Selecting optimal heat exchangers

Due to the many variables involved, selecting optimal heat exchangers is challenging. Hand calculations are possible, but many iterations¹ are typically needed. As such, heat exchangers are most often selected via computer programs, either by system designers², who are typically engineers, or by equipment vendors.

To select an appropriate heat exchanger, the system designers (or equipment vendors³) would firstly consider the design limitations for each heat exchanger type. Though cost is often the primary criterion, several other selection criteria are important:

· High/low pressure limits

· Thermal performance

· Temperature ranges⁴

· Product mix (liquid/liquid, particulates or high-solids liquid)

· Pressure drops⁵ across the exchanger

· Fluid flow capacity⁶

· Cleanability⁷, maintenance⁸ and repair⁹

· Materials required for construction

· Ability and ease of future expansion

Choosing the right heat exchanger (HX) requires some knowledge of the different heat exchanger types, as well as the environment where the unit¹⁰must operate. Typically in the manufacturing industry, several differing types of heat exchangers are used for just the one process or system to derive the final product. For example, a kettle HX for pre-heating, a double pipe HX for the ‘carrier’ fluid and a plate and frame HX for final cooling. With sufficient knowledge of heat exchanger types and operating requirements, an appropriate selection can be made to optimize the process.

1. Iteration– итерация, повторение, шаг

2. System designer - системотехник

3. Vendor – фирма-поставщик

4. Range - диапазон

5. Pressure drop – падение давления

6. Flow capacity– пропускная способность

7. Cleanability- очищаемость

8. Maintenance- техобслуживание

9. Repair - ремонт

10. Unit– установка

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Cooling towers

Cooling towers¹ are heat removal devices used to transfer process waste heat² to the atmosphere. Cooling towers may either use the evaporation³ of water to remove process heat and cool the working fluid to near the wet-bulb air temperature⁴ or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature⁵.

Common applications include cooling the circulating water used in oil refineries⁶, petrochemical⁷ and other chemical plants, thermal power stations⁸and HVAC⁹ systems for cooling buildings.

Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres tall and 100 metres in diameter, or rectangular¹⁰structures (as in Image 3) that can be over 40 metres tall and 80 metres long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning. A hyperboloid cooling tower was patented by Frederik van Iterson and Gerard Kuypers in 1918.^[1] The first hyperboloid cooling towers were built prior to 1930 in Liverpool, England to cool water used at an electrical power station that used coal.

1. Cooling tower – градирня, охладитель

2. Waste heat – отработанное тепло

3. Evaporation - испарение

Wet-bulb air temperature

Dry-bulb air temperature

6. Oil refinery – нефтеперерабатывающий завод

7. Petrochemical- нефтехимический

8. Thermal power station - теплоэлектростанция

9. HVAC - heating, ventilating and air conditioning - тепловентиляционный

10. Rectangular – прямоугольный

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HVAC

An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a chiller¹. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat ejection to tower water at or near wet-bulb temperatures². Air-cooled chillers must reject heat at the higher dry-bulb temperature³,. Large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.

Cooling towers are also used in HVAC systems that have multiple water source heat pumps⁴ that share a common piping water loop. In this type of system, the water circulating inside the water loop⁵ removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode⁶, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the atmosphere. By contrast, when the heat pumps are working in heating mode⁷, the condensers draw heat out of the loop water and reject it into the space to be heated. When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized⁸ to prevent freeze damage), and heat is supplied by other means, usually from separate boilers⁹.

1. Chiller- oхладитель

Wet bulb temperature

Dry-bulb temperature

4. Heat pump – тепловой насос

5. Water loop – водяная петля

6. Cooling mode – режим охлаждения

7. Heating mode – режим нагрева

8. Winterize -подготовить к зимней эксплуатации

9. Boiler – паровой котел

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Industrial cooling towers

Industrial cooling towers¹ can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants², petroleum refineries³,petrochemical plants⁴, natural gas processing plants, food processing plants⁵, semi-conductor plants⁶, and for other industrial facilities⁷ such as in condensers of distillation columns, for cooling liquid in crystallization, etc.^[3] The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute)^[4] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).

If that same plant had no cooling tower and used once-through cooling⁸ water, it would require about 100,000 cubic metres an hour^[5] and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. Elevated water temperatures can kill fish and other aquatic organisms (see thermal pollution). A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water. Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.

1. Industrial cooling tower – башенный охладитель

2. Power plant - электростанция

3. Petroleum refinery - НПЗ

4. Petrochemical plant – нефтехимическая установка

5. Processing plant – технологическая установка

6. Semi-conductor plant – полупроводниковая установка

7. Industrial facilities – промышленное оборудование

Once-through cooling

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Reboiler

Reboilers¹ are heat exchangers typically used to provide heat to the bottom of industrial distillation columns².They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation³.

Proper reboiler operation is vital to effective distillation. In a typical classical distillation column, all the vapor driving the separation comes from the reboiler. The reboiler receives a liquid stream from the column bottom and may partially or completely vaporize that stream. Steam usually provides the heat required for the vaporization. [

The most critical element of reboiler design is the selection of the proper type of reboiler for a specific service. Most reboilers are of the shell and tube heat exchanger⁴type and normally steam is used as the heat source in such reboilers. However, other heat transfer⁵fluids like hot oil or Dowtherm (TM) may be used. Fuel-fired furnaces⁶ may also be used as reboilers in some cases.

Commonly used heat exchanger type reboilers are:

Kettle reboilers

Kettle reboilers (Image 1) are very simple and reliable. They may require pumping of the column bottoms liquid into the kettle, or there may be sufficient liquid head⁷ to deliver the liquid into the reboiler. In this reboiler type, steam flows through the tube bundle⁸ and exits as condensate. The liquid from the bottom of the tower, commonly called the bottoms, flows through the shell side. There is a retaining wall or overflow weir⁹ separating the tube bundle from the reboiler section where the residual reboiled liquid (called the bottoms product¹⁰) is withdrawn, so that the tube bundle is kept covered with liquid.

1. Reboiler - ребойлер

2. Distillation column – ректификационная колонна, перегонный аппарат

3. Separation - разделение

4. Shell and tube heat exchanger – кожухо-трубчатый теплообменник

5. Heat transfer - теплопередача

6. Furnace -печь

7. Head - напор

8. Overflow weir - водослив

9. Tube bundle – трубный пучок

10. Bottoms product – недогон

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Thermosyphon reboilers

Thermosyphon reboilers (Image 2) do not require pumping of the column bottoms liquid into the reboiler. Natural circulation is obtained by using the density¹difference between the reboiler inlet column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid head² to deliver the tower bottoms into the reboiler. Thermosyphon reboilers (also known as calandrias³) are more complex than kettle reboilers and require more attention from the plant operators. There are many types of thermosyphon reboilers including vertical, horizontal, once-through⁴ or recirculating.

Some fluids are temperature sensitive such as those subject to polymerization by contact with high temperature heat transfer tube walls. High liquid recirculation rates are used to reduce tube wall temperatures, thereby reducing polymerization on the tube and associated foulng⁵.

Fired reboiler

Fired heaters⁶(Image 3) also known as furnaces⁷may be used as a distillation column reboiler. A pump is required to circulate the column bottoms through the heat transfer tubes in the furnace's convection and radiant sections. The heat source for the fired heater reboiler may be either fuel gas or fuel oil.