Many HVAC appliances have a component that is known as a &#;heat exchanger.&#; They come in many forms, but it is the metal that composes your heat exchanger that often makes one of the biggest impacts.
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What this piece of metal does is transfer heat from one fluid (e.g., hot water in your water heater) to another (e.g., domestic water running to your tap). There is a good amount of choice in the metal you can use for that heat exchanger, ranging from bronze and titanium to brass and carbon steel.
However, copper and stainless steel heat exchangers are the most commonly used because they are less expensive and still highly effective. One of the most common questions we are asked by our customers is some variation of: which is better a copper or stainless steel hot water cylinder, water heater, boiler, or other HVAC appliance?
The main concerns of a homeowner when choosing between copper and stainless steel should be thermal conductivity, durability, and price.
In this guide, we look at the pros and cons of copper and stainless steel heat exchangers.
The thermal conductivity of a heat exchanger determines how quickly it transfers the heat from the heating source to the distribution fluid. In this regard, a heat exchanger with copper is much faster at transferring heat than stainless steel.
Here are the basic thermal conductivity levels, measured in watts per meter pre-Kelvin, of the two different metals[1]:
On average, the thermal conductivity of copper is 20 times that of stainless steel. In practical terms, this means that copper can transfer heat 20 times faster. So, if you need quick heating, copper will work to your advantage.
Why would you need to heat something quickly? That&#;s an important question to ask if you are choosing between, say, a copper vs. stainless steel tankless water heater.
For example, if you own a swimming pool and plan on going swimming on an autumn day, a water heater with a copper heat exchanger can get your pool ready for you much faster. With a stainless steel heat exchanger, you could find yourself waiting up to 72 hours before your pool is heated to 10 degrees Celsius.
Even if you don&#;t need to heat things quickly, the higher thermal conductivity offered by copper also leads to higher efficiency. As a result, using a heat exchanger with copper will lead to lower energy costs. After all, a heater or boiler that has to run for longer to heat your home, pool, or tap water is going to cost you more.
Durability is a big concern for heat exchangers when it comes to appliances like a boiler. This is because condensing boilers (the most popular type right now), release a corrosive condensate that can eat away at the metal in the heat exchanger.
A heat exchanger that cannot stand up to the condensate will quickly corrode, requiring a time-consuming and costly replacement. As a result, you will likely want to choose a heat exchanger that can resist corrosion over the long term.
In this case, the clear winner is stainless steel. Unlike standard steel, stainless steel has a property known as &#;passivation.&#; This refers to its ability to form a layer of oxide on itself in response to contacting air.[2]
This layer of oxide protects stainless steel from corrosion and rust, allowing for a longer lifespan than regular steel. It is essentially perfect to use in any heat exchanger that will be in contact with corrosive elements.
On the other hand, copper is much more vulnerable to corrosion. The condensate turns copper atoms into copper ions, effectively dissolving the metal over time. This is a big problem for two reasons. First, because of the lower lifespan; then, because a corroded copper heat exchanger loses efficiency.
Considering that higher efficiency and thermal conductivity was the advantage for copper, having it reduced balances out the other way.
Copper tends to be cheaper than stainless steel when purchased in the same quantity, and that holds true when used in heat exchangers. While that may tempt you into getting copper for your heat exchanger, remember that it is much less durable. You will have to buy more copper replacements to maintain its efficiency levels. As a result, copper can actually end up being more expensive in the long term.
Generally, you will find that heat exchanger manufacturers will offer copper as the default choice because they&#;re cheaper. These companies are aware of the trade-off between the cost and the lifespan, where the cost is a &#;pay now or pay later&#; issue. You either pay more upfront for a stainless steel heat exchanger that will last longer or pay later to replace the copper one sooner.
The ultimate choice comes down to whether you are thinking long-term or short-term. If you plan on adding value to your home by installing high-quality HVAC equipment, go with the long-term option. (e.g., gas boilers and stainless steel heat exchangers). The long-term option will save you money and reduce the need for HVAC services and replacements.
So it should be obvious that stainless steel, the more costly of the two metals, is better for long-term thinkers. However, should you really need a heat exchanger with the highest conductivity to quickly heat large bodies of water (e.g., a pool) or larger homes, then copper might be the better choice.
Of course, stainless steel can do everything copper can, just at a slower pace and slightly higher price.
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Given the detailed nature of this content, let's break down each quiz question and provide a brief explanation or guideline to help you answer them.
### Quiz 1: Typical Values of Fouling Coefficient and Resistances
- Explanation: Fouling coefficients and resistances are crucial for the thermal design of heat exchangers. Typical values can be found in engineering handbooks and depend on the type of fluid and operating conditions. For example, water might have a fouling factor of 0. to 0. (m²·K/W).
### Quiz 2: Guidelines for Placing the Fluid in Order of Priority
- Explanation: When deciding which fluid should be on the shell side or tube side, factors such as fluid corrosiveness, fouling tendency, and pressure drop requirements are considered. Generally, the more corrosive fluid or the fluid with higher fouling tendency is placed on the tube side to simplify cleaning and maintenance.
### Quiz 3: Features of Shell and Tube Type Exchanger
- Explanation: Shell and tube heat exchangers are versatile, capable of handling high pressures, and are easily cleaned. Features include multiple pass configurations, varied baffle designs, and the ability to handle a wide range of temperatures and pressures.
### Quiz 4: Determination of Number of Tube Passes Based on Shell ID
- Explanation: The number of tube passes in a shell-and-tube heat exchanger affects the heat transfer coefficient and pressure drop. More passes can increase heat transfer efficiency but also increase pressure drop.
### Quiz 5: Supplements for Type Selection
- Explanation: Choosing the right type of heat exchanger involves considering factors such as operating pressure, temperature ranges, fluid properties, maintenance needs, and cost.
### Table 6: Common Tube Layout
- Explanation: Tube layout patterns include square, triangular, and rotated triangular patterns. The choice affects heat transfer efficiency and pressure drop.
### Quiz 7: Tube Pattern Relationship
- Explanation: Tube patterns influence heat transfer rates and pressure drop. Triangular patterns are typically more compact and provide higher heat transfer coefficients, while square patterns are easier to clean.
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### Quiz 8: Typical Heat Exchanger Parts and Connections
- Explanation: Key components include the shell, tubes, baffles, tube sheets, headers, and nozzles. Each part plays a role in directing fluid flow and enhancing heat transfer.
### Quiz 9: Standards- Comparison of Classes R, C, & B
- Explanation: These classes define different standards and codes for heat exchanger design, construction, and testing. Class R is typically for refinery and petrochemical applications, Class C for commercial, and Class B for building services.
### Quiz 10: Selection Guide Heat Exchanger Types
- Explanation: A selection guide helps choose between different types such as shell and tube, plate, air-cooled, and spiral heat exchangers based on application specifics.
### Quiz 11: Shell and Tube Exchanger Selection Guide (Cost Increase from Left to Right)
- Explanation: This guide typically compares different shell and tube configurations, showing cost implications. More complex designs generally cost more but offer better performance.
### Quiz 12: Minimum Temperature Approach for Heat Exchangers
- Explanation: The minimum temperature approach is the smallest temperature difference between the hot and cold fluids at any point in the heat exchanger. It's critical for ensuring effective heat transfer.
### Quiz 13: Typical Metal Thermal Conductivities,
- Explanation: Thermal conductivity values for metals (e.g., copper, stainless steel, aluminum) are essential for designing heat exchangers. Copper has high thermal conductivity (~400 W/m·K), while stainless steel is lower (~16 W/m·K).
### Quiz 14: Typical Heat Transfer Coefficients, U and Fouling Resistance,
- Explanation: The overall heat transfer coefficient (U) and fouling resistance (rf) depend on the specific fluids and flow conditions. For water to water, U might range from 500 to W/m²·K, while fouling resistances vary by fluid type.
### Quiz 15: Tube Dimensions
- Explanation: Tube dimensions include diameter, wall thickness, and length. Standard dimensions depend on application requirements and industry standards.
### Quiz 16: The Common Tube Pitches Used
- Explanation: Tube pitch is the center-to-center distance between tubes. Common pitches are 1.25 to 1.5 times the tube diameter to balance heat transfer and cleaning capability.
### Quiz 17: Approximate Heat Transfer Coefficients for Shell-and-Tube Heat Exchangers (KJ. Bell, )
- Explanation: Provides a range of heat transfer coefficients for different shell and tube exchanger configurations based on empirical data.
### Quiz 18: Added Surface Area for Typical Fluid Combinations
- Explanation: Sometimes additional surface area is required to achieve desired heat transfer, influenced by fluid properties and flow regimes.
### Quiz 19: Fouling Resistance vs Heat Transfer Coefficient
- Explanation: There is an inverse relationship between fouling resistance and heat transfer coefficient. Higher fouling resistance decreases the effective heat transfer rate.
### Quiz 20: Features of Some Typical Exchanger Types
- Explanation: Different exchanger types (e.g., plate, spiral, finned tube) have specific features and advantages for various applications.
### Quiz 21: Troubleshooting Checklist
- Explanation: A troubleshooting checklist helps diagnose and solve common issues like fouling, leaks, and thermal performance degradation in heat exchangers.
### Quiz 22: Selected Heat-Transfer Fluids
- Explanation: Different fluids (e.g., water, glycol, oil) have unique thermal properties and suitability for various heat exchanger applications.
### Quiz 23: Summary of Heat-Exchanger Approach Temperature Differences and Pressure Drops
- Explanation: Summarizes expected temperature differences and pressure drops for various heat exchanger types and applications.
### Quiz 24: Typical Overall Coefficient
- Explanation: Provides typical values for the overall heat transfer coefficient for different heat exchanger designs and fluid combinations.
### Quiz 25: Constants for Use in Equation
- Explanation: Lists constants for use in heat transfer equations, such as those for calculating Nusselt number, Reynolds number, and other dimensionless groups.
By understanding these concepts, you'll be well-prepared to tackle questions on heat exchanger design, selection, and troubleshooting. If you have any specific question or need detailed explanations for any particular quiz, feel free to ask!
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