Heat exchangers

Fourier’s law is the fundamental differential equation for heat transfer by conduction.

Radiation:

Unlike conduction and convection, radiation does not need matter to transfer heat. Energy is radiated from the sun, through the vacuum of space at the speed of light. When this energy arrives at Earth, some of it is transferred to the gases in our atmosphere.

For the radiation, Stefan-Boltzmann constant has the value of 5,67*10^(-12)

Everything around us constantly emits radiation, and the emissivity represents the emission characteristics of those bodies. This means that every body, in- cluding our own, is constantly bombarded by radiation coming from all direc- tions over a range of wavelengths.

For opaque objects, there is no transmission.

A black body is an idealized physical body (object) that can absorb all the incident electromagnetic radiation. A grey body is a body (object) that emits radiation at each wavelength in a constant ratio less than unity to that emitted by a black body at the same temperatures.

Electromagnetic radiation is produced whenever a charged particle, such as an electron, changes its velocity — i.e., whenever it is accelerated or decelerated.

The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes — the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays.

Infrared (sometimes called infrared light and IR) is electromagnetic radiation (EMR) with wavelengths longer than those of visible light and shorter than radio waves. It is therefore invisible to the human eye.

The greenhouse effect :

This is the effect in the atmosphere that ensures that our Earth is habitable at all. Without the greenhouse effect it would be far too cold for us. It is based on the fact that certain molecules in the atmosphere can absorb infrared radiation (e.g. CO2), which they then emit again in an undirected manner, so that part of the radiation falls back to the earth and the corresponding heat is conserved.

The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating its energy into deep space as infrared radiation. The combustion gases such as CO2 and water vapor in the atmosphere transmit the bulk of the solar radiation but absorb the infrared radiation emitted by the surface of the earth. Thus, there is concern that the energy trapped on earth will eventually cause global warming and thus drastic changes in weather patterns.

You have probably noticed that when you leave your car under direct sunlight on a sunny day, the interior of the car gets much warmer than the air outside:

The answer lies in the spectral transmissivity curve of the glass, which resembles an in- verted U, as shown in Figure below. We observe from this figure that glass at thicknesses encountered in practice transmits over 90 percent of radiation in the visible range and is practically opaque (nontransparent) to radiation in the longer-wavelength infrared regions of the electromagnetic spectrum (roughly l-3 ). Therefore, glass has a transparent window in the wavelength range 0,3 and 3 in which over 90 percent of solar radiation is emitted. On the other hand, the entire radiation emitted by surfaces at room temperature falls in the infrared region. Consequently, glass allows the solar radiation to enter but does not allow the infrared radiation from the interior surfaces to escape

Film and dropwise condensation

Condensation occurs when the temperature of a vapor is reduced below its saturation temperature (dew point of the air). It commonly results from contact between the vapor and a cool surface.

As the film thickness or size of droplets increases, the condensate provides a resistance to heat transfer between the vapor and surface. Hence, most condensors use short vertical surfaces or tube bundles to prevent a huge increase of film thickness.

In conclusion, dropwise condensation is more efficientcompared to film condensation as the heat flux and heattransfer coefficient of dropwise condensation is higher compared to filmwise. (ten times more)

Coatings such as Teflons, Silicons helps prevent the film condensation. But, it has high maintenance costs. In order to prevent that, a falling film evaporator is highly recommend.

Dropwise condensation is achieved by adding a promoter chemical into the vapor, and/or roughened surfaces and surface and surface coated with hydrophobic impurities like fatty acids and organic compounds, known as dropwise promoters.

With increasing temperature difference between bulk vapor and surface, the heat transfer coefficient will be lessened in a dropwise condensation, while it increases in film condensation.

How to calculate the condensation rate:

Calculation of leakage in heat exchangers:

The hole area should be considered as

1. 20 mm2 (ID:5 mm) or
2. Two times of the cross section.

Estimation of blocked pipeline on the heat transfer:

If a number of pipes are blocked, the velocity of medium will be increased in the tube side, causing a higher Reynolds for the tube side. The overall heat transfer coefficient would be higher, but the heat transfer area will be lower. The increase in overall heat transfer coefficient is ignorable.

Example: If 10% of the pipes are blocked with debris, the capacity can be lessened by 10%.

h values in the shell and tube side:

Shell: Steam h=17800

Tube: Oil h=2400

Shell: Oil: h=500

Tube: Cooling water: h= 1000

Shell: Cooling medium h=2500

Tube: Oil h=2500

Shell: N2 h=70

Tube: cooling water h=70000

Shell: Cooling medium h=1000

Tube: N2 h=200

Determining Heat Exchanger Heat Transfer Coefficient

To do this one must sum all the resistances to heat transfer. The reciprocal of this sum is the heat transfer coefficient. For a heat exchanger the resistances are

Tubeside fouling

Shellside fouling

Tube metal wall

Tubeside film resistance Shellside film resistance

Table 1 give ballpark estimates of film resistance at reasonable design velocities:

Heat transfer coefficient calculation for a double pipe heat exchanger:

• The hydraulic diameter must be calculated for the outer pipeline, since the cross section is not circular like in the inner pipeline.

• Based on the mean temperature of medium, the physical properties must be estimated (kinematic/dynamic viscosity, heat capacity, thermal conductivity and density)
• The Prandtl Nr. must be calculated for each side.

• The cross section area for the outer side should be calculated cautiosly.
• The Reynolds Nr. must be calculated for each side. For the outer pipeline, the hydraulic diameter is to be used.
• Depending on the laminar, transient or turbulent flow regime, the Nusselt number can be calculated.

• Nusselt-Nr: Laminer: 0–10/Turbulent: 100–1000
• The overall heat transfer coefficient will be resulted from the convective heat transfer coefficient in the inner and outer pipeline, surface area of the inner and outer side, and fouling resistances.

Selection Guide Heat Exchanger Types

Tubeside Pressure Drop:

Shellside Pressure Drop:

1. Pressure Drop across the Tube Bundle

For turbulent flow across tube banks. a modified Fanning equation and modified Reynold’s number are given.

2. Pressure Drop for Baffles:

For the additional drop for flow through the free area above, below, or around the segmental baffles use

Shell and Tube:

the most common type of heat-transfer equipment

The TEMA standards cover three classes of exchanger: class R for the petroleum; class C covers exchangers for moderate duties in commercial; and class B for use in the chemical process industries.

Tubes

• It is to mention that we have a length of 3, 6 and 12 meters for piping. It means that if we use a length of 4 meters, we have to cut the pipe! U-type is used normally for kettle type heat exchanger, while straight tube is used mostly for other types of heat exchangers In case of U-type, only one tube sheet is required.

1. Dimensions: The tube thickness (gauge) is selected to withstand the internal and external (shell-side) pressure and give an adequate corrosion allowance
2. Tube Arrangements: The triangular and rotated square patterns give higher heat-transfer rates, but at the expense of a higher pressure drop than the square pattern. A square, or rotated square arrangement, is used for heavily fouling fluids, where it is necessary to mechanically clean the outside of the tubes. The recommended tube pitch (distance between tube centers) is 1.25 times the tube outside diameter.
3. Tube-Side Passes

Shells

1. Baffles: The most commonly used type of baffle is the single segmental baffle.

It is recommended to have a central baffle spacing of 0,3–1 * shell ID. The inlet and outlet baffle spacing is dependent on the nozzle size.

2. Support Plates and Tie Rods: The baffles and support plate are held together with tie rods and spacers.

3. Tube Sheets (Plates): In operation, the tube sheets are subjected to the differential pressure between shell and tube sides. Tube sheets can have an insulation because of the hood.

4. Shell and Header Nozzles (Branches): For vapors and gases, where the inlet velocities will be high, the nozzle may be flared, or special designs used, to reduce the inlet velocities e.g. using an impingment plate to prtotect the tube bundle. It is recommended to use impigment rods instead of an impingment plate for a better distribution and prevention of a layer formation, which may cause corrosion.

MEAN TEMPERATURE DIFFERENCE (TEMPERATURE DRIVING FORCE):

using the countercurrent and cocurrent temperature difference:

Fluid Allocation:

1. Corrosion: The more corrosive fluid should be allocated to the tube side. This will reduce the cost of expensive alloy or clad components.
2. Fouling: The fluid that has the greatest tendency to foul the heat-transfer surfaces should be placed in the tubes. This gives better control over the design fluid velocity, and the higher allowable velocity in the tubes will reduce fouling. Also, the tubes will be easier to clean. The lower the fouling factor in the pipeline, the better the heat transfer coefficient would be.

3. Fluid temperatures: If the temperatures are high enough to require the use of special alloys, placing the higher temperature fluid in the tubes will reduce the overall cost.

4. Operating pressures: The higher pressure stream should be allocated to the tube side. High-pressure tubes will be cheaper than a high-pressure shell.

5. Pressure drop: For the same pressure drop, higher heat-transfer coefficients will be obtained onthe tube side than the shell side, and fluid with the lowest allowable pressure drop should be allocated to the tube side

6. Viscosity: Generally, a higher heat-transfer coefficient will be obtained by allocating the more viscous material to the shell side, providing the flow is turbulent.The critical Reynolds number for turbulent flow in the shell is in the region of 200.

7. Stream flow rates: Allocating the fluids with the lowest flow rate to the shell side will normally give the most economical design

Shell and Tube Fluid Velocities

Liquids:

• Tube side: process fluids: 1 to 2 m/s, maximum 4 m/s if required to reduce fouling; water: 1.5 to 2.5 m/s.
• Shell side: 0.3 to 1 m/s.

• Nozzle velocity: The pressure drop through nozzle should be checked, especially where pressure losses are a problem in such a low pressure system.

Vapors: the lower values in the ranges given below will apply to high molecular weight materials.

Vaccum: 50- 70m/s

Atmospheric pressure: 10–30 m/s

High pressure: 5–10 m/s

Stream Temperatures:

The optimum temperature approach will usually be in the range 10 °C to 30°C for heat exchange between process streams.

Overall Heat Transfer Coefficient:

For steam condensation, it is around 150–300 w/m2.K.

Fluid Physical Properties:

Physical properties, density, viscosity, thermal conductivity, and temperature-enthalpy correlations (specific and latent heats), are usually obtained from a process simulation model.

REBOILERS AND VAPORIZERS:

1. Forced circulation: the fluid is pumped through the exchanger, and the vapor formed is separated in the base of the column

2. Thermosiphon, natural circulation

3. Kettle type

Boiling:

• Flow boiling (forced convection boiling): boiling of water in boiler tubes under pressurized condition
• Pool boiling: boiling of water in a kettle

1. Subcooled boiling: the temperature of the volume in the pool is lower than the saturation point of the working fluid

2. Saturated boiling: the whole pool volume is at the saturation temperature of the working fluid for the given pressure

• In the industry, it is desired to increase the point C as much as possible. The more the heat flux, the lower the required surface area of kettle will be.
• In order to calculate the heat flux in pool boiling, delt T should be defined at first. For every region, there is a formula.

PLATE HEAT EXCHANGERS

• Plate exchangers typically have a larger coefficient of heat transfer, as they have more contact area between fluids
• The narrow plate pathways severely drop the pressure of the flow, requiring additional pump power

DIRECT-CONTACT HEAT EXCHANGERS

FINNED TUBES

DOUBLE-PIPE HEAT EXCHANGERS

• These can be made up from standard pipe fittings, and are useful when only a small heat-transfer area is required
• Some designs of double-pipe exchanger use inner tubes fitted with longitudinal fins.
• A hairpin exchanger is formed by inserting one or more U-tubes into two pipe sections.

AIR-COOLED EXCHANGERS

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes (forced or induced draft).

Air-cooled exchangers are packaged units, and are normally selected and specified in consultation with a manufacturer.

Louver: an opening in a door or window that has one or more slanted strips to allow air to flow in and out while keeping out rain and sun : one of the slanted strips of a louver

Plenum: one that receives air from a blower for distribution (as in a ventilation system)

Advantages of Induced Draft:

• Better distribution of air across the section.
• Less possibility of the hot effluent air recirculating around to the intake of the sections. The hot air is discharged upward at approximately 2'12 times the velocity of intake, or about 1,50Oft/min
• Less effect of sun, rain, and hail, since 60% of the surface area of the sections is covered.
• Increased capacity in the event of fan failure, since the natural draft stack effect is much greater with induced draft.

Advantages of Forced Draft

• Slightly lower horsepower since the fan is in cold air. (Horsepower varies directly as the absolute temperature.)
• Better accessibility of mechanical components for maintenance.
• Easily adaptable for warm air recirculation for cold climates.

Air-cooled Heat Exchangers: Pressure Drop Air Side (Estimation of fan horsepower)

This method will approximate required fan horse- power.

Air-cooled Heat Exchangers: Rough Rating

A suggested procedure is as follows:

1. Calculate exchanger duty (MMBtuh).
2. Select an overall U (based on finned area). Arbitrarily use o,5inch-fins, 9 to the inch for determining U.
3. Calculate approximate air temperature rise from

4. Calculate Delta T and apply appropriate correction factor F.

5. Calculate exchanger extended area from

6. Estimate number of tube rows from Hudson Company optimum bundle depth curve, Figure 1. Use 4 to 6 tube rows if curve comes close to that number.

FALLING FILM EVAPORATOR

A falling film evaporator (FFE) is a specific type of vertically oriented shell and tube (S&T) heat exchanger that is used to separate two or more substances with different boiling point temperatures. The liquid to be concentrated is supplied to the top of the heating tubes and distributed in such a way as to flow down the inside of the tube walls as a thin film. The liquid film starts to boil due to the external heating of the heating tubes and is partially evaporated as a result.

FIRED HEATERS (FURNACES AND BOILERS)

CAPITAL COST OF HEAT TRANSFER EQUIPMENT

Cost correlations for heat-transfer devices are therefore usually expressed as a function of the heat-transfer area.

Costs of shell and tube heat exchangers dependes on the pressure, shell diameter and alloy of construction. Marshall and Swift Index can be used for different years:

Cost at B = Cost at A * (Index at B/Index at A)

Scaled costs from similar purchases:

Cost = constant * (Capacity)^(n)

Kettle-type-reboiler costs are 15 to 25% percent greater than for equivalent internal-floating head or U-Tube exchangers.

For fired heaters, the heat flux through the tubes is very high. The cost of surface area is still significant, but other costs from site fabrication, refractory installation, and burners are also important. Fired heater costs are usually better correlated against heater duty than tube wall area.

Pinch Analysis:

Pinch analysis is a methodology for minimising energy consumption of chemical processes by calculating thermodynamically feasible energy targets (or minimum energy consumption) and achieving them by optimising heat recovery systems, energy supply methods and process operating conditions.

Pinch: minimum temperature difference 10K

Rating and Simulation Mode:

In other words the rating gives you whether an exchanger area is adequate for a duty requirement while the simulation tells you the max duty the exchanger can perform.

Thermal Stress

For high temperature heat exchangers, the thermal stress during the startup, shutdown and load functions can be significant

• When you start a heat exchanger the flow of cooling water should be turned on first, and when shutting down the cooling water should be shut down last. This allows the exchanger to come up to temperature gradually on startup, and prevents localised overheating
• The shell should include an expansion jointto prevent excessive stress from damaging the exchanger or pulling tubes loose from the tube sheet.

Flow-Induced Vibration:

FIV is induced by the flow of fluid inside a pipe or duct, and is typically caused either by pressure pulsing or by turbulence. Due to the increase of heat exchanger size and tube bundle support spacing, the increase of fluid flow rate, unstable operating conditions and other factors, flow-induced vibration of heat exchanger tube bundle is often caused, resulting in local failure or even overall scrapping of heat exchanger

Tube vibration mechanisms in order of their increasing severity are turbulent buffeting, vortex shedding and fluid elastic instability

it is suggested to insert baffles to reduce the velocity in the shell side ad reduce the force.

Turbulator

A turbulator breaks up, slows down and redirects the straight line or laminar core of liquids or gas (whichever the case may be) and helps to increase heat transfer efficiency. In fact, a turbulator will allow you to do the same job using less energy.

Special Heat Exchanger

Unsteady Heat Exchanger

There is a formula for the calculation of time-dependant heat duty regarding biot number

• Slop System

The more the temperature difference between hot and cold stream, the more the heat duty will be. For calculating the flow rate of hot or cold stream, it is better to calculate it with this formula KAdeltaTlmtd. Accordingly, the result can be used for the hot and cold stream.

The more temperature difference, the more duty

The more voltage difference, the more current (V=RI)

The more pressure difference, the more flow

How to calculate the heat loss of a pipeline:

The heat will be lost through the convection and radiation. There are two official websites for this calculation:

Heat Loss from Bare Pipe Surfaces (engineeringtoolbox.com)

Insulated Pipes — Heat Loss Diagrams (engineeringtoolbox.com)

Heat loss in the pipeline depends on the mass flow rate of medium. According to the Reynolds, the convective heat transfer coefficient in the pipeline can be estimated.