NATURAL DRAFT COOLING TOWERS - Their CONSTRUCTION, WORKING, TYPES AND ADVANTAGES & DISADVANTAGES.

NATURAL DRAFT COOLING TOWERS

A NATURAL DRAFT Cooling Tower is so called because it do not use fans for drawing air into the tower, instead it uses the natural air which is drawn into the tower naturally by the process of Natural Convection for cooling the hot water. It may be Cylindrical or Hyperbolic in shape. Hyperbolic shape offers superior structural strength and resistence to ambient wind loadings. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m³/hr.  These types of towers are used mainly by Utility Power Stations. 

A NATURAL DRAFT  COOLING TOWER consists of following main parts :

HOT WATER INLET

The hot water inlet is generally provided at the top of the tower, below the Drift Eliminators and above the fill. It is connected to a series of nozzles that sprays the water downwards on the fills present. 

NOZZLES

Nozzles are connected to Hot Water Inlet and are present above the fill. Uniform water distribution at the top of the fill is very much essential to achieve proper wetting of the entire fill surface.  Nozzles sprays water from the top of the fill to uniformly wet the entire fill.

COLD AIR INLET

The cold air inlet is generally provided at the bottom of the tower, above the cold basin and below the fill. The cold air enters into the towers through this.

LOUVERS

The Louvers are present at the Cold Air Inlet. The purpose of louvers is to equalise the air flow into the fill. Also they help in retaining the water within the Coling Towers. They are mainly used when the flow of water and air is crossflow. Many counter flow towers do not require Louvers.

FILL

Fills are present inside the tower below the nozzles to provide a large surface area and time for air and water contact . Most towers have fills made of plastic or wood as it facilitate good heat transfer by maximising water and air contact.

Fill can either be splash or filmtype. In splash fill, water falls in successive layers and is continuously broken it into smaller droplets, thereby wetting the entire fill surface.

Plastic splash fill provides better heat transfer than the wood splash fill.

Film fill consists of thin, closely spaced plastic surfaces(PVC) over which the water spreads uniformly forming a thin film which is in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is more efficient than the splash fill and provides same heat transfer in a smaller volume.

DRIFT ELIMINATORS

Sometimes it happens that the droplets of water are carried out of the cooling tower along with the hot exhaust air. The concentration of impurities in these droplets is same as that in the water entering the tower. These may lead to environmental pollution. To  reduce this baffle-like devices, called as Drift Eliminators are installed above the nozzles. Drift Eliminators allow only the air to pass through it thereby removing water droplets from the air.

COLD WATER BASIN

The cold water basin, is located at the bottom of the tower.  The cooled water that flows down through the tower and fill is collected by the Cold Water Basin. These collected cold water is then discharged and used for cooling other hot process fluids .

WORKING OF NDCT’s :-

The hot water that is too be cooled in the cooling tower is pumped to the top of the tower at the hot water inlet. The hot water inlet is connected to a series of nozzles which sprays this water over the fills. Simultaneously air is introduced from the bottom of the tower which flows in upward direction. The air is circulated inside the cooling tower by Natural Convection. The fills provide a large surface area and time for air and water contact . The water then flows down through plastic or wooden fills. This causes drop formation. While flowing down, the water emits heat which mixes with the air flowing upward, causing it to cool down. Part of the water evaporates, causing it to emit more heat. Water vapor can sometimes be observed over the cooling tower. The cooled water falls down into the cold water basin which is discharged and can be then used for cooling various hot process fluids.

  

ADVANTAGES OF NDCT’s :-

1. FAN IS NOT INSTALLED AS NDCT’S RELY ON NATURAL AIR
2. POWER SAVING
3. EASY TO CLEAN
4. NO MECHANICAL NOISE
5. NO CORROSION PROBLEMS
6. MAINTAINANCE IS LOW
7. NO REPAIR PROBLEMS
8. NO RECIRCULATION OF AIR OCCURS AS THE PLUME IS REJECTED AT HIGH LEVEL
9. ENVIRONMENT FRIENDLY

DISADVANTAGES OF NDCT’s :-

1. A VERY LONG CONSTRUCTION TIME IS NEEDED TO CONSTRUCT NDCT.
2. HUGE WATER FLOW REQUIRED
3. NOT FOR ALL CLIMATIC CONDITIONS
4. PERFORMANCE DEPENDS ON PREVAILING WIND VELOCITY AND DIRECTION.
5. AIR MOVEMENT DEPEND ON INDUCTION FORCES
6. UNOBSTRUCTED LOCATION NECESSARY.
7. NATURAL DRAFT COOLING TOWERS REQUIRES VERY LARGE AREA.

       The NATURAL DRAFT COOLING TOWERS are of two types :

1. SPRAY TYPE NDCT’S

2. SPLASH DECK TYPE NDCT’S.

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How to Set up a Chemical Plant - Chemical Plant Site Selection, Different Location factors & Preparing Plant Layout.

This post will answer following Questions : How to select a suitable Chemical Plant Site , Different Location factors that need to be considered & How to prepare Plant Layout - in short HOW TO SET UP A CHEMICAL PLANT.
SITE SELECTION:

                                     Site selection for a plant, keeping in view the various corporate objectives of the company requires careful considerations of the various factors that would go into making the plant contribute towards its working environment and make it into a technologically and economical viable unit. Decisions are strategic, long term and non-repetitive in nature. A certain amount of imaginative planning for the future is called for at the time of final decision about the site to be selected.

                                     Generally, sites for a new plant will be allocated according to general industrial development policies of the Government of India. The various factors which need to be considered for site selection are as follows:- 
1.     LOCATION FACTORS:

                                     The location factor deserves careful attention simply because of its long terms consequences. Any mistake in selection of a proper location could prove to be costly. Poor location could be a constant source of higher cost, higher investment, difficult marketing and transportation, dissatisfied and frustrated employees and customers, low availability of high calibre personnel, frequent interruptions of production and abnormal wastages. Once the plant is set up at a location, it is very difficult to shift later to a better location because of numerous economic, political and sociological reasons. Social reasons could include employee welfare, employment opportunities. Political reasons could be due to pursuance of a policy of regional development and planning, especially in a developing country like India.

The location factor requires the following facilities:

·        District classification:

                                     District classification includes the cause that whether the license is obtained for location of the plant in a no-industry district.

·        Transportation facilities:

                                     The raw material and end product are required to have uninterrupted receipt and dispatch facilities through good road connections and proper linking with ports and rail heads. Connection with airport is important mainly from the point of view of easy movement of professionals. This is all the more important if foreign collaboration is involved in the plant.

·        Manpower availability:

                                     Local availability of skilled and semi-skilled manpower will add to the efficient running of the plant.

·        Industrial infrastructure:

                                     All supporting services required for the successful operation of the plant like maintenance and repairing of various machines and items, availability of workshop, plant services, etc. may not be feasible to generate within factory complex. Availability of communication facilities is also an important part of the infrastructure. An existing vibrant infrastructure in the vicinity is much preferred than the need based infrastructure getting developed after a plant commissioning.

·        Community infrastructure:

                                     Generally, modern plants operate on innovative and sophisticated technology. Hence, it is essential to attract qualified professionals. This would involve ensuing a good quality living which, in turn, would depend on availability of good schools and colleges, medical services, good communication facilities, cultural and recreational opportunities, etc. Unless a good community infrastructure is available, attracting qualifying professionals is rather difficult.

·        Availability of raw water:

                                     It is recommended that the plant be located in close proximity to a perennial source of water, which will meet the requirements.

·        Effluent disposal:

                                     It would be economically advantageous if an effluent disposal facility such as gutter which can handle the produced quantity of effluent is readily available. Otherwise transporting the effluent by drainage to a safe disposal area which is far away has to tackle the legal ecological problems.

·        Availability of power:

                                     Stable and uninterrupted power of required magnitude without fluctuations in voltage and frequencies is important for the successful operation of the plant. Proximity to the available power facility will reduce the plant cost.

·        Site size and nature:

                                     The plant area and the topography should suit the plant requirements, along with the required township facilities and future expansions. The grade level of the entire area should be preferably the same. If the plot area is not flat, it has to be levelled in the most economical way to suit the plant.

                                      Load bearing characteristics of the soil plays an important role in site selection. Natural soil has more load bearing characteristics as compared to filled soil. Consequently, the cost of the civil foundation will be less in natural soil because piling to support heavy loads may not be required whereas, in case of filled soil, piling may be required.

·        Ecology and pollution:

                                     Generally, there is a great deal of awareness towards maintenance of ecological balance. Also, social obligations are to be met regarding the effect of pollution from specific type of plant. The site selected should be preferably having some advantages to meet these requirements. 

2.     QUALITATIVE ANALYSIS :

                                     The above factors are to be considered at the time of site selection, and the data collected is analysed, generally by the weight cum rating methods in order to finalise recommendations of the potential sites.

                                     In the weight cum rating method variable weight is assigned to each factor. Each site is evaluated on a 0-5 scale for cash of these factors. The assignment of points for each site for each factor is obtained by multiplying the rating of the site by the weight of each factor. The overall site rating is then obtained as the sum of the assigned points for each site. The recommendation will be for the site having maximum overall rating.

  

3.     PLANT LAYOUT :

                                     After the process flow diagrams are completed and before detailed piping, structural or electrical design can begin, the plant layout and the equipment within the process unit must be planned. This layout can play an important role in determining construction and manufacturing cost and hence, must be planned carefully with attention to potential future problems that may arise. Since each plant is a different entity and no two plant sites are exactly alike, there is no ideal plant layout. However, proper layout in each case will include arrangement of processing areas, storage areas and handling areas in efficient coordination in addition to the following factors:-

o   New site development or addition to previously developed site.

o   Type and quantity of product to be produced.

o   Type of process and product controlled.

o   Operational convenience and accessibility.

o   Economic distribution of utilities and services.

o   Type of buildings and building code requirements.

o   Health and safety considerations.

o   Waste disposal requirements.

o   Basic equipment needed.

o   Space available and space required.

o   Proximity to roads and rails.

o   Possible future expansion.

4.     PREPARATION OF THE LAYOUT:

                                     Scale drawings, complete with elevation indication can be used for determining the best location for equipment and facilities. Elementary layouts are developed first. These show the fundamental relationship between storage space and operating equipment. The next set requires consideration of the safe operational sequence and gives a primary layout based on the flow of materials, unit operations, storage and future expansion. By analysing all the factors that are involved in the plant layout, a detailed recommendation can be presented, and drawings and elevations, including isometric drawings of the piping systems, can be prepared.
                                    Templates, cut outs to a selected scale are useful in making rapid and accurate 3D models. The use of such models for making certain proposed plant layout is optimum, and has found increasing favours in recent years.

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WHAT ARE HAZARDS ? HAZARD ANALYSIS , HAZOP STUDY & HAZOP PROCEDURE

HAZARDS

Hazards are everywhere, Unfortunately a hazard is not always identified until an accident occurs. It is essential to identify the hazards and reduce the risk well in advance  of an accident.

For each process in a chemical plant  the following questions must be asked.

     1.    WHAT ARE THE HAZARDS ?

     2.    WHAT CAN GO WRONG AND HOW ?

     3.    WHAT ARE THE CHANCES ?

     4.    WHAT ARE THE CONSEQUENCES ?

The first question represents hazard identification. The last 3 questions are associated with risk assessment. Risk assessment includes a determination of the events that  can produce an accident  , the probability of those events , and the consequences . The consequences could include human injury or loss of life, damage to the environment or loss of production or damage to the equipment which ultimately leads to loss of huge capital investment.

HAZARD ANALYSIS

Hazard Analysis is nothing but the risk assessment. A risk assessment procedure that determines probabilities is frequently called Probabilistic Risk Assessment(PRA), whereas a procedure that determines probabilities and consequences is called Quantitative Risk Analysis(QRA). Hazard Analysis can be performed at any stage during the initial design or ongoing operation of a process.

HAZOP STUDY

The HAZOP study is a formal procedure to identify hazards in a chemical process facility. The procedure is effective in identifying hazards and is well accepted by the chemical industry.

The basic idea is to let the mind go free in a controlled fashion in order to consider all the possible ways that process and operational failures can occur.

Before the HAZOP study is started detailed information on the process must be available. This include up-to-date PROCESS FLOW DIAGRAM (PFD), PROCESS AND INSTRUMENTATION DIAGRAM (P&ID), detailed equipment specifications, material of construction, and mass and energy balances.

A large process might take several months of biweekly meetings to complete the HAZOP study. Obviously, a complete HAZOP study requires a large investment in time and effort, but the value of the result is well worth the effort.

The HAZOP procedure uses the following steps to complete an analysis :

1.    Begin with a detailed flowsheet. 

2.  Break the flowsheet into a number of process units (means the reactor area might be one unit and the storage tank another). 

3.    Select a unit for study. 

4.    Choose a study node (vessel, line, operating instructions). 

5.    Describe the design intent of the study node (for example, a vessel V-1 is designed to store the benzene feedstock and provide it on the demand to the reactor). 

6. Pick a process parameter (flow, level, temperature, pressure, concentration, etc). 

7. Apply a guide word to the process parameter to suggest possible deviations (for example, NO, HIGHER, LOWER, WHERE ELSE, OTHER THAN, SOONER THAN, LATER THAN, etc). 

8. If the deviation is applicable , determine possible causes and note any protective systems. 

9.    Evaluate the consequences of the deviation. 

10.  Recommend action. 

11.   Record all the information. 

12. Repeat steps 7 to 11 untill all the applicable guide words have been applied to the choosen parameter. 

13. Repeat steps 6 to 12 untill all the applicable process parameter has been considered. 

14.  Repeat steps 4 to 13 untill all the study nodes have been considered.

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