Heat Exchangers
What is a Heat Exchanger?
Put simply, a heat exchanger is a device that transfers heat from one medium to another, a Hydraulic Oil Cooler for example will remove heat from hot oil by using cold water or air. Alternatively, a Swimming Pool Heat Exchanger uses hot water from a boiler or solar-heated water circuit to heat the pool water. Heat is transferred by conduction through the exchanger materials that separate the used mediums. A shell and tube heat exchanger passes fluids through and over tubes, whereas an air-cooled heat exchanger passes cool air through a core of fins to cool a liquid.
Types of Heat Exchangers
Heat exchangers are divided into various types based on different criteria.
On the basis of the nature of the heating process:
(a) Direct contact(open) heat generators
(b) Indirect heat generators: Regenerators, Recuperators
On the basis of relative motion of fluid motion:
(a) Parallel Flow Heat Exchanger
The purpose of a heat exchanger is to transfer heat energy from one fluid to another, with the two fluids existing initially at different energies and temperatures. What it means to be a parallel is that the two fluids enter and leave from the heat exchanger in the same directions.
(b) Counter flow heat exchanger
Counter flow heat exchangers use flows in the opposite direction of each other. Shell and tube, and double pipes heat exchangers are examples of common exchangers using counter flow configurations. The best design for shell and tube and double-pipe exchanger is counter flow configuration, and the heat transfer between the fluid is the maximum. In counter flow, the efficiency is higher than the parallel, and temperature in the cooling fluid outlet can exceed the warmer fluid inlet temperature.
(c) Cross flow heat exchanger
On the basis of Design and construction feature:
(a) Shell and Tube Heat Exchanger
Shell and Tube; Shell and Tube Heat Exchangers consist of a large number of small tubes which are located within a cylindrical shell. The tubes are positioned into the cylinder using a tube bundle or "tube stack" which can either have fixed tube plates (permanently fixed to the body) or, in the case of Thermex Heat Exchangers a floating tube stack which allows the tube bundle to expand and contract with varying heat conditions as well as allowing the tube bundle to be easily removed for servicing and maintenance.
(b) Plate Heat Exchanger
Plate Type; Plate Heat Exchangers operate in very much the same way as a shell and tube heat exchanger, using a series of stacked plates rather than tubes. Plate heat exchangers are usually brazed or gasketed depending on the application and fluids being used. Their compact stainless steel construction makes them an ideal choice for use with refrigerants or in food and beverage processing.
(d) Plate Fin Heat Exchanger
A plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between fluids. It is often categorized as a compact heat exchanger to emphasize its relatively high heat transfer surface area to volume ratio. The plate-fin heat exchanger is widely used in many industries, including the aerospace industry for its compact size and lightweight properties, as well as in cryogenics where its ability to facilitate heat transfer with small temperature differences is utilized.
Aluminum alloy plate fin heat exchangers have been used in the aircraft industry for 50 years and in cryogenics and chemical plants for 35 years. They are also used in railway engines and motor cars. Stainless steel plate fins have been used in aircraft for 30 years and are now becoming established in chemical plants.
Others:
Air Cooled Heat Exchanger
Source: Thomasnet
Air Cooled; Air Cooled Heat Exchangers are commonly used in vehicles or other mobile applications where no permanent cool water source is available. Thermex designs and supplies combination cooling packs (or combi-coolers) which combine an engine jacket water cooler, oil cooler and charge air cooler into a single unit reducing space requirements and improving efficiency. Cool air is provided either by a fan or by air flow caused by the movement of the vehicle.
Regenerative Heat Exchanger
In a regenerative heat exchanger, the same fluid is passed along both sides of the exchanger, which can be either a plate heat exchanger or a shell and tube heat exchanger. Because the fluid can get very hot, the exiting fluid is used to warm the incoming fluid, maintaining a near-constant temperature. A large amount of energy is saved in a regenerative heat exchanger because the process is cyclical, with almost all relative heat being transferred from the exiting fluid to the incoming fluid. To maintain a constant temperature, only a little extra energy is needed to raise and lower the overall fluid temperature.
Adiabatic Wheel Heat Exchanger
In this type of heat exchanger, an intermediate fluid is used to store heat, which is then transferred to the opposite side of the exchanger unit. An adiabatic wheel consists of a large wheel with threads that rotate through the fluids-both hot and cold-to extract or transfer heat.
Designing a Heat Exchanger
How is a Heat Exchanger designed?
Primary circuit fluid type, temperature and flow rate (usually the hot fluid)
What you want to take out of the primary circuit (Heat dissipation or a target outlet temperature)
Secondary circuit fluid type, temperature and flow rate (usually the coolant)
The fields above are only the basics. When putting an enquiry together you should also make Thermex aware of any pressure loss limitations and any other special requirements.
Marine Heat Exchangers
The operating principles of a marine heat exchanger are the same as a cooler designed for fresh water use, the main consideration for the designer however is that the marine heat exchanger must be resilient to erosion or corrosion caused by sea water. This means that materials that come in to contact with the sea water must be suitable, such as 90/10 Cupro-Nickel, 70/30 Cupro-Nickel, Bronze and Titanium.
There are other factors which need to be taken in to consideration when a marine heat exchanger is being designed. One is the velocity, if it is too low then there is a risk that sand and other particles will block the tubes. If it is too fast on the other hand then those same particles can rapidly erode the tube plate and tubes.
Additional protection can be provided by installing a sacrificial anode which Thermex can include upon request. This will be installed in to the threaded hole normally used for a drain plug and is in direct contact with the sea water flow.
What fluids can a Heat Exchanger operate with?
The suitability of a fluid with a heat exchanger will depend on the type of heat exchanger being used and the materials which are available. Standard Thermex Heat Exchangers are suitable for most fluids including Oil, Water, Water Glycol and Sea Water. For more corrosive fluids such as chlorinated salt water, refrigerants and acids other materials such as Stainless Steel and Titanium will need to be used instead.
What is temperature Cross Over?
Temperature cross over is a term used to describe the scenario where the temperatures of both circuits in a liquid cooled heat exchanger begin to cross over. This can be an important factor in a heat exchanger design as the efficiency of a cooler will be significantly reduced when the temperatures cross over. In many cases a plate heat exchanger is the best option for applications where temperature cross over can't be avoided.
Oil Water
Inlet Temperature 80 30
Outlet Temperature 50 51.5
Flow Rate 25 L/min 15 L/min
The table above demonstrates that the cooling water outlet temperature is slightly higher than the outlet temperature of the oil. One simple way to combat this and increase the efficiency of the oil cooler is to increase the flow rate of the coolant. In this particular example increasing the water flow rate to 25 L/min would reduce the water outlet temperature to 43°C
What is a heat exchanger "pass" and how do I know how many passes I need?
A Heat Exchanger Pass refers to the movement of a fluid from one end of the heat exchanger to the other. For example, when referring to the "through tubes" circuit (usually the coolant); · Single Pass – Fluid enters one end of the heat exchanger, and exits at the other end. · Double Pass – Fluid enters and exists the heat exchanger at the same end. · Triple Pass – Fluid travels the length of the heat exchanger body three times before exiting.
A greater number of passes increases the amount of heat transfer available, but can also lead to high pressure loss and high velocity.
With a full set of operational data, Thermex can select the most efficient heat exchanger possible whilst working within the pressure loss and velocity limits.
The number of passes on the primary circuit can also be adjusted to optimise thermal performance and efficiency by changing the baffle quantity and pitch.
How to make a heat exchanger more efficient.
Heat exchanger efficiency can be defined in many ways, in terms of thermal performance there are several key factors to consider;
Temperature differential - As discussed in point 3 (temperature cross-over) the difference between the hot fluid and coolant is very important when designing a heat exchanger. The coolant always needs to be at a lower temperature than the hot fluid. Lower coolant temperatures will take more heat out of the hot fluid than warmer coolant temperatures. If you had a glass of drinking water at room temperature for example, it is much more effective to cool it down using ice rather than just cool water, the same principle applies to heat exchangers.
Flow rate - Another important factor is the flows of the fluids in both the primary and the secondary side of the heat exchanger. A greater flow rate will increase the capability of the exchanger to transfer the heat, but a greater flow rate also means greater mass, which can make it more difficult for the energy to be removed as well as increasing velocity and pressure loss.
Installation - The heat exchanger should always be installed based on a manufacturers' guidelines. Generally speaking the most efficient way to install a heat exchanger is with the fluids flowing in a counter-current arrangement (so if the coolant is travelling left to right, the hot fluid travels right to left) and for shell and tube heat exchangers the coolant should enter at the lowest inlet position (as shown in the diagrams above) to ensure that the heat exchanger is always full of water. For air cooled heat exchangers it is important to consider the air flow when installing a cooler, any part of the core which is blocked will compromise cooling capacity.
Industry-specific Heat Exchangers available in market
Sometimes, a specific heat exchanger type can be selected based on the industry it will be used for. Some examples include;
Hydraulics and Industrial Process
Hydraulic Oil Coolers (Shell and Tube)
Brazed Plate Heat Exchangers
Air Cooled Oil Coolers
Marine
Marine Oil Coolers
Header Tank Heat Exchangers
Manifold Heat Exchangers
Titanium Heat Exchangers
Food Process Equipment
Stainless Steel Shell and Tube Heat Exchangers
Brazed Plate Heat Exchangers
Mobile Plant
Air Cooled Heat Exchangers
Hydraulic Oil Coolers
Power Generation
Exhaust Gas Heat Exchangers
Manifold Heat Exchangers
Header Tank Heat Exchangers
Air Cooled Heat Exchangers
Mining
Stainless Steel Shell and Tube Heat Exchangers
Carbon Steel Heat Exchangers
Spas and Swimming Pools
Swimming Pool Heat Exchangers
How to increase the lifetime of a Heat Exchanger
Heat exchangers are manufactured from robust materials, have no moving parts and operate at a variety of different pressures and temperatures, therefore if a heat exchanger is used in the correct way then there is no reason why it shouldn't be able to remain operational for many years. To help increase the operational lifetime of a heat exchanger there are several steps that should be taken;
Make sure the design data is accurate - If you are sending data to our engineers for heat exchanger selection, then it is best to make sure that it is as accurate as possible. Not only will this ensure that your heat exchanger is thermally efficient but also that it will be able to operate for a long period of time. If the flow rates are too high then erosion could be a problem, if the pressures are too high then leaks could occur and if there are any unusual chemicals in the fluids (such as acids in coolant water) then please contact us to check the compatibility. If our standard materials aren't suitable then we can usually supply an alternative which is.
Regular Maintenance and Servicing - All shell and tube heat exchangers are designed to allow for easy maintenance and servicing. The end caps can be removed allowing the internal tube bundle to be removed for cleaning.
Heat Exchanger Failure
There are many possible causes for heat exchanger failure, some examples for shell and tube include;
Oil and Water Mixing, O Ring Failure
Normally a seal kit is all that is required to solve the problem, but temperatures should be checked to make sure that the seals purchased are suitable.
Before purchasing an Oil Cooler, make sure that Thermex are aware of the pressures and temperatures being used.
Split Tube, Erosion on the tube plate face
Water velocity is too high, Check water flow rate and advise Thermex. In some cases a higher grade of material such as 70/30 CuNi or Titanium may be required.
When using seawater, make sure that the flow rate doesn't exceed the Heat Exchanger's capabilities. Contact us for advice.
Header / End cap split, Frozen water expands within the heat exchanger / Oil Cooler. A new header kit and Seal Kit will be required. Drain the water system during the winter if the cooler isn't being used.
Insufficient thermal capacity, or reduced performance.
Operating conditions not matching design conditions Review design conditions and operating conditions with Thermex engineers, a new tube stack or pump may be required. Ensure that operational data provided to Thermex is as accurate as possible.
Sediments, dirt or other blockages inside the heat exchanger Remove headers to check tubes, and clean with a suitable fluid or pipe cleaner. Use a filtration system to prevent blockages within the tubes, or increase the cooling water flow rate.
Heat Exchanger Abbreviations and Terminology
Below are some abbreviations and terms often used in the heat exchanger industry;
ETD Extreme Temperature Differential
EGC Exhaust Gas Cooler
MGO Marine Gas Oil Cooler
HTHE Header Tank Heat Exchanger
CAC Charge Air Cooler
ACOC Air Cooled Oil Cooler
PHE / PHEX Plate Heat Exchanger
Shell Side Flow/Connections - Shell Side refers to the flow of a fluid, such as oil, over the external surface of the tube stack
Tube Side Flow/Connection - Tube Side refers to the flow of fluid through the tubes of the tube stack, usually the coolant Pressure Loss / Drop The level of back pressure generated by pumping the fluids through the heat exchanger circuits.
Heat exchangers transfer thermal energy from one fluid to another. In manufacturing facilities, steam is a common source of heat for many reasons, some of which are discussed in the Introduction. There is a wide range of heat exchanger designs that use steam, largely due to the wide range of products that are heated with steam.
Many processes and product considerations must be incorporated into the selection of a heat exchanger. Some more types of Heat Exchangers:
Tubular
Plate and frame
Jacketed
Coil
Tubular Heat Exchanger
Shell and Tube Heat Exchanger
Aside from classifying heat exchangers based on fluid direction, there are types that vary mainly in their composition. Some heat exchangers are comprised of multiple tubes, whereas others consist of hot plates with room for fluid to flow between them. It’s important to keep in mind that not all heat exchangers depend on the transfer of heat from the liquid to liquid, but in certain cases use other mediums instead.
Heat Exchanger Analysis
LMTD: Logarithmic Mean Temperature Difference
Effectiveness of Heat Exchanger
The effectiveness of a heat exchanger is defined as the ratio of actual heat transfer to the maximum possible heat transfer.
Number of Transfer Units(NTU)
Compiling non-dimensional parameters together, effectiveness of a heat exchanger can be expressed as a function of 3 non-dimensional constants. This method is called as Number of Transfer Units or NTU.
Subject: Heat Transfer - Home Assignment
Group ID: D02
Group Members:
Vivek Sanap(06)
Sanket Lavalekar(08)
Arya Sapre(09)
Samvedh Shetty(16)
Siddharth Dhakar(20)
Nice Blog!!!!!
ReplyDeleteNice blog
ReplyDeleteVery informative blog 👍
ReplyDeleteGreat work guy's
ReplyDeleteNice information guys!
ReplyDeleteNice thorough study! Looking forward for more such technical blogs from you guys✨
ReplyDeletegreat explaination 👍🏻
ReplyDelete