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Heat recovery: Types of heat recovery equipment 

There is a broad range of heat recovery systems. Virtually all heat recovery involves the use of heat exchangers, together with associated items of equipment such as ducts, pipes, pumps, fans and controls.

You can use the Heat recovery overview guide (PDF) to help you understand the principles and practicalities of heat recovery and identify what opportunities it may provide to improve the energy efficiency of your business.

 
 

Introduction
 Practical issues
Types of heat recovery equipment
 Publications and resources
Common applications

Heat recovery systems and equipment

The choice of heat recovery system for a particular application depends on a number of factors. Understanding them will help you to identify the most appropriate and economic solution. They include:

Source of heat

  • Medium (liquid or gas) and physical properties
  • Temperature of the available heat (known as its ‘grade’)
  • How much heat is available and when
  • Physical location of the source of the heat
  • Contamination risk

Proposed uses for recovered heat

  • Medium (liquid or gas) and physical properties
  • Grade of heat required (temperature)
  • How much heat is required and when
  • Physical location of the potential use for the heat

Find out more about three types of heat recovery systems equipment below:


To learn more about heat recovery systems for light industrial facilities or warehouse based business, register for our Heat Recovery Systems webinar, 8 September 2011


Heat exchangers

Heat exchangers are devices for transferring heat from one fluid to another. A fluid may be a gas or a liquid. It is generally possible to transfer heat either between examples of the same type of fluid, for example gas-to-gas or liquid-to-liquid, or between different types, for example gas-to-liquid. An example is the cross flow heat exchanger in the animation on the introduction page.

Equipment types


What is a heat exchanger? Read on to find out about three types of heat exchangers: Gas-to-gas heat exchangers, Gas-to-liquid heat exchangers, and liquid-to-liquid heat exchangers.


Gas-to-gas heat exchangers


  • Run around coils (one in each airstream – as shown in the animation below)
  • Plate heat exchanger or recuperator
  • Rotating regenerator (or thermal wheel)

Details of all of the above exchanger types can be found in the Heat recovery overview guide (CTG057).


Animation showing how Run around coil works:



Run around coil

When the supply and extract fans are not physically close together, you can use a run around coil to transfer heat from the hot air exiting the building to the cool fresh air entering the building. This involves the addition of two heat exchangers, one in the supply air handling unit (AHU) and one in the extract AHU. The addition of the heat exchangers allows cool fresh air to be pre-heated, meaning that the boiler needs to supply less heat to raise the air to the required temperature, saving energy and carbon.

Without a Run around coil With a Run around coil
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Other types include the heat pipe (thermosyphon) – where the working fluid is a refrigerant that evaporates under heat and condenses when cooled – the heat exchanger and the tubular recuperator.

Gas-to-liquid heat exchangers


  • Economiser (as shown in the animation below)
  • Condensing economiser

Animation showing how an Economiser works:


Economiser

Boilers create heat for use in a variety of applications, such as space heating, through the combustion of fuel. Typically, the fuel will be natural gas, oil or biomass. The combustion process produces very hot combustion gases, which are discharged to atmosphere via the boiler flue.


In some cases, it is possible to add an economiser, which captures some of the heat from the flue gases and transfers it to heating system water returning from the building before it enters the boiler. This reduces the amount of heat which needs to be added by the boiler, saving energy and carbon.

Without an Economiser With an Economiser
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4


Other, less common gas-to-liquid types are the spray condenser (or spray recuperator) and shell-and-tube heat exchangers.

Liquid-to-liquid heat exchangers


  • Shell-and-tube
  • Plate (not to be confused with its gas-to-gas counterpart)


Heat pumps


In the context of heat recovery, heat pumps are a technology that allows waste heat to be upgraded (that is, its temperature raised sufficiently for re-use) without excessive energy input. In some cases they can also be used to move recovered heat from one place to another (such as from exhaust to supply air streams in air handling units which aren’t adjacent to each other).


Regenerative devices


Regenerators operate on the principle of short-term heat storage as part of a cycle. During the first part of the cycle, hot waste gases heat a thermal store. Incoming cold air is then pre-heated by being passed over the heat store. Regenerators are mainly used in high temperature industry, with the exception of the rotary regenerator (typically referred to as a ‘thermal wheel’), which is also employed in heating, ventilation and air conditioning (HVAC) and other applications. Regenerators have high operating efficiencies, but can be relatively expensive and also require significant space.



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