SAFETY DEVICES IN AIR CONDITIONING AND REFRIGERATION PLANTS

written by: Chief Engineer Mohit Sanguri edited by: Lamar Stonecypher updated: 4/2/2010

Compressors in air conditioning and refrigeration plants have to be provided with protective devices

and circuits to protect them from damage. The basic protective devices are the high pressure cut out,

low pressure cut out, oil low pressure cut out, and oil separator or oil extractors.

Compressor Safety

A compressor in a refrigeration or air conditioning plant has to be provided with some safeties to

protect it from operational faults. The three common safeties provided are the high pressure trip, the

low pressure trip, and the low oil pressure trip among the others. A compressor has to be protected

against high pressure that can cause structural failure therefore a high pressure cut out is provided,

similarly any deficiency in the oil pressure can damage the bearings and a low oil pressure cut out has

to be provided, a lower atmospheric in the pipe line can cause air ingress and therefore must be

avoided. In this article we discuss the different safeties one by one.

High Pressure Cut Out

High pressure can be caused in a refrigeration plant due to various causes like over charge, loss of

cooling water, high ambient temperature, air, or other incompressible gases in the system, and

obstruction in the discharge line of the compressor. For protecting the compressor from high pressure

and subsequent failure, a high pressure cut out is provided that take a pressure tapping from the

discharge line and when it detects an over pressure, it stops the compressor. The HP cut out is not

resettable automatically but has to be reset manually by the operator. This is because the high

pressure is a serious fault and the cause must be investigated and corrected before the plant is

started again.

Construction of High Pressure Cut Out

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Operation of a High Pressure Cut Out

The high pressure cut out as shown in the diagram is of a simple construction. It has a bellows that is

set against a spring. The nut at the end of the spring is used to adjust the cut out pressure. When

the high pressure gas enters the bellow, the bellow expands and presses the spring. At the cut off

pressure the movement of the bellow against the spring releases the catch and the contact is broken

and the compressor cuts off.

The switch arm can be pressed and the cut out reset after the cause of the over pressure has been

found and rectified.

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Low Pressure Cut Out

To protect the compressor against low pressure in the system and to avoid the ingress of air into the

system if a vacuum is generated in the lines a low pressure cut out is provided. Also when the

refrigerated compartments are cut off by the solenoids and there is no return gas, the low pressure

cut out is activated. When the solenoid of the refrigerated compartments open, the return gas comes

in the inlet of the compressor and the suction pressure rises, and then the low pressure switch cuts in

the compressor.

Unlike the high pressure cut out, the low pressure cut out is self-resettable and does not need to be

reset manually.

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Low Pressure Cut Out Layout

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Low Oil Pressure Cut Out

The oil is pumped under pressure by an attached oil pump that supplies oil to the bearings for

lubrication. Any problem in the lube oil pressure can jeopardize the bearings and therefore a tapping

is taken from the pump outlet and fed to the oil pressure switch. Any fall in the pressure will activate

the cut out which will stop the compressor.

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Oil Separator

As oil is miscible with the refrigerant and often goes out of the compressor with it, it can go to the

evaporator where it can cause a decrease in heat transfer. To avoid the oil from going to the

evaporator where it can form a layer inside or cause obstruction an oil separator is used. It basically

consists of baffle plates that separate the oil from the refrigerant and feed it back to the compressor.

A float valve is provided so that short circuiting of the refrigerant should not take place.

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Oil Separator Construction

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Conclusion

The refrigeration plant compressor has to be protected against unnatural working conditions by safety

devices and controls. The high pressure cut out, the low pressure cut out, and the low oil pressure

cut out are some of the basic protective devices provided. In large complex circuits other additional

safety devices are provided according to the complexity of the circuit.

CONSTRUCTION OF THE THERMOSTATIC EXPANSION VALVES written by: Haresh Khemani edited by: Lamar Stonecypher updated: 8/20/2013

This articles describes all the parts of the thermostatic expansion valve and also their working.

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Construction of the Thermostatic Expansion Valve

Construction details of the internally equalized thermostatic expansion valve are shown in the figure

to the right. It is comprised of a metallic body which encloses the following parts:

1) Metallic Diaphragm: The metallic diaphragm is a flexible metallic plate that can expand due to

pressure of refrigerant inside the refrigeration or air conditioning plant.

2) Power Element: This is the upper portion of the thermostatic expansion valve which is filled with

gas. In the case of the air conditioning system, the gas filled in element is the same as the gas filled

in the air conditioning system. For instance if the air conditioning system is has R22 refrigerant, the

gas filled in the power element is also R22. The power element is connected to the feeler bulb, via

thin tubing. The same gas is also filled in the tubing and the feeler bulb. Thus the feeler bulb, the

connecting tubing and the power element all form a single flexible chamber.

The feeler bulb is connected to the evaporator and senses the temperature inside the evaporator of

the refrigeration system. The volume of the gas inside the power element changes as per the

temperature sensed by the feeler bulb inside the evaporator. If the temperature inside the evaporator

is high the gas in the feeler bulb will expand; the gas in power element will also expand and its

pressure will increase. The gas pressure inside the power element tends to open the thermostatic

expansion valve.

3) Valve Seat and Needle: The valve seat and the needle are located in the lower side of the

thermostatic expansion valve. The valve seat is the metallic plate that provides passage for the flow

of the refrigerant. The needle is connected to the lower part of the diaphragm and it moves inside the

opening of the valve seat. When the diaphragm moves down due to high pressure inside the power

element the needle also moves down thus opening the thermostatic expansion valve and when the

needle moves up the valve closes. The valve seat and the needle form the orifice that allows the flow

of the refrigerant through it.

4) Spring: The spring is located at the bottom of the thermostatic expansion valve. It is under

compression and tends to move the needle of the valve in an upward direction and close the valve.

The pressure of the spring is adjusted by the manufacturer and it depends on the degree of

superheat in the evaporator. You should purchase the TEV of the required spring pressure. Though

there is a screw for changing the spring pressure, it is preferrable not to change its setting unless you

are sure that the plant is working satisfactorily.

5) Liquid Inlet Port and Outlet to Evaporator: The liquid inlet port is connected to the tubing

coming from the condenser. The refrigerant enters the thermostatic expansion valve via this port.

After passing through the orifice of the valve seat and the needle the refrigerant leaves to the

evaporator in flashed conditioned via outlet to the evaporator.

7) All refrigeration systems require equalization lines. For more information, see this source:

http://www.swtc.edu/ag_power/air_conditioning/lecture/expansion_valve.htm

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Reference

Information and Images from Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan,

Second Edition, Tata Mc-Graw-Hill Publishing Company Limited. See the trial version of the book here.

Working of the Thermostatic Expansion Valve written by: Haresh Khemani edited by: Lamar Stonecypher updated: 2/16/2010

This article describes the working of commonly used thermostatic expansion valve or TEV. TEV is used as the throttling device in number of refrigeration and air conditioning plants of higher cooling capacities.

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Pressures Acting Inside the Thermostatic Expansion Valve or TEV The working of the thermostatic expansion valve can be explained with the help of the attached image of the valve. The valve comprises of external body inside which various parts as shown in the figure are enclosed.

There are three pressures acting inside the thermostatic expansion valve. P1 is the pressure at the top of the thermostatic expansion valve acting inside the power element above the diaphragm. Due to this pressure the diaphragm tends to move down due which the needle also moves down and the valve tends to open. When the evaporator temperature becomes higher the gas in the feeler bulb expands due to which the gas pressure inside the power element increases. This causes the downward movement of the needle to open the valve

The pressure P2 is the pressure acting on the lower side of the diaphragm due to the refrigerant pressure inside the evaporator. This pressure tends to move the diaphragm upwards and close the opening of the valve.

The pressure P3 is the spring pressure that tends to close the opening of the valve. This pressure remains constant. The pressures P2 and P3 act against the pressure P1. The pressure P1 tends to open the valve while the pressure P2 and P3 tend to close the thermostatic expansion valve. Thus if the valve has to open P1 should be greater than combined forces of P2 and P3.

How Thermostatic Expansion Valve Works? During the normal working of the refrigeration plant the thermostatic expansion valve remains opened in certain position. When the refrigeration load increases, the temperature inside the evaporator also increases. In such cases there is need of the more refrigerant to take care of the increased load. The increased temperature in the evaporator is sensed by the feeler bulb of the thermostatic expansion valve. This leads to the expansion of the gas in the feeler bulb and also in the power element of the TEV leading to the increase in pressure P1. Due to this the diaphragm of the TEV moves down and tends to open the valve further to increase the flow of the refrigerant to the evaporator.

At the same time the pressure P2 below the diaphragm also increases due to superheating of the refrigerant inside the evaporator. This pressure tends to close the valve. There is also spring pressure P3 below the diaphragm that opposes the opening of the valve. If the increase in the refrigeration load is much higher the pressure P1 overcomes pressure P2 and P3 leading to the further opening of the thermostatic expansion valve. This allows for the increased flow of the refrigerant to the evaporator to take care of the extra load.

When the refrigeration load reduces, the magnitude of pressure P1 reduces and the combined pressures P2 and P3 overcome pressure P1 that allows for partial closing of the valve so the flow of the refrigerant to the evaporator reduces. Thus the TEV maintains the flow of the refrigerant inside the evaporator as per the refrigeration or air conditioning load. The TEV constantly modulates the flow to maintain the superheat for which it has been adjusted by the spring.

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Reference 1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited

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Images Courtesy 1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited

2) e-Refrigeration

ROOM AIR CONDITIONING SYSTEMS THAT ARE EASY TO CONSTRUCT

written by: Swagatam edited by: Lamar Stonecypher updated: 3/14/2013

Are you planning to buy a conventional type of air conditioner from the market? Wait a moment: the

low cost, efficient, and eco-friendly homemade air conditioner design project shown here may just

change your mind. The drawbacks of Freon and ammonia air conditioner units are also included.

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Spring doesn't stay long, and then its back to the sticky hot summer days. As summer heat becomes

unbearable, air conditioners in every house become active and yet again the electric bills start

soaring, disgustingly adding salt to the wound. And if your country happens to be within the tropical

boundaries, this further makes the situation hostile.

Conventional air conditioners are efficient- there's no doubt about that- but they come with huge

costs in the form of electric consumption and the associated utility bills. More electric usage from

such units would mean that more hydro-electric and nuclear power plants will have to be installed in

the future, resulting in the destruction and occupation of precious land and other natural resources in

the process.

However, interestingly of late, scientists have started looking for many different renewable methods

for replacing the above rather costly counterparts. Air conditioning is no exception and is at the core

of their research.

Water as we know is the basis of life on this planet and moreover has unconditionally provided us

with simple solutions to most of the energy related problems. Whether its a hydroelectric power

station or a cheaply operated evaporative air conditioning unit, water forms the main and ubiquitous

element everywhere - a magical fluid thats available FREE of cost and plentifully on this planet.

Before moving into the proposed design of our homemade air conditioner, lets study a few of the

following conventional AC designs and their drawbacks:

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Air Conditioners Incorporating Freon

The chemical compound Freon is the trade name for ChloroFluroCarbon (CFC) by Du Pont. Freon in

ACs work in the following process:

An electrical compressor compresses cool Freon in liquid form and in the process Freon heats up and

turns into gas.

The gaseous Freon is then forced through a set of coils placed externally behind the AC cabinet

where it dissipates its heat into the outer atmosphere. The cooled Freon gas is then forced back

through another set of coils where it is allowed to expand.

In the process it evaporates and cools down to freezing temperatures before it reaches back to the

compressor to repeat the cycle. This cooling effect is radiated inside the room through a fan to

produce the required effect inside a room.

Drawbacks: In case CFC or Freon leaks out can escape into the uppermost layer of our atmosphere

where through a chemical reaction can help depleting and rupturing the precious ozone gas envelope

- making our planet and us exposed to the lethal UV rays from the sun.

Running the compressor of the above room air conditioners involves high electricity consumptions

and bills.

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Air Conditioners that Use Ammonia and Water

Probably one of the better ways of producing cooling effect in ACs, the procedure taking place inside

a ammonia air conditioner can be understood with the following points:

Here ammonia acts as the chiller component whereas water acts as the absorbent.

From an ammonia water mixture, ammonia is forced out by heating the mixture in a gas-fired burner.

The separated ammonia is then passed through an outdoor coil and condensed.

The above condensed or the liquid ammonia is forced through another set of low pressure coils

where it expands and evaporates to generate the required cooling after which its finally reabsorbed

in water to repeat the cycle.

Drawbacks: Although ammonia is a highly degradable chemical agent and also does not affect the

ozone layer in any way, it is definitely harmful to us in case it leaks out into our rooms. Therefore ACs

using ammonia need to be positioned such that any leaking gas is kept well away from the room

where its installed, quite a complicated and critical affair.

The running cost is also high, equal to the Freon ACs.

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Air Conditioner Based on Evaporation Principle

The important property of water of absorbing the ambient temperature while evaporating has been

exploited to cool houses since the early ages. Even today the coolers based on evaporative

technology use the above feature of water extensively.

The figure alongside shows the internal mechanism of a standard air cooler, also called a swamp

cooler, based on the evaporative principle.

As we can see the entire process takes place inside a rather large box type enclosure.

The side walls of the enclosure have slotted ventilations for the surrounding warm air to get in.

A layer of thick spongy pad is placed immediately after the side ventilations inside the box.

A small electric water pump is used to transfer water from the bottom reservoir over these pads from

the top so that they are properly soaked. The excess water drips down back into the reservoir.

A motor/pulley mechanism is used to rotate the propeller of a large fan at the front of the box. When

it starts moving, the surrounding air is sucked inside the box through the side ventilations.

The air is dragged in through the water soaked pads where due to evaporation the sucked air loses

its temperature and becomes cool.

The cooled air is ultimately forced out by the fan outside into the room, for the desired cooling.

The above principle may be enforced or implemented through different ways and may also be

enhanced through new innovations.

For example, water sprinklers are used over the roofs of many houses where the water droplets

evaporate helping the house to lower their internal temperature. The principle has been thoroughly

studied and improved by NREL and they have succeeded in building a super efficient air conditioner

entirely based on evaporative technology.

Although quite cheaper than an ammonia air conditioner, a swamp cooler may be accompanied with

certain drawbacks. Here, the evaporated water molecules are distributed into the premise,

unnecessarily increasing the relative humidity of the room. This can become a big nuisance especially

for the coastal countries where the RH remains already on the higher side for most months. Also the

efficiency of these units drastically fall as the surrounding humidity increases.

In our present design of homemade air conditioner, we rather take a different approach and use cold

water to produce the required cooling effect, but not in a evaporative manner. This is a bit costly than

the swamp coolers, but surely way ahead as far as the efficiency is concerned.

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Building the Homemade Air Conditioning System

The idea of our homemade air conditioner with cool water is very simple. Get a few bottles of

freezing cold water, empty them in a tank, and use it to cool the room by circulating it through a

simple coil/fan mechanism set up.

Lets learn the procedure through the following steps:

The set-up is shown alongside, as we can see, it utilizes very ordinary low cost items such as a

plastic/thermacole (EPS) basket, copper tube or coils, a table fan, and a few bottles of cold water.

An over head tank made up of plastic and lined up with thermacole, which is probably one of the best

bad conductors of heat (coldness being also a temperature) is used as the main storing compartment

for the passive energy in the form of the cold water.

A conduit copper pipe is attached to the base of this tank which further forms into a large spirally

wound coil having a diameter equal to the procured table fan for the purpose. The end of this copper

spiral ultimately terminates into another tank for retrieving the melted or the exhausted ice water.

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How to Run the Homemade Air Conditioner

The over head tank is specifically made of plastic, ABS to be precise. This box is further placed inside

a thermacole container such that the ABS box snugly slides inside it.

The tank is first filled with freezing cold water; you can simply get it from in your home refrigerator

itself.

The thick thermacole lining around the container helps keep the near-freezing temperature of the

water intact and allow its exhaution very slowly so that the low temperature of the dripping water

through the copper coils is sustained for a longer period time.

As soon as the cold water fills-up the whole copper spiral, copper being extremely good conductor

of heat instantly starts radiating the coolness through its outer walls into the atmosphere around it.

However this makes only the air immediately around the coils colder and will not help to serve the

purpose we are up to.

We need to shoot or rather spread the low temperature created around the coils over the entire

enclosed area or the room where it has been installed.

This is simply done using a table fan. Make sure that the diameter of the fan exactly matches to that

of the copper spiral or vice versa for maximum efficiency.

You will find that by keeping the fan very close to the copper spiral and switching it ON amazingly

initiates the generation of fresh cold breeze all over the place.

A control cock fitted at the end of the copper piping is used to control the flow of the seeping water

into the collector tank. This is directly proportional to the rate of cooling the room: the faster the

water escapes from the tap, the greater the radiation of the cooling effect.

The design has a striking advantage over the age old ordinary evaporative type of coolers where the

cold air is also accompanied with moisture undesirably raising the RH of the premise to uncomfortable

levels. Here copper being non porous allows only the freezing temperature to circulate, restricting the

water content from shedding into the surrounding.

The following data will confirm the high efficiency of the above homemade air conditioner using cool

water, compared to a conventional air conditioning system.

o Set- up cost not more than 20% of the commercially available units.

o Electric comsumption less than 50% compared to the conventional ACs.

o The main cooling agent being cold water is available very cheaply or is derived from the

home refrigerator itself costing less than a dollar per month.

o Each filling of the tank with cold water lasts for more than 15 hours of air cooling.

This is nothing but a Cooler very commonly available in the market in India with 100s of variants. The

major problem is the availability of the "cold water". Water is hot everywhere when it is summer. The

other major issues are: humidity from such cooling systems, seepage of water and to maintain and

regular continuous water supply, oh yes, cold water.

I am somehow convinced that this idea will work and bought copper tubes/rubber tubes, ties, table

fan and ice chest. I just want to know the power pump specs, what would be its watts or power

ideally needed to run this simple setup

o Sure it will work, the problem is that you're not going to find cool water on a hot day. If you

attempt to use the exact same freon liquid/gas evaporation methodology used in a

conventional air conditioner in a much more inefficient way by using a refrigerator to cool the

water then you'll be producing more heat at the back of the fridge which heats up the house

than the cold air produced (heat extracted) inside the fridge to cool the water. Basically,

you'll just end up heating up your house by using a fridge to make cool water.

o There's only one scenario where this cool water spot cooling makes sense. You have to have

an electric plan that charges high prices during peak hours and cheap rates off-peak. Cool the

water during the cheap rates and use the cool water during the peak rates. If nobody is

home during the day to make use of the cool air, then it wouldn't make as much sense. It's

only practical if you're going to be home during peak rates and just want to spot cool a small

area where you are sitting.

This only works if you have a supply of near freezing water available to you naturally. The refrigerator

will put off more heat while cooling the water than you will get in cooling power by this device.

Physics tells us, you can't get something from nothing. In this case, cooling power cannot be created

inside the house your trying to cool.

In most western countries the refrigerator will be running anyway. Further, it's not getting something

from nothing: it's getting something extra from electricity purchased externally. That is, energy

purchased from a power company is transferred into the chilled water which is then used to lower the

ambient temperature.

The extra electricity is not free. Heat seeks equilibrium, so if you put that warm water in,

it will make the cold air around it warmer which will make the thermostat on the fridge

turn the compressor on. Even when the fridge cycles off because the air inside cooled

down, the water is still warmer than the air so the water will heat up the air again and

make the fridge cycle over and over until the water is eventually the same temperature

as the air.

The refrigerator also creates more heat in the home than it cools. Go ahead and put a

refrigerator in a small unconditioned room, leave the door open on the fridge, then

monitor the room to see if it gets colder or warmer. All conversions of energy produce

heat because of inefficiencies which is always below 100%. You can have the same

failure to cool off a room if you put a window AC unit in the middle a sealed room (not

hanging out of a window).

The idea of the proposed home made air-conditioner is a low tech design intended for

providing some sort of relief during summer heats.

The idea uses common sense and yet it definitely works, and it would be quite foolish wasting

valuable time in calculations for such a simple design.

hi,

u designed very good idea, but it is not useful. Do you have any calculations about cooling load,

amount of cooling effect require to keep room temperature below than atm temp. and if u calculate

all then you will find that it is really expensive than window AC, so next time design with calculation.

sorry.