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INTRODUCTION









Introduction

Refrigeration

Refrigeration is removing heat from required place (or required object) to some other needless place and controlling the temperature of that required place or required object.
Refrigeration system uses firstly for the food preservation. There were some other food preservation methods earlier. But that food preservation systems change color, smell, taste and the quality of foods. There for, people tried to control bacteria by reducing the temperature of foods. With the result of this the refrigeration systems are came up.
In this case natural refrigeration systems uses firstly. They use natural ice for this. With the increasing of production and usage, mechanical refrigeration systems take place. Now there are a lot of these types of refrigeration systems and those are use for various types of productions and for various requirements.
At present mainly we can see vapor compression system for the mechanical refrigeration.
When using refrigeration for food preservation, we use refrigeration under four main requirements.

  1. For Industrial Requirements.
  2. For Transport & Marine Requirements.
  3. For Commercial Requirements.
  4. For Domestic Requirements.

Air-Conditioning

In air-conditioning uses same method, which use in refrigeration. There are some differences within refrigeration and air-conditioning. We can divided air-conditioning in to two categories according to the requirement.

Ø  For industrial Prepossess.
Ø  For Human Comfort.

When we do air-conditioning for above prepossess, we should pay our attention on below five basics.

  1. Temperature Control.
  2. Humidity Control.
  3. Air Filtration.
  4. Air purification.
  5. Air Circulation.

Basic Refrigeration Cycle (Vapor Compression System)

It needed refrigerant for mechanical refrigeration system. That refrigerant should vaporize in a low boiling point and it should be liquidize in high boiling point. In that case for the mechanical refrigeration system, the most applicable refrigerant should vaporize in a low pressure and low boiling point and, it should liquidize in high pressure and high boiling point. According to this we use this type of refrigerant for the mechanical refrigeration system and to do refrigeration possess we use vapor compression system.
It needs four essential equipments for the vapor compression type refrigeration system. 





01.  Compressor
02.  Condenser
03.  Metering Device
04.  Evaporator


The Function of Basic Refrigeration Cycle



Basic Refrigeration Cycle


Compressor compress low pressure, low temperature vapor which comes to the compressor and that vapor convert in to high pressure, high temperature vapor. This high pressure, high temperature vapor goes to the condenser. In the condenser this high pressure, high temperature vapor dissipate heat to the out side and the refrigerant vapor convert in to high pressure, normal temperature liquid. In the condenser, it dissipates heat to the out side in three steps.
Ø  De-supper heat
Ø  Latent heat (large amount of heat)
Ø  Sub cool
 In this case, the vapor which is in high boiling point de supper heat and it be a vapor which is in low boiling point. This vapor dissipates head by latent heat and it will be a liquid. This liquid dissipates heat by sub cool and it will be a liquid which is in low boiling point. In this case refrigerant remove heat from refrigerant to the out side by conduction and convection.
This high pressure, normal temperature refrigerant liquid goes to the metering device through the liquid receiver. In the metering device high pressure, normal temperature liquid convert in to low pressure, low temperature liquid (flash gas). This is an adiabatic process. In this process, there is no any heat transfer and it drops pressure only.
Then low pressure, low temperature refrigerant liquid goes to the evaporator. In the evaporator this low pressure, low temperature refrigerant liquid absorbs heat from relevant place, which should be cooled. In this case the refrigerant absorbs heat by two steps.




Ø  Latent heat (large  amount of heat)
Ø  Supper heat (small amount of heat)
By absorbing the heat, the low pressure, low temperature refrigerant liquid convert in to low pressure, low temperature refrigerant vapor. This low pressure, low temperature refrigerant vapor goes again to the compressor. this cycle functioning again and again.

Simple Saturated Refrigeration Cycle






Simple Saturated Refrigeration Cycle drawn in Pressure-Enthalpy Chart

                          
                          A                        B = Compression
                          B                        C = Condensation
                          C                        D = Expansion
                          D                        A = Evaporation

In a pressure-enthalpy chart, we called this as a simple saturation refrigeration cycle; because it starts compression from saturated vapor line and it ends condensation from saturated liquid line. (But practically refrigeration systems function in different way). In here low pressure, low temperature saturated vapor (A) compress by the compressor. This compression happens on constant entropy line up to discharge pressure. There is a high pressure, high temperature refrigerant gas in position (B). This refrigerant gas enters to the condenser. In condenser, it keeps the pressure constantly and firstly removes the heat by de-supper heat up to the boiling point relevant to the pressure. Then it removes heat by latent heat until the refrigerant gas become to saturated liquid state. Then this high pressure, normal temperature saturated refrigerant liquid leaves the condenser (C) and enters to the metering device. The duty of the metering device is to drop the pressure up to relevant suction pressure without exchanging the heat. This is called adiabatic expansion. This low pressure, low temperature liquid enters to the evaporator (D). In this process small amount of the refrigerant liquid vaporized (This is a loss and it should minimize). There fore the contamination of refrigerant that enters to the evaporator is small amount of refrigerant vapor and large amount of refrigerant liquid (flash gas). The refrigerant liquid that enters to the evaporator is in low pressure. In this pressure the refrigerant liquid absorb latent heat relevant to the boiling point and it leaves the evaporator as a saturated vapor and it enters to the compressor (A) again. This refrigeration cycle function again and again.



Refrigerator





There are four basic equipments in gas cycle of a domestic refrigerator. Those are:
Ø  Compressor
Ø  Condenser
Ø  Capillary Tube
Ø  Evaporator.
In addition there is a filter drier and suction accumulator as accessory equipments.

Compressor

Mostly there are reciprocating type compressors uses in refrigerators. Sometimes we can see rotary compressors also in refrigerators. These compressors can be 1/5 hp, 1/6 hp and 1/8 hp in capacity (domestic refrigerators).These are fully hermetic compressors. The compressor compress the low pressure, low temperature refrigerant vapor and convert in to high pressure, high temperature refrigerant vapor and let the refrigerant goes to the condenser.  



 
Cut view of a Fully Hermetic Piston Type Compressor

Condenser

There is a wire and tube condenser in refrigerator gas cycle. Material of the condenser tube is steel. The condenser mounted to the refrigerator in a slant. The reason to mount the condenser in a slant is to increase the condensation efficiency. The color of the condenser is black. The reason for this also to increase the efficiency of condensation. Thigh temperature, high pressure refrigerant gas enters to the condenser from the top of the condenser. The condenser removes the heat of the refrigerant vapor to the out side and convert high pressure, high temperature refrigerant vapor to high pressure, normal temperature refrigerant liquid. This refrigerant liquid goes to the capillary tube through the filter drier.

Filter Drier

There is a pencil type filter drier in refrigerator gas cycle. This pencil type filter drier can be double end or single end.  The filter drier filter unnecessary particles of the refrigerant and absorbs the moisture in, the gas cycle. To filter the unnecessary particles there are strainers in the filter drier and to absorb the moisture there are activated alumina in the filter drier.

Capillary Tube

This is a small (small in in-side and out side diameter) and lengthy copper tube. This drops the pressure of the refrigerant which comes from the condenser. By relating the capillary tube with the suction line, it has created a heat exchanger in the refrigerator gas cycle.

Evaporator

 There is a plate type evaporator in refrigerator. Low pressure, low temperature refrigerant liquid enters to the evaporator. This low pressure, low temperature refrigerant liquid absorbs the heat from place that must be cooled and vaporize. This refrigerant vapor goes to the compressor back through the suction accumulator.

Suction Accumulator

The location of the suction accumulator is the end of the evaporator. This does not let to go refrigerant liquid to the compressor.




No-Frost Type Refrigerator   
     


 
No-Frost Type Refrigerator   



There is compressor, condenser, filter drier, capillary tube, evaporator and suction accumulator in no-frost type refrigerator.

Compressor

There are reciprocating type fully hermetic compressors in no-frost type refrigerators. The compressor compress the refrigerant gas and let the refrigerant gas goes to the condenser.

Condenser

There is plate type condenser in no-frost type refrigerator gas cycle. We can not see the condenser from out side. This condenser condenses the heat of the refrigerant to the out side through the refrigerator body. There fore when we touch the body of the refrigerator we feel heat. After removing heat of the refrigerant gas, the refrigerant gas converts in to refrigerant liquid. This is a high pressure, normal temperature refrigerant liquid. This refrigerant liquid goes to the capillary tube through the filter drier.






Filter Drier

This can be a single end or double end pencil type filter drier. This filter drier the refrigerant and absorb the moisture from the refrigerant.

Capillary Tube

Capillary tube drop the pressure of the refrigerant liquid and let low pressure, low temperature refrigerant liquid to go to the evaporator.

Evaporator

There are fin and tube evaporators in no-frost type refrigerators. In the evaporator refrigerant liquid absorb the heat from the place which should be cooled and vaporized. This low pressure, low temperature refrigerant vapor goes to the compressor through the suction accumulator.

Suction Accumulator

Though the suction accumulator located at end of the evaporator in frost type refrigerator, we can see separate suction accumulators in no-frost type refrigerators. This does not let to go refrigerant liquid to the compressor.


Window Type Air-Conditioner

There are four basic equipments in window type air-conditioner gas cycle. Those are compressor, condenser, capillary tube and evaporator. To increase efficiency of the air-conditioner and reduce faults, there are accessory equipments. Mostly we can see filter and suction accumulator as accessories. We can see window type air-conditioners in below capacities.
 
  •   6,000 Btu/hr
  • 9,000 Btu/hr
  •   12,000 Btu/hr
  • 18,000 Btu/hr
  •   24,000 Btu/hr



  Window Type Air-Conditioner



 
Window Type Air-Conditioner

Compressor

Three type of compressors use for window type air-conditioners. Those are: reciprocating compressors, rotary compressors, scroll compressors. If rotary compressor uses for the window type air-conditioner, must use suction accumulator.
Compressor compress the low pressure, low temperature refrigerant vapor and let them goes to the condenser.

Condenser

Fin and tube force draft air cool condensers use for window type air-conditioners. Condenser removes the heat from the high pressure, high temperature refrigerant vapor and covert it into high pressure, normal temperature refrigerant liquid and let the liquid goes the capillary tube through the filter.

Filter

This filter the refrigerant liquid and let the refrigerant goes to the capillary tube.

Capillary Tube

This drop the pressure of the high pressure, normal temperature refrigerant liquid and let refrigerant goes to the evaporator.

Evaporator

There are fin and tube force draft evaporators in window type air-conditioners. The evaporator absorbs the heath from place, which should be cooled. Because of absorbing heat refrigerant liquid vaporize and leaves the evaporator. This low pressure, low temperature refrigerant vapor enters to the compressor through the suction accumulator.

Suction Accumulator

This do not let to goes refrigerant liquid in to the compressor. If rotary compressor in the system, the suction accumulator must be in the refrigerant cycle. 
 


Split Air-Conditioner

If we install a window type air-conditioner:

  •   It brings noise.
  •  Cant install in a room, whish is inside of a building.
  •   Have to damage walls to make cowl.

Split air-conditioner has made fulfilling above shortage. The split air-conditioner has two units. Those are; in-door unit and out-door unit. In-door unit install in, inside of the room and out-door unit install outside of the room.

 Split Air-Conditioner










We can see various types of split air-conditioners according to installing system of in-door unit.

Ø  Floor Mount
Ø  Wall Mount
Ø  Ceiling Mount
Ø  Cassette Type

And also we can see split air-conditioners 6,000 Btu/hr to 60,000 Btu/hr in cooling capacity. There are four basic equipments that means, compressor, and condenser, metering device and evaporator in the gas cycle of split air-conditioner. And also there are accessory equipments as oil separator, filter and suction accumulator in gas cycle of a split air-conditioner.

Compressor

We can see reciprocating, piston and scroll compressors in split type air-conditioners basically. The compressor compress the low pressure, low temperature refrigerant vapor and convert the refrigerant in to high pressure, high temperature refrigerant vapor and let them goes to the condenser through the oil separator.

Oil Separator

This separate the refrigerant oil from the refrigerant and do not let refrigerant oil goes to the refrigerant gas cycle.






Condenser

Condenser of the split air-conditioner is a fin and tube force draft air cool condenser. Condenser removes the heat of the high pressure, high temperature refrigerant vapor and convert refrigerant in to high pressure, normal temperature refrigerant liquid. This high pressure, normal temperature refrigerant liquid goes to the evaporator through the filter.

Filter

This filter the refrigerant and let the refrigerant goes to the metering device.

Capillary Tube

Capillary tube wok as metering device. Capillary tube drops the pressure of the high pressure, normal temperature refrigerant liquid. This low pressure, low temperature refrigerant liquid goes to the evaporator.

Evaporator

Type of the evaporator of the split air-conditioner is fin and tube.  This absorbs the heat from the place, which should cool. Because of absorbing heat the refrigerant liquid vaporize. This low pressure, low temperature refrigerant vapor goes to the compressor through the suction accumulator.

Suction Accumulator

Suction accumulator do not let goes the refrigerant liquid in to the compressor.




Package Type Air-conditioner

Package type air-conditioner use to cool more area than the area cooled by split and window type air-conditioners. The cooling capacity of package type air-conditioners above 50,000 Btu/hr. package type air-conditioners are categorized according to below factors.

Ø  Body parts arrangement.
Ø  Condenser cooling method.
Ø  Cool air distribution.

  
Package Type Air-conditioner


 
Package Type Air-conditioner

We can see basic four equipments (compressor, condenser, metering device and evaporator) in package type air-conditioner gas cycle also. And also we can see below accessory equipments in package type air-conditioner gas cycle.

Ø  Oil separator
Ø  Filter
Ø  Liquid distributor
Ø  Suction header
Ø  Suction accumulator
Ø   Moisture and liquid indicator

Compressor

Mostly reciprocating, rotary and scroll compressors use for package type air-conditioners. Most of the compressors in the package type air-conditioners are three phase. The compressor compress low pressure, low temperature refrigerant vapor. Because of the compression low pressure, low temperature refrigerant vapor will be high pressure, high temperature refrigerant vapor. This high pressure, high temperature refrigerant vapor goes to the condenser through the oil separator.

Oil Separator

When the compressor functioning, refrigerant oil can be go out with the discharge refrigerant vapor. Because of this it can be drop the oil level of the compressor. And also it can be block the some places in the gas cycle. To avoid this, we use oil separator. Oil separator does not let to go oil to the system and returns oil to the compressor.

Condenser

We can see various types of condensers in package type air-conditioners.

Ø  Fin and tube force draft air cool condensers.



Ø  Shell and tube water cool condensers.
Ø  Shell and coil water cool condensers.
Ø  Tube in tube water cool condensers.

Water use as the condenser cooling medium, in water cool condensers. For the water cool condensers it uses cooling tower to remove the heat of the water.
For force draft air cool condensers, it uses fan motors to cool the condenser. These fan motors are three phase.
Condenser remove the heat of the high pressure, high temperature refrigerant vapor. Because of the condensation, the high pressure, high temperature refrigerant vapor will be high pressure, normal temperature refrigerant liquid. This refrigerant enters to the metering device though the filter.

Filter

The filter filtrate the refrigerant.

Moisture and Liquid Indicator

Moisture and liquid indicator is located ahead of each expansion valve and it must be full of refrigerant liquid to properly indicate moisture content. We should operate the system at least 30 minutes before attempting to determine moisture content.

Expansion Valve

Thermostatic expansion valve uses as the metering device of package type air conditioners. Most of these valves are with external equalizer. This drops the pressure of the high pressure, normal temperature refrigerant liquid.

Liquid Distributor

This distribute the refrigerant liquid to several places (coil sets) of the evaporator.

Evaporator

There are fin and tube evaporators on the package type air-conditioners. These evaporators are in several types. As an example some evaporators are “V” type. This absorbs the heat from the place, which should cool. Because of absorbing heat the refrigerant liquid vaporize. This low pressure, low temperature refrigerant vapor goes to the compressor through the suction accumulator.

Suction Header

The all of the tubes which are out from the evaporator connected to the suction header and one tube goes to the compressor from the suction header.

Suction Accumulator

Suction accumulator do not let goes the refrigerant liquid in to the compressor.


Central Air-Conditioning System




Air Cooled Chiller


The central air-conditioning system also made with same equipments which are in vapor compression system. But the central air-conditioning systems are very large. The temperatures are different according to the medium that is cool by this system. In this case these large machines called as chillers. The chillers cool normal water for the air-conditioning purposes.
When water uses for the central air-conditioning systems as the cooling medium, it maintain the temperature of the water about 7°C-8°C. But when uses brine as the cooling medium, the temperature of the brine vary with the temperature that required.
Chillers are named as below according to cooling system of the condenser and compressor type in the system.

Ø  Reciprocating Water Cooled Chiller.
Ø  Reciprocating Air Cooled Chiller.
Ø  Screw Water Cooled Chiller.
Ø  Screw Air Cooled Chiller.
Ø  Centrifugal Water Cooled Chiller.
Ø  Centrifugal Air Cooled Chiller.





When uses a water chiller, secondary refrigerant cooled by the primary refrigerant. This type of cooling system called as indirect cooling. When uses normal water as the secondary refrigerant, it cooled by the chiller and supply to the required places through the insulated P.V.C. by the water pumps. According to the requirement that chill water supply to a fan coil unit (F.C.U.) or air handling unit (A.H.U.).
Fan coil units made with copper tube coil and a blower. The chill water comes to the fan coil units through the insulated P.V.C. lines. The temperature of the chilled water is about 7°C-8°C. When air circulates through the fan coil unit, the heat of the air absorbs the chill water. Because of this the temperature of the chill water increases. The condition of the return water to the chiller is about 11°C-15°C. Again this water cool by the chiller.
Complete air-conditioning process done by the air handling unit. Have to supply chill water to the coil of the fan coil unit and if had to heat the air have to supply hot water to the hot water coil.

Compressor

Compressor of the chiller can be semi hermetic or open type compressor. These compressors are working with three phase current supply. There are oil heaters to heat oil of the compressor. If the oil temperature is not up to the required value, the compressor can not switch on. To get the temperature of the oil in the compressor, there are oil sensors in the compressors.
Compressor compress the low pressure, low temperature refrigerant vapor and let to goes to the condenser through the oil separator.  

Oil Separator

Oil separator separates the oil from the refrigerant and let the refrigerant to go to the condenser.

Condenser

We can see below type’s condensers in central air conditioners.

Ø  Water cool shell and tube condensers.
Ø  Water cool shell and coil condensers.
Ø  Air cool fin and tube condensers.

If there is a water cool condenser, there is a cooling tower to cool the water and there are water pumps to pump the water. If there is an air cool condenser there is a fan to cool the condenser. Sometimes natural draft air cool condensers are also available.
Condenser condense the heat of the high pressure, high temperature refrigerant vapor and let the refrigerant go to the metering devise through the liquid receiver, filter and sight glass as high pressure, normal temperature refrigerant liquid.

Liquid Receiver

This stores the liquid refrigerant and supply to the metering devise according to the requirement.

Filter

Filter filtrate the refrigerant.

Sight Glass

Sight glass must be full with the refrigerant liquid. If there are bubbles, that represents lack of refrigerant in the system

Metering Devise

This drops the pressure of the high pressure, normal temperature refrigerant liquid let to go the refrigerant in to the evaporator (chiller).

Evaporator (Chiller)

There are two types of evaporators in chillers. Those are:

Ø  Flooded Evaporators
Ø  Direct Expansion Evaporators

The evaporator absorbs the heat of the water. Because of this water will cool and refrigerant liquid will vaporize. This low pressure, low temperature refrigerant vapor enters to the compressor through the suction accumulator.

Suction Accumulator

Suction accumulator do not let to refrigerant liquid goes to the compressor.

 Cold Room




 
A Model of a Cold Room


We can see four main equipments in cold room gas cycle also, which are in basic refrigeration cycle. In addition there is a oil separator, liquid receiver, core type filter drier, heat exchanger and suction accumulator as accessory equipments.

Compressor

Mostly there are open type compressors or semi hermetic compressors in cold room gas cycle. These compressors are working with three phase electrical supply.
Compressor compress the low pressure, low temperature refrigerant vapor and convert it in to high pressure, high temperature refrigerant vapor and let the refrigerant vapor goes to the condenser through the oil separator.

Oil Separator

Oil separator separates the oil from the refrigerant and returns oil to the compressor and let the refrigerant vapor goes to the condenser.



 Discharge Line Oil Separator


Condenser

There are water cool condensers or air cool condenses in cold room gas cycle. There are cooling towers to cool the water and air cool condensers are forced draft.
Condenser condenses the high pressure, high temperature refrigerant vapor and converts it in to high pressure, normal temperature refrigerant liquid. This refrigerant liquid goes to the metering device through the liquid receiver and filter drier.

Liquid Receiver

This stores the refrigerant liquid and supply liquid refrigerant according to the requirement.

Filter Drier

Mostly there are core type filter driers in cold rooms. This filters the refrigerant and absorbs the moisture in the refrigerant if there is.





Core Type Filter Drier



Metering Device

Mainly there are thermostatic expansion valves with external equalizer in cold room gas cycle.
The expansion valve drops the pressure of the high pressure, normal temperature refrigerant liquid and let to goes the refrigerant to the evaporator as the low pressure, low temperature refrigerant liquid.




T.E.V. with External equalizer


Evaporator

Mainly we can see force draft air cool fin and tube evaporators in cold rooms. The evaporator absorbs the heat from the outer area which should be cooled. Because of that the refrigerant liquid vaporizes. This low pressure, low temperature refrigerant vapor goes the compressor back through the heat exchanger and suction accumulator.



Fan Coil Unit of a Cold Rom


Heat Exchanger

This exchanges the heat among the liquid line and the suction line of the gas cycle.



Cross Section of Heat Exchange

Suction Accumulator

This do not let to goes liquid refrigerant to the compressor.





Suction Accumulator


Auto Air-Conditioner

  
Auto Air-Conditioner





Auto Air-Conditioner



There is compressor, condenser, metering devise and evaporator in the gas cycle of the auto air-conditioner. In addition there is a receiver drier in the auto air-conditioner gas cycle.

Compressor

We can see below compressors in an auto air-conditioner gas cycle.

Ø  Open Type Reciprocating Compressor.
Ø  Open Type Rotary Compressor.
Ø  Open Type Screw Compressor.
Ø  Open Type Scroll Compressor.

Compressor compress the low pressure, low temperature refrigerant vapor. Because of the compression the refrigerant vapor will be high pressure, high temperature refrigerant vapor. This high pressure, high temperature refrigerant vapor goes to the condenser.

Condenser

There are fin and tube forced draft condenses in auto air-conditioner gas cycles. The condenser condenses the high pressure, high temperature refrigerant vapor. Because of the condensation the high pressure, high temperature refrigerant vapor will be high pressure, normal temperature refrigerant liquid. This refrigerant liquid goes to the metering device through the receiver drier.

Receiver Drier

This filter the refrigerant liquid, store the refrigerant liquid and supply the refrigerant liquid to the metering devise according to the requirement.

Metering Devise.

There are thermostatic expansion valves or fixed orifice tube as the metering devise of auto air-conditioners. This drops the pressure of the high pressure, normal temperature refrigerant liquid. Because of the pressure drop, the refrigerant liquid will be low pressure, low temperature refrigerant liquid. His low pressure, low temperature refrigerant liquid goes to the evaporator.

Evaporator

The evaporators of the auto air-conditioners are forced draft fin and tube evaporators. The evaporator absorbs the heat of the area which should be cool. Because of the heat absorbing, the low pressure, low temperature refrigerant liquid will vaporize. This low pressure, low temperature refrigerant vapor goes to the compressor back. 


Safety

It can be happen various types of accidents in the industry (work shop). It is very important to awareness of those accidents before happen. It is very important safety sings (mandatory sings, warning sings and caution sings) for this. To protect from the accidents it is very important to know about these safety sings.

Mandatory Sings



Warning Sings

 





(1)   Wear year protections: We can see this sign at the places which are with abounding noises. We should wear year protections to protect our years from abounding noises.

(2)   Wear helmets: We can see this sign at the working places which are far above the ground. When we work at this type of a place we should wear the helmets.

(3)   Wear eye protections: We can see this sign near the drilling and drilling machines. When we grind or drill work piece (metal), we should wear the eye protections o protect our eyes.

(4)   Wear safety boots: We can see this sign at store room. To protect our legs we should wear the safety boots.

(5)   Wear gloves: We can see this sign near the welding plants, drill machine and oxy acetylene kit. To protect our hands we should wear the hand gloves.

(6)   No smoking: We can see this sign at gas filling stations. Awareness of this sign is very important to prevent from fire hazards.

(7)   Fire hazards: We can see this sign near the gas cylinders and gas store rooms. Awareness of this sign is very important to prevent from fire hazards.

(8)   A weight on top of head: This sign tells us about a weight which is top of head. This weight can be fall and we should be careful at that type of a place.

(9)   Poisons: We can see this sign at poison bottles. We should use these types     of bottles carefully.

(10)     Electrical hazard: We can see this sign at the places which are with high voltages like generator rooms and near the transformers.

(11)     Caution: We can see this sign at he places which are accidents can be happen. When see this type of a sign we should be careful.

(12)     Fork lifts: We can see this sign at the places which fork lift are traveling.

(13)     A roof can be break: This sign tells us the roof can be break and it is risky to walk on the roof.