Calculation of air cooling apparatus

Introduction

The processes of oil and gas processing have undergone both qualitative and quantitative changes in their development, resulting from the tasks of the development of our country's economy. Currently, combined processes are widely used in the oil and gas processing and petrochemical industries, which are characterized by the use of multifunctional devices with simultaneous reaction stages, heat and mass transfer. Particularly relevant is the use of multifunctional devices in small-sized low-tonnage hydrocarbon processing plants to bring the quality indicators of target products to the requirements of standards.

To increase the depth of oil refining, it is necessary to increase the share of secondary processes, develop and implement more efficient catalysts and progressive equipment. [9, c.6]

A significant expansion of the range of petroleum products and a further increase in their quality requirements due to the intensive development of technology necessitated the use of a wide range of chemical technology processes in oil and gas processing; such processes as rectification, absorption, extraction, adsorption, drying, settling, filtration, centrifugation, etc., as well as various chemical and catalytic processes: pyrolysis, catalytic cracking, reforming, hydrotreating and others. This made it possible to orient oil and gas processing to provide all industries not only with fuel, oils and other commercial products, but also cheap raw materials for chemical and petrochemical industries producing various synthetic products: plastic masses, synthetic rubbers, chemical fibers, alcohols, synthetic oils and others. [25]

The implementation of such a variety of processes in oil and gas processing required the use of equipment operating in a wide range of changes in operating parameters.

In the world market of chemical products, three main centers of its development are distinguished: the USA, Western Europe and Japan. All of these centres have significant chemical company strategies. The United States of America is the unqualified and sole leader in the chemical industry. The United States has all the chemical raw materials necessary for development. The world's most modern chemical equipment fleet is located there. [22]

This imposes high requirements for calculating the equipment and justifying the operating parameters of the process.

Theoretical justification

One of the main types of technological equipment in oil refining, petrochemical, chemical and related industries is heat exchange equipment, which accounts for about 3040% by weight of all equipment. A significant share of all heat exchangers is condensation and refrigeration equipment designed for condensation of vapors and cooling of liquid products of technological processes.

Currently, air-cooled condensers and refrigerators using atmospheric air as a cooling agent are increasingly being used in the oil refining and petrochemical industries.

The cooling process is constructed as follows: water is first heated in the jackets of fuel units, then supplied to the radiators of the apparatus. Water is cooled by fans and street air. At the end, it is cooled to the required parameters, and only then it is supplied to the consumer.

Air cooling devices are popular in oil refineries and petrochemical enterprises, their use is especially effective where there is a shortage of cooling water. The devices can be operated using air - this refrigerant has no cost. [30]

Air cooling unit (air cooling unit) is designed for cooling or condensation of process flows of gas and condensate. It is operated at open process sites in areas with a temperate or cold climate. Process flow temperature from − 40 to 300 ° С, pressure up to 7.5 MPa. Heat exchange pipes are made from 1.5 to 8 m long with finning in the form of rolled mono or bimetallic tape and are arranged in a section. The finning ratio (ratio of the total surface of the finned tube to the outer surface of the tube to the diameter of the base of the fins) is 9 or 14.6. The power of the installed electric motors is 3-100 kW, which provides airflow speeds of 5-15 m/s in narrow sections of sections. Number of strokes in pipe space from 1 to 8. [21]

Air cooling devices are versatile and economical, due to which they have a wide field of application. ABO work in ammonia synthesis, hydrocarbon reforming and cracking plants, in the production of organochlorine products, methanol and many others.

The correct choice of the size and type of each apparatus, as well as its competent installation and rational operation, significantly affects the sum of the initial and subsequent costs for the construction of the installations. Special attention should be paid to the conditions and method of temperature adjustment of process flows. [26]

The main feature of air cooling devices is the use of finned pipes with an extremely developed heat exchange surface.

Thus, during the operation of AVO, excess heat is sent directly to the atmosphere, and low thermal hydraulic properties of air - low density, low thermal conductivity and low speed - determine small values ​ ​ of heat transfer coefficients from the air side, then a heat exchange surface with a high ratio of the area in contact with air to the area washed by another heat carrier is used. The most modern methods of finning heat exchange pipes make it possible to achieve a ratio of the area of ​ ​ the external (finned) surface of the pipe to the area of ​ ​ its internal (smooth) surface in the range of 15-25 times.

According to their operational characteristics, the proposed AVO corresponds to the best foreign samples.

Data

Air cooling devices are easy to operate, cleaning and repairing them do not require large labor costs. Contamination of the external surface of heat exchange with a refrigerant (inlet) is practically absent even under conditions of blowing them with dusty air and with a significant degree of finning of heat exchange pipes. There is also no corrosion of the outer surface characteristic of all condensers and refrigerators, which allows you to quite easily choose the material of pipes for air condensers. Therefore, it becomes possible to use the required inhibitors (ammonia water and others), which reduce corrosion not only of the condensers themselves, but also of the technologically connected devices.

Operation of air condensers, which do not use water as a refrigerant, eliminates the danger of accidents as a result of sudden termination of its supply to the plant. In case of power outage, these devices provide removal of 2530% of the heat due to natural air convection, which reduces the likelihood of a fire for the time required for emergency shutdown of the plant.

In the northern regions of the country, the use of air condensers allows you to reliably and economically cool all technological flows leaving the plant. In the southern regions, cooling of low-boiling streams is advisable in two stages: air to 60℃ and then water in submerged or shell-and-tube condensers. In foreign practice cases when operation of air coolers is justified at a gradient of temperatures between the cooled environment and air 1015 ℃ are known. [11]

Due to the variable nature of the apparatus load, depending on the process mode, temperature and humidity of the air, fans should be able to control the air flow in a wide range.

The control system shall provide the requirements of the technology regardless of the change of the fan operation mode. Air flow control is performed in several ways:

1) changing the flow rate of cooling air supplied to the heat exchange sections;

2) air heating (in winter) at the entrance to the AVO;

H) restart-up of a part of a process stream on bypass lines via control valves;

4) humidification of cooling air and heat exchange surface, which allows to reduce temperature of cooling air at its high temperature in summer period.

The most common control method is to change the flow rate of cooling air, which is carried out:

- by using two-speed motors, which allows to have two local values of air flow and the third - the minimum value with the fan stopped (in winter at low ambient temperature the device can work with the blower turned off, while the product is cooled due to natural convection). This method is the most practical and economical;

- by smooth control of fan rotation speed using electric motor with variable number of revolutions, hydraulic drive, hydraulic couplings, variator, speed box, etc.;

- by adjusting the rotation angle of the fan blade. The angle is changed manually when the fan is stopped by reinstalling each blade separately or automatically when using a pneumatic or electromechanical drive. Stepwise change of blade turn angle with fan stop is provided for seasonal adjustment. Automatic control allows to maintain the output temperature of the cooled product to within ± 1 wasps;

- installation of special louver devices located both before the fan and after the heat exchange sections. Louvers may be provided with a manual or pneumatic actuator. When turning the louver elements, the air flow rate decreases and the range of operating modes increases, but this adjustment is accompanied by a decrease in fan efficiency. [16]

The use of ABO contributes to maintaining the purity of rivers and reservoirs, as well as saving alloyed expensive steels, which are required for corrosion protection from cooling water. [22]

Everyone knows that air is not an aggressive environment; it is not costly; its stocks are not limited; it is a pure cooled gaseous medium which does not require treatment and special storage conditions. [17]

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