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Heat Exchangers for Solar Hot Water Systems

 

Solar water heating systems use heat exchangers to transfer solar energy absorbed in solar collectors to the liquid or air used to heat water or a space.

Solar Hot Water heating systems

 

Liquid-to-liquid

 

This heat exchanger uses a heat transfer fluid that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to water in a storage tank.

 

Storage tank                                                                                                         

                                                                                                      Evacuated Tube Solar Collector

Heat-transfer fluids, such as antifreeze, protect the solar collector from freezing in cold weather. Liquid-to-liquid heat exchangers have either one or two barriers (single wall or double wall) between the heat-transfer fluid and the domestic water supply.

 

Types of Heat Exchangers

A single-wall heat exchanger is a pipe or tube surrounded by a fluid. Either the fluid passing through the tubing or the fluid surrounding the tubing can be the heat-transfer fluid, while the other fluid is the potable water.

Double-wall heat exchangers have two walls between the two fluids. Two walls are often used when the heat-transfer fluid is toxic, such as ethylene glycol (antifreeze).

Double walls are often required as a safety measure in case of leaks, helping ensure that the antifreeze does not mix with the potable water supply.

An example of a double-wall, liquid-to-liquid heat exchanger is the "wrap-around heat exchanger," in which a tube is wrapped around and bonded to the outside of a hot water tank. The tube must be adequately insulated to reduce heat losses.

While double-wall heat exchangers increase safety, they are less efficient because heat must transfer through two surfaces rather than one. To transfer the same amount of heat, a double-wall heat exchanger must be larger than a single-wall exchanger.

 

             Cutaway of Stiebel Eltron Tank

Coil-in-tank

The heat exchanger is a coil of tubing in the storage tank. It can be a single tube (single-wall heat exchanger) or the thickness of two tubes (double-wall heat exchanger). A less efficient alternative is to place the coil on the outside of the collector tank with a cover of insulation.

Shell-and-tube

 

The heat exchanger is separate from (external to) the storage tank. It has two separate fluid loops inside a case or shell. The fluids flow in opposite directions to each other through the heat exchanger, maximizing heat transfer.

In one loop, the fluid to be heated (such as potable water) circulates through the inner tubes. In the second loop, the heat-transfer fluid flows between the shell and the tubes of water.

The tubes and shell should be made of the same material. When the collector or heat-transfer fluid is toxic, double-wall tubes are used, and a non-toxic intermediary transfer fluid is placed between the outer and inner walls of the tubes.

Tube-in-tube

 

In this very efficient design, the tubes of water and the heat-transfer fluid are in direct thermal contact with each other. The water and the heat-transfer fluid flow in opposite directions to each other. This type of heat exchanger has two loops similar to those described in the shell-and-tube heat exchanger.

 

 

 

 

 

 

 

 

 

 

 

Example of a heat exchanger design

Sizing

 

A heat exchanger must be sized correctly to be effective. There are many factors to consider for proper sizing, including the following:

• Type of heat exchanger

• Characteristics of the heat-transfer fluid (specific heat, viscosity, and density)

• Flow rate

• Inlet and outlet temperatures for each fluid.

• Usually, manufacturers will supply heat transfer ratings for their heat exchangers (in Btu/hour) for various fluid temperatures and flow rates. Also, the size of a heat exchanger's surface area affects its speed and efficiency: a large surface area transfers heat faster and more efficiently. 

Installation

 

For the best performance, Eco Depot USA follow's the manufacturer's installation recommendations for the heat exchanger. And we choose a heat-transfer fluid that is compatible with the type of heat exchanger you will be using.