The Growing Role of Heat Exchangers

Heat Exchangers Based on Their Working Mechanism

Stratview Research 16-05-2023

Have you ever been travelling down the road and noticed smoke rising from a smokestack? The smoke is wasted energy that could be put to better use.

By the means of Combined Heat and Power (CHP), also called Co-generation, heat escaping from smokestacks can be turned into electricity and passed to grids and can be used for a variety of commercial, non-commercial, or industrial purposes.

More than 70% of U.S. greenhouse gas emissions come from burning fossil fuels for electricity, heat, and transportation, U.S. Energy Information Administration (EIA) states. Combined Heat and Power's potential is remarkable, as it can help replacing nearly 30% of power generated by burning fossil fuels, reduce CO2 emissions by 20%, and can also save billions of expenditures, according to the experts.

How CHPs work? Heat exchangers, here play a vital role.

Gasses exiting from the exhaust are passed through the heat exchanger’s central tube and cold water is circulated around the outer shell, travelling over and around the tube core, the exhaust gases are cooled and much of the heat in the gases is transferred to the water circuit to convert it into energy.

The heat exchangers are one of the common yet vital elements of power, utility, and technological installations. They are used in almost all the industries like Oil and Gas, Petrochemicals & Chemicals, Pharmaceuticals, etc.

For instance –

  • Refrigerators use heat exchangers to throw the heat from inside the fridge to the room it is in.
  • Overheating engine can cease any vehicle, so vehicles use heat exchangers and throw waste heat to the atmosphere. For instance, a car radiator is a type of heat exchanger.
  • In a food and beverage processing plants, heat exchangers are used to reduce/eliminate microbials to prevent spoilage and making items safe and healthy for consumption.
  • As oil and gas industries, almost all processes have heating, cooling, or condensation process that require heat exchangers.
  • Heat Exchangers are used in chemical and pharma industries for purposes like heating or cooling a solution, manufacturing pharmaceuticals, and to produce steam, etc.

 

Petrochemical & chemical industries generate the nearly one-fourth of the demand for heat exchangers globally. Global liquid fuels production is estimated to increase by 1.1 million barrels per day (b/d) in 2023.

Refer to fig.1. depicting the forecast of world production and consumption of liquid fuels.

 

FIG.1. The forecast of world production and consumption of liquid fuels.

 

How Heat Exchangers Work

Heat exchangers, regardless of their type and design, exchange heat using the principles of thermal heat conduction, namely Zeroth, First, and Second laws of thermodynamics. These laws describe and dictate the transference of heat from one fluid to other.

 

How exactly do they work? Now assume a situation where you need to heat a container full of oil but putting it directly over the flame can be hazardous. To solve this, a heat exchanger can be used. Fig.2. depicts the working of a heat exchanger in this case.

Fig.2. Working of a heat exchanger.

If we boil some water and run it using a metal rod/pipe through a simple heat exchanger, the oil will safely absorb the heat of water through the rod of the heat exchanger. The same process applies to other heat exchangers that are used in different machineries for thermal energy exchange.

Despite the fact that all heat exchangers serve the same function, there are many different types with different applications. Let's have a look at different types of heat exchangers used in various industries across the globe.

Classification of Heat Exchangers Based on their Design Characteristics

The main attributes by which heat exchangers can be categorized are:

1. Based on the number of fluids - The heat exchanger may incorporate two, three, or more than three heat transfer fluids in the system.

 

  • Be it heating, cooling, heat recovery, or heat rejection, every process involves transfer of heat between two fluids. Hence, two -fluid heat exchangers are the common ones.
  • Three-fluid heat exchangers are widely used in chemical processes such as air separation systems, a helium–air separation unit, purification and liquefaction of hydrogen, etc.
  • Heat exchangers with multifluid streams are used in some of the chemical process applications. The design theory of these type of heat exchangers is said to be very complex.

 

2. According to Flow Configuration - The heat exchanger can be single or multiple pass. It can include crossflow, counterflow, or co-current flow. Heat exchangers often use a combination of these flow patterns to maximise thermal efficiency.

  • In single pass heat exchangers, the fluid goes in one end of each tube and out from the other once in the system.
  • Fluids in multi-pass heat exchangers are those which allow fluids to pass through the heat exchangers multiple times before exiting. This flow produces a thermally longer heat exchanger while preserving the same smaller footprint.

Fig.3.1. Illustrates the flow in single and multi-pass heat exchanger.

 

- Fluids flow in opposite directions towards each other in counter flow heat exchangers.

- Fluids flow perpendicular to each other in cross flow heat exchangers.

- Fluids flow parallel to each other in co-current flow heat exchangers.

Fig.3.2. illustrates the flow in cross, counter, and co-current flow heat exchangers.

 

  1. Types of Heat exchangers depending on their working mechanism.

There are various heat exchanger versions available based on the features mentioned above. Some of the popular variations used in industry by working mechanism include:

  • Shell and tube heat exchangers
  • Double pipe heat exchangers
  • Plate heat exchangers
     

Shell and tube heat exchangers

 

Double pipe heat exchangers

 

Plate heat exchangers

The structure of the shell-and-tube heat exchanger is best in terms of strength, but not ideal from the perspective of heat exchange as the fluid flows through the bypass and does not fully participate in the heat exchange.

Also called hairpin heat exchangers, these heat exchangers can be used under severe fouling conditions because of the ease of cleaning and maintenance. The outer surface of the inner tube can be finned, and then, the tube can be placed concentrically inside a large pipe.

There is no bypass in the plate heat exchanger, and the corrugation of the plates can cause the fluid to produce turbulence at a smaller flow rate. Therefore, the plate heat exchanger has a higher heat transfer efficiency.

Ideal solution for hydraulic power packs, Oil and gas machinery, sea water cooled vessels and swimming pool heating, etc.

This is suited for high pressure applications because of their smaller diameter.

They can be used in the hot-water sections of millions of combination boilers.

Such as oil, gas & chemical industries.

Restricted to solutions with low solids contents and viscosities under 10,000 cPs.

N/A

Liquids with less than 5% of nonabrasive solids, generally micron size range. Viscosities less than 20,000 cPs.

Temperature (oF): >1,000

Pressure (psig): >1,500

Flow Rate (Nominal/Maximum): No Limit

Temperature (oF): >1,000

Pressure (psig): >1,000

Flow Rate (Nominal/Maximum): No Limit

Temperature (oF): 300 to 450*

Pressure (psig): 150 to 300

Flow Rate (Nominal/Maximum): >500 gal/min

The thickness of the heat exchange tube of the shell-and-tube heat exchanger is 2.0-2.5mm.

In this type, the inner tube of inner diameter is ~1.5 cm and outer diameter is ~1.9 cm

The plate heat exchanger is just 0.5mm.

Plate exchanger is the most efficient heat exchanger type, due to turbulent flow on both sides. High heat-transfer coefficient and high turbulence due to even flow distribution are always kept in mind by the business owners while deciding on the best heat exchanger. These heat exchangers can also increase their cooling capacity, unlike other types.

Increasing Role of Heat Exchangers with Rising Emissions

With the rising CO2 emissions, the demand for heat exchangers is also shooting up.
Global fossil CO2 emissions were 5.1% higher in 2021 than in 2020, and Earth System Science Data (ESSD) states that global fossil CO2 emissions were expected to rise by 1.0% in 2022 to reach 10.0 GtC (36.6 GtCO2). If current demography, emission-intensity continues, it is expected that we could spew >60 GT carbon by 2030.

 

To recover and achieve the target of net zero by 2050, the emissions rate must reduce at a faster pace. Heavy investments in development of green technologies in countries like China, India, and Japan, etc. depicts the rising awareness in the sustainability formula.

Heat exchangers play a vital role in reducing CO2 emissions. They are counted as green energy, as they use less energy, can also provide energy, and generate no pollution. To improve the efficiency, the need of compact heat exchangers is growing rapidly, as they reduce footprint as well as CO2 emissions.

Compact heat exchangers, according to Alfa Laval, one of the major players in the heat exchangers industry, can help plants consume less energy, reduce CO2 emissions, and increase uptime. This group has also signed a contract with SSAB, to develop the world's first plate heat exchanger to be made using fossil-free steel, which aims to have the first unit made with hydrogen-reduced steel ready for 2023.

Increased focus on efficient thermal management across all industries, meeting growing energy demand, etc. are some more major factors that push the demand for heat exchangers. The market of global heat exchangers is estimated to grow at a healthy annual growth rate of more than 5% to cross US$18 Billion by 2027.

Though the rising prices of raw materials like stainless steel, aluminium, etc. is a big challenge that the heat exchangers industry has witnessed, but with the rising demand for energy efficiency, growing industrialization, etc., the role of heat exchanger will only increase. At the same time, continuous developments and innovations in the segment will make machineries environmentally friendly.

 

Authored by Stratview Research, and originally published on Stainless Steel World Americas.


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