How Does a Heat Exchanger Work?

Have you ever wondered how does a heat exchanger work? Well, a heat exchanger is an essential part of your furnace. Its purpose is to transfer heat from one medium to another, such as from hot oil to cold water, or between a fluid and air. Heat exchangers are also used in swimming pools and to control fluid temperatures in food, beverage, and pharmaceutical processing applications.

How Does a Heat Exchanger Work?

In general, here’s how a heat exchanger works:

  • Allows heat from a fluid to pass to a second fluid.
  • The fluids do not mix or come in direct contact with one another.
  • Heat is transferred without the fluid changing its path.

Heat Exchanger in a Natural Gas/Propane Burning Furnace

  • Exhaust and combustion by-products (Flue gas) enter the heat exchanger.
  • Metal is heated by the hot gas as it travels towards the exhaust.
  • Air circulating around the heat exchanger is heated by the hot metal.

The primary heat exchanger contains the hottest flue gas. It’s the most prone to heat stress and cracking. High-efficiency furnaces have a secondary heat exchanger, which releases even more heat from the gas when water vapor turns into a liquid.

Heat Exchanger Design

There are several types of heat exchangers. Each has the same basic design principles, including:

  • Primary Circuit: Typically, the hot fluid that flows at a specific rate.
  • What’s Taken Out of It: Dissipated heat or a desired temperature at the outlet.
  • Secondary Circuit: Generally the coolant, its temperature, and flow rate.
Other design considerations include:
  • Temperature Crossover: The temperature of both circuits cross over, reducing the efficiency of a cooler.
  • Heat Exchanger Pass: In a single pass system, fluid enters one end and exits another; in a double pass unit, fluid enters, reverses direction, and exits the same side. In a triple pass system, fluid travels the length of the heat exchanger three times.

Heat Exchanger Fluids

  • Water
  • Oil
  • Sea Water
  • Water Glycol
  • Chlorinated Salt Water

Types of Heat Exchangers

  • Shell and Tube: A set of metal tubes carries one fluid. The second fluid passes through and within a shell encasing the assembly. This design is suited for viscous fluids and those with a higher density of particulates.
  • Plate/Fin: Thin metal plates or fins create a large surface area to exchange a greater amount of heat more quickly, such as in gas boilers. Stainless steel stack plates are often used instead of tubes.
  • Air Cooled: Don’t use a permanent cool water source; instead, a fan delivers cool air, or the movement of a vehicle drives air flow, which is why this type is often used in automotive or mobile applications.

Double tube-sheet heat exchangers, used in pharmaceutical applications, prevent mixing between the product and heating/cooling medium, and make leaks easy to spot. Scraped surface heat exchangers used in manufacturing can process fluids with a high viscosity or high number of particulates, thereby increasing efficiency.

What Are Heat Exchangers Made Of?

  • Metals, such as steel, copper, or iron, are often used.
  • Ceramic materials are better suited for high-temperature applications as they resist melting.
  • Composite materials may be used for heat exchangers.
  • Plastics are lightweight, corrosion-resistant, and have good thermal conductivity. They’re more cost-effective and well-suited for most residential applications.

Contact Black Hills Inc. Home Services

The heat exchanger in a home furnace can crack or rust over time. Since it is at the core of the appliance, we will recommend replacing your heater. Replacement can result in improved reliability, efficiency, and safety. With a new furnace, you’ll get more even heating and can save on your energy bills. To learn more or request 24/7 HVAC services, contact us at 888-538-5821 today.