HEAT EXCHANGERS
Introduction
Heat Transfer:
Energy in the form of heat is an essential part of oil and gas processing. Efficient operation may depend on transferring heat from one place to another. Heat energy can be transferred between fluids (gases and liquids) whenever their temperatures are different. The direction of transfer is always from the hotter fluid to the colder fluid. There are basically four benefits for transferring heat in the petroleum industry.
- To increase process efficiency
- To reduce maintenance
- To conserve energy
- To provide employee safety.
Main Factors Affecting Heat Transfer:
Transfer of heat between fluids is affected by the following factors:
Type of Material: Some materials transfer heat better than others, e.g., metals are better conductors than non-metals.
Thickness of Material: The thinner a material, the faster heat will transfer.
Surface Area: The greater the contact surface area, the more heat transfer takes place.
Type of Fluids: Some fluids transfer heat better than others, e.g., liquids are better than gases.
Gravity of Fluids: The thinner a fluid, the faster heat will transfer.
Flow Rate of Fluids: The slower the flow rate, the more heat transfer takes place.
Turbulence of Fluids: The greater the turbulence, the better heat will transfer.
Temperature Difference: The greater the difference in fluid temperature, the faster heat will transfer.
Contamination: Contaminants in a fluid or on the walls of a heat exchanger will reduce heat transfer.
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Functions of Heat Transfer Equipment:
The term “heat exchanger” is commonly used to describe different types of equipment used in heat transfer service. Functions of heat transfer equipment common to the petroleum industry are:
Cooler: Coolers reduce the temperature of liquid or gas using water or air to remove heat.
Heater: Heaters increase the temperature of a liquid or gas by adding heat using condensing steam, hot oil, etc.
Condenser: Condensers remove heat from gas, changing it to a liquid.
Vaporizer: Vaporizers add heat to a liquid, changing it to a gas.
Re-boiler: Reboilers provide heat to a liquid in the bottom of a distillation tower. The heat may be supplied by either steam or a hot process stream.
Chiller: Chillers cool a liquid or gas using a refrigerant instead of water.
Exchanger: Exchangers perform basically two functions. They can heat a cold process stream by using a hot process fluid, or they can cool a hot process stream by using a cold process fluid.
Types of Heat Exchangers:
There are many different types of heat exchangers. All heat exchangers are used to transfer heat from one fluid to another. Heat is transferred through the walls of tubes, pipes, or plates within the heat exchanger, keeping the two fluids from coming in direct contact with each other. The most common types found in the petroleum industry are: shell and tube, double pipe, plate, and aerial coolers.
Shell and Tube:
Shell and tube heat exchangers are some of the most widely used in the petroleum industry. These heat exchangers consist of a bundle of tubes encased in a larger “pipe” or shell.
Double Pipe:
Double pipe heat exchangers consist of a smaller diameter pipe inserted inside a larger diameter pipe. These heat exchangers are well suited for high pressure applications.
Plate:
Plate heat exchangers consist of a series of plates separated by gaskets. These heat exchangers are versatile because the contact surface area can be changed by adding or removing plates. However, their service may be limited by pressure ratings.
Aerial Cooler:
Aerial coolers consist of a series of tubes containing a hot process fluid exposed to a stream of air moving across the tubes. A motor-driven fan is used to create the air movement. Aerial cooler usage has increased in recent years due to increased costs of water and concerns over water pollution.
- Heat Exchanger Operation:
Heat transfer in heat exchangers depends on the difference in temperature between the two fluids. The greater the temperature difference, the more heat transferred. Two other factors that affect heat transfer are: flow path and tube arrangement. These two factors determine the length of time, amount of contact surface area and turbulence between the two fluids. The longer the two fluids are in contact and the greater the area of contact, the more heat will be transferred.
Flow Path:
Fluids flowing through a heat exchanger can take one or a combination of these paths: parallel flow, counter flow, or cross-flow.
Parallel Flow:
In parallel flow, fluid flowing inside the tubes flows in the same direction as the fluid flowing outside the tubes. This flow pattern yields the least amount of heat transfer because it does not maintain a high temperature difference between the fluids. Suppose the hotter fluid is flowing inside the tubes and the colder fluid outside the tubes. At the inlets the temperature difference is the greatest, but at the outlets the colder fluid has absorbed enough heat from the hotter fluid that the temperature difference is relatively small. Therefore, heat transfer at the outlets drops off considerably.
Counter flow:
In counter flow, sometimes called reverse flow, fluid inside the tubes flows in one direction while the fluid outside the tubes flows in the opposite direction. This flow pattern yields the most heat transfer because temperature difference remains relatively high all the way through the heat exchanger.
Suppose the hotter fluid is flowing inside the tubes and the colder fluid outside the tubes. Although the colder fluid picks up heat along its path, it will exit the heat exchanger at the point where the hotter fluid is entering at its highest temperature. At the point where the hotter fluid has been cooled and is exiting the heat exchanger, the colder fluid is entering at its lowest temperature. Therefore, the temperature difference between the fluids remains higher throughout the heat exchanger.
Cross-Flow:
In cross-flow, fluid outside the tubes flows at right angles to fluid inside the tubes. This flow pattern creates more turbulence in the fluid outside the tubes which increases the amount of heat transfer. Cross-flow is commonly used in conjunction with parallel flow and/or counter flow fluid paths.
Tube Arrangement:
Heat exchangers are built so fluids will have one pass, two passes, or multiple passes through the exchanger, depending on the arrangement of the tubes. They may also have a combination of flow paths.
The tubes in heat exchangers have either straight tube arrangements or U-tube arrangements. In straight tube heat exchangers, fluid enters one end of the tubes, flows straight through and exits the other end. This is called a single pass heat exchanger
In U-tube heat exchangers, fluid enters one end of the tubes and flows to the other end. However, instead of exiting, the tubes bend back in the shape of a U. Fluid flows around the bend and back to the first end, then exits the heat exchanger. This is called a double pass heat exchanger
Heat exchangers can also be designed for multiple passes. Tubes are built to change the direction of flow through the heat exchanger several times before the fluid exits. The more passes between the fluids, the more heat can be transferred.
Double Pipe:
The simplest heat exchanger is the double pipe. It consists of one pipe inserted within another pipe and then secured at each end by either welding or with a packed stuffing box. One fluid flows inside the inner pipe while the other fluid flows inside the outer pipe. These fluids may flow in the same direction (parallel flow) or in opposite directions (counter flow).
Double pipe heat exchangers are commonly used for counter flow service but can be used for parallel flow equally as well. They can have straight tube arrangements as in heat exchangers for heater transfers or U-tube arrangements as in double pipe coils used for gas plant applications.
These heat exchangers are well suited for high pressure service because their small diameters and cylindrical shape require minimum wall thickness. Some applications may require fins on the outside wall of the inner pipe to increase contact surface area.
Plate:
Plate heat exchangers are made up of a series of plates separated by gaskets. The plates provide space between them for fluids to flow. The gaskets are set in channels around each plate which directs and contains fluid flow. Ports for inlet and outlet of both fluids are stamped into the corners of each plate. When aligned these ports form four distribution headers through the plates. Hot process fluid flows through every other plate, and cold process fluid flows through the other plates. Distribution of hot and cold fluids to alternating plates is achieved by the gaskets around each port. If the fluid is to flow through a plate, the gasket is removed from around the port. If the fluid is to bypass a plate and flow to the next plate, the gasket is left intact. It is very important for the gaskets to be made of a material that is compatible with the process fluids or leaks may develop between plates.
Shell and Tube:
Shell and tube heat exchangers consist of a bundle of heat transfer tubes encased in a larger “pipe” or shell. The main parts of these heat exchangers are: the shell, tubes, tube sheets, baffles, and heads. All components are usually constructed of steel; however, corrosion-resistant alloys are used for certain applications.
Shell:
The shell is the outer case which contains the tube bundle, tube sheets, and baffles. The space between the shell and tubes contains the shell-side fluid.
Tubes:
Tubes are small diameter pipes located inside the shell and attached to the tube sheets. They are arranged in either square or triangular patterns. The triangular pattern provides more turbulence and better heat transfer on the outside of the tubes; however, the square pattern provides clear lanes for cleaning the outside surfaces of the tubes. Process fluid flowing through the tubes is called the tube-side fluid.
Tube sheets:
Tube sheets are located at both ends of the heat exchanger. They secure the tubes and separate shell-side and tube-side fluids. Tube sheets can be fastened to the shell (fixed) or not fastened to the shell (floating) which allows the tube bundle to be removed for cleaning.
Aerial Coolers:
Aerial coolers are heat exchangers composed of a series of tubes, containing a hot process fluid exposed to air moving across the tubes. The tubes usually have fins on their outer walls to increase contact surface area. A chamber, called the plenum, directs air across the tubes by means of a motor-driven fan. The movement of air across the tubes is referred to as draft. There are two types of draft: forced draft and induced draft.
Forced-draft aerial coolers have the fan mounted so that air is blown or pushed across the tubes. The tubes are designed for at least two passes of the hot fluid through the tubes before leaving the heat exchanger. Many draft coolers have six to eight passes.
Induced-draft aerial coolers have the fan mounted to draw or pull the air across the tubes instead of pushing it. For certain service, induced-draft is more desirable than forced-draft because it reduces the chance for warm exhaust air being drawn across the tubes.
Typical applications for aerial coolers include: removing heat from compressor cooling water, cooling compressor discharge or inter-stage cooling, and cooling compressor lubricating oil.
Self Review
- Which are the Main Factors Affecting Heat Transfer:
- . . . Functions of heat transfer equipment common to the petroleum industry are:
Cooler: ———————————————————————————————————————————————.
Heater: ———————————————————————————————————————————————-
Condenser: ———————————————————————-.
Vaporizer: ———————————————————————-.
Reboiler: —————————————————————————————————————————————————————————————————————————————————————————————.
Chiller: ———————————————————————–
————————————————————————
Exchanger: ————————————————————————————————————————————————————————————————————————————————-
3.Name the types of heat exchangers.
- Fluids flowing through a heat exchanger can take one or a combination of these paths: ————-, ——————-, or ———–
- Write the parts of a shell and tube heat exchanger