Designing a Heat Exchanger

Designing a Heat Exchanger

1. How is a Heat Exchanger designed?

To be able to select a heat exchanger, we need to know;

• 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)

2. 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.

3. 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.

4. 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.

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

5. 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. The images below will help to demonstrate this;

Single Pass (1 pass)

Double Pass (2 pass)

Triple Pass (3 pass)

• 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.

6. 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. To find out more about counter-flow and why it is more efficient, please read our blog post “Why counter flow heat exchangers are more efficient”

7. Which industry-specific Heat Exchangers can Thermex provide?

Sometimes, a specific heat exchanger type can be selected based on the industry it will be used for. Some examples include;

Sourced by thermex.

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