Excerpt
TABLE OF CONTENTS
Acknowledgement
List of Table
List of Figures
List of Photographs/ Plates
List of Symbols, Abbreviations & Greek Letters
Abstract
1 Introduction
1.1 Need of Project
1.2 Objectives
1.3 Scope
1.4 Problem statement
1.5 Methodology
2 Literature Survey
3 Basic of refrigeration and air conditioning
4 Factors affecting condenser capacity
5 Classification of condenser
5.1 Air cooled condenser
5.1.1 Natural convection air cooled condenser
5.1.2 Forced convection air cooled condenser
5.2 Water cooled condenser
5.2.1 Tube in tube condenser
5.2.2 Shell and coil condenser
5.2.3 Shell and tube condenser
5.3 Evaporative condenser
6 Configuration type
7 Water distribution system
8 Fan system
8.1 Induced draft
8.2 Forced draft
9 Fouling factor
10 Scaling
10.1 How it occurs
10.2 Types of scaling
10.3 Adverse effect of scaling
10.4 How to prevent scaling
10.5 Types of descaling
11 The water quality
11.1 Water quality guidelines
12 Design philosophy and principle of operation
12.1 Design calculation
13 Cad modeling
14 Results & Discussion
15 Observation
16 Requirement of industry
16.1 Descaling cost analysis
16.2 Descaling cost
17 Conclusion
18 Reference
List of Table
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List of Figures
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List of Symbols & Abbreviation
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Acknowledgement
We wish to express our deep sense of gratitude and honor towards our respected guide Prof. P.G. Anjankar and Industrial Guide Mr. Arvind Surange Sir (ACR) for his inspiring guidance and constant encouragement. His committed devotion, dedication and encouragement with full faith on us were like a lamp in our path which keeps us constant throughout project work.
We also express our honor and gratitude to our Head of Mechanical Department Dr.N.R.Deore for consistent encouragement for completing our project work successfully.
We are also thankful to our respected Principal Dr.A.M.Fulambarkar and the P.C.E.T's (Pimpri Chinchwad Education Trust) for supporting our project. We are thankful to all Teaching and Non-Teaching staff member of the institute and our classmate who had directly or indirectly made me enthusiastic for the project work.
As we conclude, we would like to state that just as a positive attitude pays off our hard efforts to bring this project to successful end, would also pay off. We hope that this project would be one of the most significant steeping stones for our career and would fulfill our aspiration in every aspect.
Abstract
Evaporative cooling takes advantage of the potential of the outside air in dry climates to absorb moisture, which results in a temperature reduction of the air stream.
But one of the major drawback is continuous scale built up on condenser tubes which makes barrier between tubes and water sprayed on them, which in turn drastically reduces the heat transfer. This results in loss of plant efficiency and increases annual refrigeration cost as the compressor work increases for compensating the pressure drop due to scaling.
In HVAC around 27% worldwide energy is consumed only for different HVAC applications. This project mainly focuses on this problem statement, if the scale is 0.06mm then condenser performance decreases by 16%. Mainly evaporative condenser is used in different HVAC applications. .
In order to overcome this problem regular maintenance of condenser should be done by descaling the coils. Various descaling methods are being used in industries for descaling like mechanical descaling, chemical descaling, etc. But manual descaling is preferred over other methods due to its simplicity, less cost and reliability. Yet there are some problems in manual descaling like the number of rows of condensing tubes are very large in numbers and also the pitch between them is less so it becomes very difficult to reach in deep portion of evaporative condenser in order to clean them thoroughly and effectively scale removal.
1. Introduction
The phenomenon of cooling by evaporation is well-known and it has found many applications. The ancient Egyptians used porous clay containers to^ keep water cool thousands of year ago. Today evaporative cooling is used extensively in industry, ranging from the cooling of power generating plants to the cooling of condensers in air conditioning Systems. In evaporative cooling, the medium which is being cooled can theoretically reach the air wet bulb temperature whereas the minimum temperature which can be reached in dry cooling would be the air dry bulb temperature. The use of evaporative cooling can lead to major cost •savings and improvements in thermal efficiency because of the lower temperatures which can be reached. In a conventional direct contact cooling tower the water to be cooled flows through the cooling tower where it is cooled by counter flow or cross-flow airstream. The cooled water is then passed through a heat exchanger or a condenser to cool a process fluid or condense a vapor this requires two separate units, i.e. the cooling tower and the heat exchanger or condenser.
An evaporative cooler or condenser combines the heat exchanger or condenser and the cooling tower in one unit with the evaporative cooler or condenser tubes replacing the packing of the cooling tower. The operation of an evaporative cooler or condenser can be described as follows:
Re-circulating water is sprayed onto a bank of horizontal tubes containing a hot process fluid or a vapor which is to be condensed while air is drawn across the wet tube bank. The re-circulating water is heated by the hot process fluid or the condensing vapor inside the tubes while it is cooled from the airside by a combined heat and mass transfer process.
The airflow through the evaporative cooler or condenser may be horizontal, in which case the unit is referred to as a cross-flow evaporative cooler or condenser or vertically upwards through the tube bundle where it is known as a counter flow evaporative cooler or condenser.
1.1. Need of project
One of the major drawback is continuous scale built up on condenser tubes which makes barrier between tubes and water sprayed on them, which in turn drastically reduces the heat transfer. This results in loss of plant efficiency and increases annual refrigeration cost as the compressor work increases for compensating the pressure drop due to scaling.
In order to overcome this problem regular maintenance of condenser should be done by descaling the coils. Various descaling methods are being used in industries for descaling like mechanical descaling, chemical descaling, etc. But manual descaling is preferred over other methods due to its simplicity, less cost and reliability. Yet there are some problems in manual descaling like the number of rows of condensing tubes are very large in numbers and also the pitch between them is less so it becomes very difficult to reach in deep portion of evaporative condenser in order to clean them thoroughly and effectively scale removal.
1.2. Objectives
- To design evaporative condenser of any required capacity.
- To increase spacing between two consecutive condenser tubes and observe the effect on work required.
- To provide arrangements for easy descaling of condenser tubes manually.
- To overcome the drawback of chemical descaling.
1.3. Scope
- Implementation of this design in industries for reduction in power consumption.
- Reduction in annual cost of refrigeration.
1.4. Problem Statement
- Design of Evaporative Condenser with Arrangement for Easy Manual Descaling.
1.5. Methodology
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2. Literature survey
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3. Basics of Refrigeration and Air conditioning
- Refrigeration Ton
Ton is therefore amount of heat required to melt 1 ton of ice in 24 hours from 0[0]C ice into 0[0]C water.
- Humidity
Humidity is the amount of water vapor present in the air
- Refrigerants
There is no ideal refrigerant which can be used for all type of applications. A refrigerant is said to be ideal if it has all of the following properties.
1. Low boiling point
2. High critical temperature
3. High latent heat of vaporization
4. Low specific heat of liquid
5. Low specific volume of vapor
6. Non-corrosive to metal
7. Non-flammable and non-explosive
8. Non-toxic
9. Low cost
10. Easy to liquefy at moderate pressure and temperature
11. Easy of locating leaks by odour or suitable indicator
Classification of Refrigerants:
The refrigerants may, broadly, be classified into the following two groups.
1. Primary refrigerants
2. Secondary refrigerants.
The refrigerants which directly take part in the refrigeration system are called primary refrigerants whereas the refrigerants which are first cooled by primary refrigerants and then used for cooling purposes, are known as secondary refrigerants.
The primary refrigerants are further classified into the following four groups.
1. Halo-carbon refrigerants,
2. Azeotrope refrigerants,
3. Inorganic refrigerants,
4. Hydro-carbon refrigerants.
- Simple Vapour Compression Refrigeration System
It consists of the following essential parts:
1. Compressor:
The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve, where it is compressed to a high pressure and temperature. This high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery or discharge valve.
2. Condenser:
The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the surrounding condensing medium which is normally air or water.
3. Receiver:
The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is supplied to the evaporator through the expansion valve or refrigerant control valve.
4. Expansion Valve:
It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporized in the evaporator at the low pressure and temperature.
5. Evaporator:
An evaporator consists of coils of pipe in which the liquid-vapour. Refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which is to be cooled.
- Refrigeration Processes
1. Compression:
The heat that was absorbed in the Evaporation stage must be released into the surroundings, but this will not happen unless the temperature of the refrigerant is higher than the outside air. This is the purpose of the Compression stage. A device, predictably called a compressor, raises the pressure of the refrigerant vapor. Due to basic thermodynamic principles, this causes the temperature of the refrigerant to rise, leaving the stage as a superheated vapor. Energy is needed to power the compressor, which is why electricity is required to operate a refrigerator. The vapour refrigerant at low pressure p1 and temperature T1 is compressed isentropically to dry saturated vapour as shown by the vertical line 1-2 on the T-s diagram and by the curve 1-2 on p-h diagram.
2. Condensation:
The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure p2 and temperature T2 as shown by the horizontal line 2-3 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing through the condenser, gives its latent heat to the surrounding condensing medium.
3. Expansion:
The liquid refrigerant at pressure p3 = p2 and temperature T3 = T2, is expanded by throttling process through the expansion valve to a low pressure p4 = p1 and Temperature T4 = T1 as shown by the curve 3-4 on T-s diagram and by the vertical line 3-4 on p-h diagram. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporized in the evaporator. We know that during the throttling process, no heat is absorbed or rejected by the liquid refrigerant.
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Fig 3.1. Simple refrigerating system.
4. Evaporation:
During this stage, the refrigerant travels through a device called an evaporator that has a large surface area and typically consists of a coiled tube surrounded by aluminum fins. The refrigerant exits this stage as a saturated vapor. The liquid-vapour mixture of the refrigerant at pressure p4 = p1 and temperature T4 = T1 is evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporization. From the medium (air, water or brine) which, is to be cooled, This heat which is absorbed by the refrigerant is called refrigerating effect and it is briefly written as RE. The process of vaporization continues up to point 1 which is the starting point and thus the cycle is completed.
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Fig.3.2. p-h diagram of a simple refrigerating system.
Condenser
The condenser is an important device used in the high pressure side of a refrigeration system. Its function is to remove heat of the vapour refrigerant discharged from compressor. The hot vapour refrigerant consists of the heat absorbed by the evaporator and heat of compression added by the mechanical energy of the compressor motor. The heat from the hot vapour refrigerant in a condenser is removed first by transferring it to the walls of the condenser tubes and then from the tubes to the condensing or cooling medium. The cooling medium may be air or water or a combination of the two. The selection of a condenser depends upon the capacity of the refrigerating system, the type. Refrigerant used and the type of cooling medium available.
Working of a Condenser
The compressor draws in the superheated vapour that the refrigerant contains the heat it absorbed in the evaporator. The compressor adds more heat (i.e the heat of compression) to the superheated vapour. This highly superheated vapour from the compressor is pumped to the condenser through the discharge line. The condenser cools the refrigerant in the following three stages.
1. First of all, the superheated vapour is cooled to saturation temperature (desuperheating) corresponding to the pressure of the refrigerant. This is shown by the line 2-3 in Fig.2.2. The de-superheating occurs in the discharge line and in the first few coils of the condenser.
2. The saturated vapour refrigerant gives up its latent heat and is condensed to a saturated liquid refrigerant. This process, called condensation, is shown by the line 3-4.
3. The temperature of the liquid refrigerant is reduced below its saturation temperature (i.e. sub cooled) in order to increase the refrigeration effect. This process is shown by the line 4-5.
4. Factors Affecting the Condenser Capacity
The condenser capacity is the ability of the condenser to transfer heat from the hot vapour refrigerant to the condensing medium. The heat transfer capacity of a condenser depends upon following factors.
1. Material:
Since the different materials have different abilities of heat transfer ‘h', the size of a condenser of a given capacity can be varied by electing the right material. It may be noted that higher the ability of a material to transfer heat, the smaller will be the size of condenser.
2. Amount of contact:
The condenser capacity may be varied by controlling the amount of contact between the condenser surface and the condensing medium. This can be done by varying the surface area of the condenser and the rate of flow of the condensing medium over condenser surface. The amount of liquid refrigerant level in the condenser also affects the amount of contact between the vapour refrigerant and the condensing medium. The portion of the condenser used for liquid sub-cooling cannot condense any vapour refrigerant.
3. Temperature difference :
The heat transfer capacity of a condenser greatly depends upon the temperature difference between the condensing medium and the vapour refrigerant. As temperature difference increases, the heat transfers rate increases and therefore the condenser capacity increases. Generally, this temperature difference cannot be controlled. But when temperature difference becomes so great that it becomes a problem, devices are available that will change the amount of condensing surface and the air flow rate to control condenser capacity. Most air-cooled condensers are designed to operate with a temperature difference of 14° C.
5. Classification of Condensers
According to the condensing medium used, the condensers are classified into the following three groups:
1. Air cooled condensers
2. Water cooled condensers
3. Evaporative condensers
5.1. Air Cooled Condensers
An air-cooled condenser is one in which the removal of heat is done by air. It consists of steel or copper tubing through which the refrigerant flows. The size of tube usually ranges from 6 to 18 mm outside diameter, depending upon the size of condenser.
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Fig.5.1. Air cooled condenser.
Generally copper tubes used because of its excellent heat transfer ability.The tubes are usually provided with plates or fins to increase the surface area for heat transfer, the fins are usually made from aluminium because of its light weight. The fin spacing is quite wide to reduce dust clogging. The condensers with single row of tubing provide the most efficient heat transfer. This is because the air temperature rises at it passes through each row of tubing. The temperature difference between the air and the vapour, refrigerant decreases in each row becomes effective.
There are two types of air-cooled condensers:
5.1.1. Natural convection air-cooled condensers:
In natural convection air-cooled condenser the heat transfer from the condenser coils to the air is by natural convection. As the air comes in contact with the warm condenser tubes, it absorbs heat from the refrigerant and thus the temperature of air increases. The warm air being lighter, rises up and the cold air from below rises to take away the heat from the condenser. As the rate of heat transfer in natural connection condenser, they require a larger surface area as compared to forced convection condensers. These are used only in small capacity applications such as domestic refrigerant freezers, water coolers and air-conditioners.
5.1.2. Forced convection air-cooled condensers:
In the fan (either propeller or centrifugal) is used to force the air over the condenser coils to increase its heat transfer capacity.
5.2. Water Cooled Condensers:
A water cooled condenser is one in which water is used as the condensing medium. They, are always preferred where an adequate supply of clear inexpensive water and means of water disposal are available. These condensers are commonly used in commercial and industrial refrigerating units. The water cooled condensers operate at a lower condensing temperature then an air cooled condenser this is because the supply water temperature is normally lower than the ambient air temperature, but the difference between the condensing and cooling medium temperatures is normally the same (i.e. 14°C) Thus, the compressor for a water cooled condenser requires less power for the same capacity. The water cooled condensers are classified into three groups.
1. Tube - in - Tube or double tube condensers
2. Shell and coil condensers
3. Shell and tube condensers
5.2.1. Tube-in-tube or double tube condensers:
It consists of a water tube inside a large refrigerant tube. The hot vapour refrigerant enters at the top of the condenser. The water absorbs the heat from the refrigerant and the condensed liquid refrigerant flows at the bottom. Since the refrigerant tubes are exposed to ambient air, therefore some of the heat is also absorbed by ambient air natural convection.
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Fig 5.2.1. Tube-in-tube condenser
The cold water in the inner tubes may flow in either direction. When the water enters at bottom and flows in the direction opposite to the refrigerant, it is said to be a counter flow system. On the other hand, when the water enters at the top and flows in the same direction as refrigerant, it is said to be a parallel flow system. The counter flow system is preferred in all types of water cool condensers because it gives high rate of heat transfer. Since the coldest water is used for final cooling of the liquid refrigerant and the warmest water absorbs heat from the hottest vapour refrigerant, the temperature difference between the water and refrigerant remains fairly constant throughout the condenser. In case of parallel flow system, as the water and refrigerant flow in the same direction, the temperature difference between them increases. Thus the ability of water to absorb heat decreases at it passes through the condenser.
5.2.2. Shell and coil condensers:
Shell and coil condenser consists of one or more water coils enclosed in a welded steel shell (Fig. 21). Both the finned and bare coil types are available. It may be either vertical or horizontal. In vertical type of condenser, the hot vapour refrigerant enters at the top of the shell and surrounds the water coils. As the vapour condenses, it drops to the bottom of the shell which often serves as a receiver. Most vertical type shell and coil condensers use counter flow water system as it is more efficient than parallel flow water system.
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Fig 5.2.2. Shell-coil condenser.
In the shell and coil condensers, coiled tubing is free to expand and contract with temperature changes because of its spring action and can withstand any strain caused by temperature changes. Since the water coils are enclosed in a welded steel shell, the mechanical cleaning of these coils is not possible. The coils are cleaned with chemicals. They are used for units upto 50 tonnes capacity.
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