Process 1-2 shows isentropic compression of working fluid. The piston moves from BDC to TDC. The entropy of system is constant during this process hence it is called as isentropic compression.
• Isochoric heat addition- Process 2-3 represents heat addition in the system. The piston remains at TDC and shows ignition of the working fluid.
• Constant entropy expansion- Process 3-4 represents isentropic expansion (constant entropy expansion) where the piston moves from TDC to BDC. Since, the entropy remains constant throughout this process it is called as isentropic expansion.
• Isochoric heat addition- Process 4-1 represents heat addition to constant volume. The piston remains stationary at BDC while heat gets rejected to atmosphere.

This cycle keeps repeating as piston moves to TDC.

Brayton cycle vs Otto cycle efficiency

Both cycles different processes and different working fluids. This affects the efficiency of the cycles.

The comparison of thermal efficiencies of Brayton cycle and Otto cycle is shown in the table below-

Thermal efficiency of Brayton cycle	Thermal efficiency of Otto cycle

Where,

rp is the compression ratio and Y is specific heat ratio.

Hence, for constant values of compression ratio, both the efficiencies have same values.

But in practice, Brayton cycles are used for larger values of compression ratios and Otto cycle is used for small values of compression ratio. Hence, the formula of efficiency may be same but their applications are different.

Why is Brayton cycle more suitable than Otto cycle?

Brayton cycle uses a gas turbine and compressor whereas Otto cycle uses piston cylinder arrangement for its working. Otto cycle is preferred for SI engines where one cannot fit a gas turbine and compressor in the vehicle.

Following points explain in detail about advantages of Brayton over Otto cycle-

• For same values of compression and work output, Brayton cycle can handle a larger volume at small range of temperature and pressure.
• A piston cylinder arrangement can’t handle large volume of low pressure gas. Hence, Otto cycle is preferred in vehicles.
• In Otto cycle, the working parts are exposed to maximum temperature for a very short period of time and also it takes time to cool down. Whereas in gas turbine cycle, the working parts are exposed to high temperature all the time. In steady state process, the heat transfer from the machinery is difficult in constant volume process (ie Otto cycle) than at Constant pressure (ie Brayton cycle).

The topic Brayton cycle Vs Rankine cycle gives us an idea that they both must be similar in some aspects. Both cycles are used to generate mechanical energy out of thermal energy.

The major difference between these cycles is the working fluid used. Rankine cycle uses liquid (mostly water) as working fluid whereas Brayton cycle uses gas (mostly air) as working fluid. This article does a comparative analysis on Brayton cycle vs Rankine cycle.

Major components used in Brayton cycle

Every cycle needs a set of machinery that helps achieve the desired output.

Brayton cycle consists of following working parts-

• Compressor- Compresses the air isentropically.
• Mixing chamber- Heat is added to the compressed air that increases the temperature isobarically.
• Turbine- Air is expanded in turbine, as turbine shaft rotates the air pressure reduces and temperature reduces. This process is isentropic expansion.

 Working of Brayton cycle

Brayton cycle generally uses atmospheric air as its working fluid. It takes minimum three processes to complete this cycle (An open cycle has three processes and closed cycle has minimum four processes).

The different processes that the working fluid undergoes in closed Ideal Brayton cycle are-

• Isentropic compression- Ambient air is drawn inside the compressor and compressed isentropically.
• Isobaric heat addition- Heat is added to the compressed air at constant pressure.
• Isentropic expansion- Air is expanded in a turbine isentropically.
• Isobaric heat rejection- Heat is rejected from the system at constant pressure.

Isentropic compression and expansion processes denote an ideal cycle. Usually, the process is not completely isentropic due to irreversibilities and friction losses in turbine and compressor. The isentropic efficiency of turbine and compressor denote the magnitude of useful output that can be obtained from given conditions.

Parts used in Rankine cycle

Rankine cycle produces mechanical energy from thermal energy of the working fluid. This is achieved by many components working in harmony.

The working components used in Rankine cycle are-

• Pump- The low pressure liquid is pumped to boiler increasing its pressure.
• Boiler- Heat is added to the working liquid inside the boiler. The heat addition process is isobaric. The high pressure liquid gets converted to high pressure steam inside the boiler.
• Turbine- Steam is expanded in turbine. The high pressure steam is responsible for producing mechanical energy which is achieved by turbine shaft rotation.
• Condenser- The low pressure steam is condensed inside the condenser. Condenser is nothing but a heat exchanger that extracts heat from the steam to convert it into liquid.

Working of Rankine cycle

Rankine cycle is used to produce mechanical energy from thermal energy of working fluid (in this case water) which in turn is used for generating electricity (shaft power of turbine is used to produce electricity).

Rankine cycle also works on four major processes. They are-

• Isentropic compression (process 1-2): Pressure of working fluid increases in this process.
• Isobaric heat addition (process2-3): High pressure liquid is subjected to heat inside the boiler where it gets converted into steam. The steam exits at high pressure and enters the turbine at point 3.
• Isentropic expansion (process 3-4): The high pressure steam rotates the turbine propellers as a result turbine shaft starts rotating. During this process, the high pressure steam gets converted to low pressure steam. The low pressure steam enters the condenser.
• Isobaric heat rejection- The steam gets converted back to liquid state inside the condenser. The heat is rejected from the steam at constant pressure as a result of which the steam gets converted to liquid.

Note that condenser and boiler are devices that change the state of working fluid without changing the temperature and pressure.

Rankine cycle and Brayton cycle efficiency

Efficiency is the measure of cycle’s effectiveness. The amount of output a cycle can deliver in a given amount of input is called the efficiency of a cycle.

Before discussing the efficiencies of Rankine cycle and Brayton cycle, lets have a look on turbine and compressor efficiency-

Turbine isentropic efficiency is given by-

Compressor isentropic efficiency is given by-

The comparison of Rankine cycle efficiency and Brayton cycle efficiency is given below-

Subject of comparison	Rankine cycle	Brayton Cycle
Ideal efficiency
Actual efficiency
T-s Diagram

How to increase efficiency of Brayton cycle and Rankine cycle?

Efficiency is the ratio of output to input. To increase the efficiency of any cycle, one needs to increase output at constant input or decrease input for constant output or increase the output while reducing the input.

In both the cycles, same methods can be used to improve the efficiency. These methods are-

• Regeneration– Steam from condenser is passed through turbine to increase inlet temperature before the steam enters the boiler.
• Reheat- A secondary turbine is used that results in more work output.
• Intercooling– Intercooler cools the gas after compression thereby making it available to be compressed again. This way the compressor work is reduced.
• Combined regeneration, intercooling and reheat cycle– This cycle uses combination of regenerative cycle, reheat cycle and intercooling.

What are the two main types of Brayton cycle?

Brayton cycle may use regeneration, reheat, intercooling or sometimes all of them. But the foundational cycle in which such methods can be used are of two types.

The two basic forms of Brayton cycle are-

• Open Brayton cycle- In Open Brayton cycle, the exhaust gases are spat out to the atmosphere. Each cycle uses new set of gas or working fluid.
• Closed Brayton cycle- In Closed Brayton cycle, the exhaust gases are cooled and sent back to the compressor to be used again. This forms a complete cycle.

What is a combined cycle?

One combines two things to get more output or increase the efficiency of particular system. In a combined cycle, both Brayton and Rankine cycles are combined to derive more output from a given set of input.

Brayton cycle produces more power so it is called as topping cycle. The exhaust gases from this cycle are so hot enough that it can be used as source for a comparatively low power producing cycle that is Rankine cycle. In this case, it is also known as bottoming cycle.

The heat from the exhaust gases is recovered by waste heat recovery boiler in bottoming cycle. The steam/water gets heated to complete Rankine cycle.

This way the waste exhaust gases from one cycle can be used as source for another cycle.

To the uninitiated, Liquid Refrigerant and Coolant sound like two names for the same automobile fluid.

However, both these fluids serve completely different purpose in your car. Refrigerants are the primary working fluid in a refrigeration or Air conditioning system. Coolant on the other hand is a blend of water and an antifreeze.

Is liquid coolant the same as antifreeze?

Liquid coolant and antifreeze are sometimes used interchangeably.

They are not the same. Antifreeze is the chemical ingredient that lowers the freezing point and increases a water-based liquid’s boiling point. Coolant is the mixture of antifreeze agents and water which regulates the engine’s temperature.

The coolant primarily maintains the temperature of a system and prevents it from overheating. It acts as a heat transfer fluid in manufacturing applications, automobile and as a cutting fluid in metalworking, machining processes and industrial rotary machinery.

Coolant is a 50-50 split of antifreeze and water, which means antifreeze is nothing but a coolant component.

So why do we add antifreeze?

Water-cooled engines must be protected from freezing, heating, and corrosion.

However, water absorbs a larger amount of heat in comparison to most other liquids. But it freezes at a relatively high temperature, and also it is corrosive.

A mixture of antifreeze and water gives an adequate coolant solution with :

• Improved anticorrosive properties
• lower freezing point
• higher boiling point of water

Ethylene glycol is a chemical that performs very well as antifreeze. It mixes properly with water and due to having a low viscosity, allows it to circulate simply through the cooling system.

Which liquid is used as refrigerant?

For a fluid to be used as refrigerant it must have few properties that are difficult to find in a liquid at room temperature.

The only refrigerant that is found in liquid form under normal atmospheric conditions is water (R718). However, commercial use of water as a refrigerant is minimal.

In order to delve into further details we must understand…

What Refrigerants do?

Refrigerants are the primary heat transfer agents in an HVAC system.

They absorb heat during evaporation, causing the refrigeration effect at low temperature and pressure, and release heat to cooling media, which is normally water or ambient air during condensation at high temperature and pressure. A schematic diagram of a refrigeration system is shown below:

In a refrigeration system, it is desired that during the evaporation cycle (which sees the lowest pressure), the refrigeration system pressure is maintained above atmospheric so that no non-condensing gas (read air) ingresses into the system and render the system inefficient.

The evaporating pressures (40°F) and condensing pressures (100°F) of all the commonly used refrigerants are above atmospheric (Source: p410, Handbook of air conditioning and refrigeration, Auth Shan K. Wang, Mcgraw-Hill pub). It implies all the refrigerants that are usually being used in the industry are gases at normal atmospheric pressure and temperature.

Types of Refrigerants

The earliest refrigerants used were air and ammonia. Then came the CFCs (Chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) and were extensively used till the 1980s. Due to the environmental concerns of CFCs and HCFC, they are  gradually phased out and replaced with new formulations, which can be classified as follows:

• Hydrofluorocarbons: HFCs are a combination of hydrogen, fluorine, and carbon atoms. Due to the absence of chlorine atoms, they are environmentally safe, and there is no chance of ozone depletion. They are chosen by the prefix HFC.
• Azeotropic: Azeotropes are mixtures or blends characterized by constant boiling points. The blends of refrigerants are called azeotropic if there is no change in composition at any point in the vapor-liquid mixture similar to that of a single component. They evaporate and condensate at a fixed temperature under constant pressure conditions.
• Near Azeotropic and Zeotropic: These blends of refrigerants behave as a single component while phase change is taking place. The phase change, however, doesn’t take place at a single temperature, and it happens over a range. This range is lower for near azeotropic mixtures and higher for Zeotropic blends.

Selection of proper refrigerant is important for efficient and safe operation of a HVAC system.

Criteria for selection of Refrigerants

A good refrigerant must fulfill specific properties to be commercially and environmentally viable and safe for use in an inhibited place. Factors that are considered for the selection of a refrigerant are:

• Safety requirements: Leakage of refrigerants may occur from pipe joints, seals, or different parts during the installation period, operations, or accident. Hence, refrigerants must be adequately safe for humans and manufacturing processes, without toxicity or flammability. Ammonia is an example of toxic refrigerant.
• Refrigeration Capacity: Refrigeration capacity is defined as the volume (measured in cfm) of refrigerant required to produce 1 ton of refrigeration. Depending upon the properties of refrigerant, such as its latent heat and its specific volume, the volume of refrigerant would be different, effecting the size of the compressor required and thus affecting both fixed as well as operating cost.
• Physical Properties: Physical properties of a refrigerant, such as its heat capacity, thermal conductivity, dielectric properties etc., also play an essential role.

Why is gas line larger in size than liquid size in AC

The design of any component can be done based on the phase of matter used in it.

Gasses occupy more volume for the same mass compared to liquid by virtue of their lower density. Liquid state needs to be pumped through a smaller pipe diameter to maintain the same velocities.

In other words, for the same mass flow rates, in order to maintain the same velocities,  fluid in its liquid state needs to be circulated through an area lower than that compared to the same fluid in its vapor state.

That is exactly what is happening inside an AC or refrigeration system. Hence, to maintain system pressure drop and velocity across the refrigeration system, gas pipelines are sized larger than liquid.

How line sizing is decided?

The line sizing is decided based on pressure drop, velocity and phase changes of the refrigerants taking place.

As the fluid changes from liquid to vapor phase the velocity increases. As the velocity increases the pressure drop increases. Hence, in order to maintain pressure drop as well as velocity the line sizes are different for liquid and vapor phase.

Let us look at the refrigeration system and see how the refrigerant travels through the four sections of an Air conditioning system.

• Evaporator to Compressor:  Low-pressure Saturated Vapor
• Compressor to Condenser:  High-pressure Superheated Vapor
• Condenser to Expansion device: High-Pressure Sub-cooled liquid.
• Expansion valve to evaporator:  a low-pressure liquid-vapor mixture

A figure of the refrigeration system is shown below:

As shown in the figure above, the refrigerant enters the evaporator from the expansion device in the form of cold, low-pressure liquid with some amount of vapor as a result of expansion cooling or the Joules-Thompson effect. Due to heat transfer from the refrigerant to the warm air outside, the refrigerant turns into a vapor by boiling.

The cold low-pressure vapor is then compressed by the compressor, increasing its temperature and pressure. This hot, high-pressure vapor condenses in the condenser.

The outlet of the condenser is sub-cooled liquid. This sub-cooled liquid refrigerant then flows from the condenser to the expansion valve and the cycle continues.

What are the Design Goals of Piping system?

The main design goals of refrigeration piping are to maximize system reliability and reduce installation costs.

To accomplish the same, the refrigerant must be transferred at proper velocity across the system to maintain the design aspects and also at minimum capital and operating cost.

The primary design goals are as follows:

• Returning of the lubricating oil to the compressor at the proper rate.„
• There is no flashing of liquid taking place before the refrigerant  enters the expansion device „
• System pressure drops are within acceptable limits, and no capacity loss is taking place.
• Total refrigerant charge in the system is economical.„

Lubricating oil is required to lubricate and seal the moving parts of a compressor. Since the refrigeration process is a closed system, the oil is present along with the refrigerant and is pumped along with the refrigerant throughout the system. Thus it is important that the refrigerant, whether in liquid or vapor form, should have sufficient velocity to carry the oil along with it.

Let’s start with the Suction line or the line connecting the Evaporator to Compressor. This gas line must have sufficient velocity to carry the entrained oil droplet to the compressor.

Next is the compressor discharge line, which operates at high pressure and high temperature and delivers vapor to the condenser. Thus maintaining the mass flow rates across the system to maintain similar velocities, the discharge line operating at higher vapor densities (because of higher pressure) is comparatively smaller than the suction line.

The most critical piping in the refrigeration system is the liquid line which connects the condenser to the expansion device. Out of the three pipes, the liquid line has the most significant impact on the quantity of refrigerant required to charge the system, and hence its proper sizing becomes critical.

A Larger pipe size would call for a higher refrigerant flow requirement to fill up the pipe. On the other hand, lowing the size of the pipe would cause pressure drop issues. The pressure drop in the line must be small enough so that no vaporization occurs in the pipe before the entry of refrigerant into the expansion device.

Thus to sump-up, the gas-liquid piping in a refrigeration system is designed to minimize the pressure drop and thus reduce compression power cost. Appropriate velocities are to be maintained mainly in the gas phase to carry the entrained oil droplets required for lubrication along with the refrigerant.

Gas being lighter and having low densities need a larger pipe size than liquid for the same mass flow of refrigerant. Finally, liquid line size is minimized to reduce the refrigeration requirement. However, its size is limited by the pressure drop allowed in the pipe to prevent it from flushing before reaching the expansion device.

Knowing the position of crankshaft drive in the engine is essential. Cam shaft position sensor is used to get the required information.

This information is necessary to calculate the ignition point and injection point. This article gives a deep insight on functions of cam shaft sensor code, cam shaft sensor and steps required to follow after replacement of camshaft sensor.

Cam shaft position sensor

Camshaft sensor and crankshaft sensor work in harmony to know the exact position of crankshaft drive. The combination of readings from both sensors helps the engine control unit in determining the exact time when the first cylinder is in the top dead point.

Cam shaft sensor works on the Hall principle. A ring gear is located on the camshaft whose rotation is scanned by the sensor. Rotation of this ring gear is responsible for change in the Hall voltage of Hall IC in the sensor head. This change in voltage is translated to the required data by the engine control unit.

• Sensor code P0340
• Sensor code P0340 symptoms
• Sensor code P0340 causes
• How serious is P0340?
• Camshaft sensor code P0016
• Symptoms of P0016 code
• Causes of P0016 code
• How serious is P0016?
• What to do after replacing cam shaft sensor?
• Cam shaft sensor code after replacement

Sensor code P0340

The camshaft position sensor is essential for calculating the exact time of ignition and injection.

Without this sensor, the engine will not know when to ignite the fuel, leading to increase in fuel consumption and sometimes engine damage.  

Sensor code P0340 symptoms

There are many ways by which P0340 can be identified/suspected.

Major symptoms of code P0340 are-

• Check engine light on dashboard
• Poor acceleration
• Engine stalling
• Car jerking
• Problems shifting gear
• Low fuel mileage
• Ignition problems

Sensor code P0340 causes

There can be many causes behind the setting of P0340 code.

Following are the reasons behind P0340-

• Defective sensor
• Defective ring gear on the camshaft
• Damaged or corroded wiring in Camshaft sensor circuit
• Fault in crankshaft sensor
• Damaged or corroded wiring in crankshaft sensor circuit

How serious is P0340?

Any alarm is dangerous which is why it is called “alarm”. The intensity of problem maybe low in the beginning but if the alarm is ignored for a longer period then it may lead to severe damages to the engine.

The engine will initially start running erratically. The engine will give low fuel efficiency or mileage. If left untreated, then engine parts can be damaged due to improper ignition timing.

Camshaft sensor code P0016

Another code relating to camshaft sensor is code P0016.

Code P0016 is a generic OBD-II code which indicates the cam shaft position sensor whether bank 1 correlates to the signal from the crankshaft position sensor or not.

Symptoms of P0016 code

There are many ways through which this code can be identified/suspected.

Some symptoms of P0016 code are-

• Check engine light turns on.
• Engine runs abnormally/erratically.
• Engine mileage decreases.
• Reduction in power

Causes of P0016 code

 There are many ways through which this code can appear.

Major causes of P0016 are-

• Oil control valve has restriction in Oil control valve filter
• Camshaft timing is out of position.
• Camshaft phaser is out of position because of fault with phaser.

How severe is P0016?

As discussed for problems pertaining to code P0034, P0016 code has similar problems.

The engine will start stalling or running erratically. Then fuel mileage will go down. At last leading to severe damages to the engine depending upon the failed part.

What to do after replacing camshaft sensor?

The camshaft sensor must be installed in correct orientation. After orienting in the correct direction, one must reset the sensor before using the vehicle.

• The resetting procedure is simple. Firstly, one has to focus on switch ON and OFF function, these switches are connected to magnets that need to be adjusted first.
• After doing this, engine light, crank sensor and engine block needs to be checked for damages. Then, trouble codes also need to be checked with the help of a code reader to see if the problem still persists or not.
• After doing this, turn off all the parts that are connected to battery and start driving vehicle at 70 Kmph-80 Kmph for five minutes and then decelerate it to 50-60 kmph. This way the timing chain is changed or the sensor is reset.

If one faces problems while resetting then he/she can consult a mechanic to perform this procedure.  

Camshaft sensor code after replacement

It is not necessary that replacing the camshaft sensor will solve the issue. The error light might still be ON in some cases This happens when there is a fault in sensor wiring harness.

If the error doesn’t show after replacing the sensor then it is safe to test drive the vehicle, if the error still shows then it is desired to seek professional help. A professional mechanic can have a check engine light inspection which will ensure him whether the issue is fixed and can reset the code. There is no need of calibrating the sensor as it has been installed in correct orientation.

After installation of sensor, OBD II reader must be used to reset the codes. Most likely the sensor is fine. The only problem lies in resetting of codes that turns off the light in the sensor.

The terms hygroscopic and hydroscopic may sound similar but their meanings completely differ from one another.

Hygroscopic substance refers to the substance that can take and hold moisture from the surroundings. Hydroscope is an instrument used to see objects deep underwater. This article discusses about hygroscopic vs hydroscopic substances in detail.

Hygroscopic vs Hydroscopic:

Hygroscopic substances

Hygroscope refers to the phenomenon of attracting water molecules via absorption or adsorption. Hygroscopic substances are capable of taking away moisture from the surroundings and holding it. This decreases the relative humidity of the surrounding. The relative humidity of substance is directly proportional to the amount of moisture the substance can hold.

Engineering materials like ABS, Cellulose , Nylon etc are hygroscopic in nature.In some composites, due to difference in hygroscopic properties of two materials, there can be detrimental effects such as stress concentration. The amount of moisture taken by a substance is a function of temperature and humidity of the surrounding.

The rate of transfer of moisture decreases as it approaches equilibrium. This happens because of two reasons- the driving force behind moisture transfer decreases and the diffusional resistance to mass transfer increases as the surface taking up moisture nears to equilibrium.

Image credits: Wikipedia

Storage of hygroscopic materials

Hygroscopic materials are usually stored in sealed bags. These bags are simply kept in those places where the moisture content has to be regulated. A common example is silica gel which is used to take away moisture content from the products such as water bottles, lunchboxes, water filters etc.

If these materials are not properly stored, the desired moisture content will not be achieved. Moisture content is an essential factor for determining a machine’s life. If it is not regulated properly then simply because of improper moisture content, life of machines will be altered.

Hygroscopic materials in different pressure conditions

The partial pressure of hygroscopic materials and the ambient pressure can affect the moisture of the system directly.

When the material is subjected to high pressure (isothermally beyond saturation point), then the specific humidity will decrease and relative humidity will keep on increasing. The added moisture will affect material’s quality. An example of such pressure fed system is pneumatic system wherein the hygroscopic material is conveyed through air.

When the material is subjected to negative pressure, the specific humidity remains constant and relative humidity decreases as pressure of the air in conveyer decreases.

Applications of deliquescent materials

The phenomenon of absorbing moisture up to such an extent that the substance dissolves completely in the water to make a solution. Liquids like Sulfuric acid and and salts like Sodium Chloride are examples of deliquescent substances.

In chemical industries, deliquescent materials are used for absorbing water content from chemical reactions. These materials are also known as desiccants. Desiccants like silica gel are used for absorbing moisture from the surrounding environment.

Hydroscopy

Hydroscopy is completely different from hygroscopy. Hydroscopy is the practice of looking and observing things underwater. This can be done by using the instrument called hydroscope. The original hydroscope was invented by Greek scholar and scientist philosopher, Hypatia of Alexandria.

Hydroscope itself is not any instrument. Hydroscope refers to the type of any instrument that is used to measure properties related to water. The hydroscope is generally made out of tubes and a transparent cap at the end made of plastic or glass for viewing.

It is difficult for humans to see underwater without using hydroscope. When we try looking underwater with naked eye, water rushes on the surface of eyeball and distorts the light coming to the pupil. Hydroscope prevents this distortion by providing a transparent material which allows light to enter the eye and avoiding contact with water. If required, we can also achieve magnification underwater.  

Examples of hydroscopy

Complexity of hydroscope varies from application to application. It can be as simple as a tube with two lens and as complex as a computer controlled lens with variable magnification.

Some examples of hydroscopy are as follows-

• For viewing objects near the surface of oceans, a long tube is fitted with lenses so as to see the objects that can’t be seen otherwise.
• In defence practices, subsurface water is detected by the use of surface nuclear magnetic resonance technique.

Applications of hydroscope

Hydroscopy is an important technique that allows us to study aquatic life and perform underwater tasks. Everything that requires deep water excursions is achieved by using hydroscope.

Following are the applications of hydroscope-

• Scientists use hydroscopes for looking at marine life which dwells deep inside the ocean. Many marine animals and plants have been discovered with the use of hydroscopes.
• Archaeologists use hydroscopes to search for ancient remains which might have submerged deep underwater.
• Hydroscopy is used for inspection of ship hulls and underwater pipelines to check for corrosion.
• Rescue missions in caves which are flooded by seawater.

Chillers are machines used to dehumidify or cool fluids. There are various types of chillers classified on the basis of working fluid used, working mechanism used etc.

This article explains how does a chiller work, different types of chillers used in industry and general information about compressors used in air cooled chillers.

How does an air-cooled chiller work?

Ever seen multiple fans installed on the top of a building? They are used for cooling purposes inside the building. These fans are a part of a bigger system known as air cooled chiller.

Chiller is a machine that absorbs heat using vapour compression cycle, vapour absorption cycle or vapour adsorption cycle. The cool fluid can be passed through a heat exchanger for further applications. Concepts of thermodynamics are used in air cooled chillers to cool the fluid or dehumidify air.

Chillers collects heat from water and sends it back to air handling unit which uses cool water for its operation. After AHU’s operation, the water temperature rises and is brought back to the air chiller.

How does an industrial chiller work?

The main purpose of industrial air chiller is to cool the water and send it back to the AHU (Air Handling Unit). After AHU does its specified task, the water inside the AHU becomes warm. This warm water is sent back to the inlet of chiller. This cycle continues till the end of AHU’s operation.

The air chiller absorbs heat from the processed water that comes into the inlet of the chiller. Heat is absorbed with the help of chiller’s evaporator.

After the liquid refrigerant passes through evaporator, its phase changes to gas and pressure decreases in this process. After compression, the refrigerant that leaves has high pressure and high temperature.

This gas enters the condenser where it is cooled by condensing fans. The cooling fans blow away the heat into ambient hence it is suggested to install air chillers outside the room or at a place where dumping heat is not an issue.

An industrial air chiller has following components- Evaporator, condenser, compressor, pump and cooling fans.

• Evaporator-It takes away heat from the water to change the phase from liquid to gas.
• Compressor-Temperature and pressure of the gas is increased by compressing the gas in compressor.
• Condensing fans/cooling fans-The cooling fans blow away the heat from the refrigerant reducing the temperature of gas.
• Condenser-The phase changes back to liquid inside the condenser.

What are industrial chillers used for?

Industrial chillers are used for cooling mechanisms, products and a wide range of machinery. It can be centralized where one chiller can be used for multiple applications or decentralized where each and every application has one dedicated chiller.

Chillers are used in plastic industries, metal cutting work oils, injection and blow moulding, cement processing. They are also used in gas turbine cooling system, high heat applications such as MRI and lasers in hospitals.

Liquid cooled chillers are used for indoor operations due to as liquid absorbs the rejected heat. Air cooled chillers are meant for outdoor installations because the heat is rejected in the ambient. Hence, most air cooled chillers are installed at the top of buildings.

Types of compressors used in air cooled chillers

There are various types of compressors that can be used in chillers depending on the load requirements in the application. Following are the compressors that can be used in chillers-

• Reciprocating compressor-A simple positive displacement pump which used a piston to deliver gas at high pressure. The gas enters the cylinder in the suction stroke when the piston is at bottom dead centre. The gas is compressed in the next stroke when the piston move towards the top dead centre. Compressed gas leaves through the delivery valve. This type of compressors deliver compressed gas in pulsations.
• Rotary screw compressor-Rotary compressors are used in large sized refrigeration applications such as chillers. These have rotary type positive displacement mechanism and provide continuous delivery of compressed gas unlike reciprocating compressors which have pulsations. Rotary compressors are more quiet in operation.  
• Vane compressor-Most common type of compressor is the vane compressor. It uses centrifugal force to compress the gas. These compressors uses vanes instead of helical screws to generate compressed air.
  
• Scroll compressor-A scroll compressor uses two spiraled scrolls for compressing the gas or refrigerant. Usually one scroll is fixed and other orbits with a little offset without rotating. The tapped gas between the scrolls get compressed due to the relative motion between scrolls. Its efficiency is slightly higher than reciprocating compressors.

Water cooled chillers

As the name suggests, water cooled chillers use water instead of air for cooling. It uses latent heat for cooling purposes.

External cooling towers supply water that is used to cool the gaseous refrigerant in the condenser. Inside the condenser, refrigerant’s phase changes. The gaseous refrigerant turns into liquid refrigerant and is then re-circulated in the system.

Advantages and disadvantages of water cooled chillers

Every mechanical component has its own pros and cons. Designers have to make a trade off between pros and cons to make the best design suitable for the particular application. Following are the advantages and disadvantages of water cooled chillers

Advantages of water cooled chillers-

• They are more efficient than air chilled coolers.
• They don’t create much noise while operating.
• They can be used in both small scale and commercial scale applications.

Disadvantages of water cooled chillers-

• Due to continuous requirement of water, water cooled chillers are not feasible to use in areas having water shortage problems.
• As the number of components are increased (cooling tower and pumps), installation cost of water cooled chillers is more.

Vapour compressed chillers vs vapour absorbed chillers

Vapour compressed and vapour absorbed chillers are both air cooled chillers. The principle difference between vapour compressed chiller and vapour absorbed air chiller is the way of cooling.

It is clear that vapour absorbed chiller has more parts but it is cheaper to operate as it does not need any compressed air for operation.