Heat is the most predominant type of energy. Due to its being the end product of energy conversion processes, it is also the most wasted. A heat exchanger removes wasted heat from one process and reuses it in another. It may be in one pass or multiple passes. A heat-recovery ventilator captures heat from air inside the home that is about to be vented and transfers it to fresh air being brought in from outdoors.
Watch the video on why it is a good idea to have ventilation in the home, particularly in damp regions. It goes over some solutions such as extractor fans which waste heat, require trickle vents, are noisy and create drafts. Passive stack ventilation avoids using electricity but has a lack of pressure differences affecting air flow leading to a need to open windows or an additional central fan in warm weather, also wastes heat and may cause drafts. Central extract ventilation (MEV) removes contaminated air from wet rooms which is replaced with fresh air from trickle vents. It has all the disadvantages of wasted heat, drafts, and need for tricle vents.
Mechanical ventilation with heat recovery, MVHR, is the best solution where contaminated stale air is continuously replaced by fresh, filtered air. In the warm months, the air is without being heated by
the heat exchange. A very efficient heat exchange can recover 90 percent of the stale air’s heat and transfer it to the incoming air. The final result of MVHR is a lower carbon footprint, a constant supply of fresh air and controlled condensation. It can increase property value and lower electric costs.
Ground air heat exchange (GAHX) uses underground 40 meter long pipes 1 1/2 meters below the surface to bring in constant stabilized air temperature into the MVHR system. It preheats winter air or
pre-cools summer air.
Heat exchanger choices are based on construction materials; pressure and temperature; performance parameters such as flowrates and pressure drop; fouling tendencies; ease of inspection, cleaning, extension and repair; types of fluids and costs. Some types include tubular heat exchangers (THE) and compacts such as gasketed plate heat exchanger (PHE), spiral heat exchanger (SHE) and lamella heat exchanger (LHE).
Discussion of heat exchangers involves lots of scientific calculations, formulas, and terms. As a simple introduction, their performance is rated in heat transfer units (HTU), or the ratio of temperature change of one fluid to the mean temperature of the temperature difference between the two fluids. Flow rate is determined by the total flow that may be passed through the channels without extreme pressure drop.
Pressure drop is the price paid for heat transfer. Losses in return bend connections and inlet/outlets should be minimized to avoid pressure drop. The more sophisticated the construction material, the higher the economic pressure drop.
Fouling is the depositing of low thermal conductivity material on heat transfer surfaces, reducing the heat exchange efficiency. Factors depend on fluid velocity and influence on turbulence. The PHE rates the highest for ease of inspection and cleaning. Due to its excellent fluid distribution and very high turbulence, it is relatively little affected by fouling. It can be cleaned mechanically because its affected surfaces are readily accessible and can be easily removed, or by chemical means, “Cleaning-In-Place” (CIP).
Residence time is the amount of time in the heat transfer process that the product is exposed to a hot or cold surface. It is the time it takes for a fluid particle moving at mean fluid velocity to pass through an exchanger. It can be important is chemical reactions or controlling function rapidity.
The THE requires more construction material than the compact for the same job. Welded heat exchangers require more manhours in manufacturing. The PHE requires more expensive construction material. Although the cost per unit area of the compacts may be higher, the greater efficiency of the THE results in smaller area requirements so it all evens out.
Heat recovery ventilators can be installed in Greenville, South Carolina by Jennings-Dill, Inc., 33 Grand Avenue in Greenville, SC 29606, 800.235.2622 or 864.235.2518; SC Air Tightness & Ventilation, phone 046 907.4344, Email: email@example.com. A Broan HRV can be purchased on Amazon for less than $500. Or watch the detailed YouTube video on how to make your own HRV out of Coroplast for a do-it-yourself project.