Heat Exchangers: Why Efficiency Matters

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Information about Heat Exchangers: Why Efficiency Matters
Education

Published on May 13, 2014

Author: ZehnderAmerica

Source: slideshare.net

Description

Geothermal Heat Exchangers: The optional Zehnder ComfoFond geothermal heat exchanger utilizes the constant temperature of the earth for heat recovery and tempering the supply air by means of a Ground Loop HDPE or Pex. tubing installed in the ground. Geothermal energy is collected in the fluid in the tubing and used to preheat or cool the supply air. The energy it contains is transferred to the fresh supply air via a heat exchanger upstream of the ventilation device.


GEOTHERMAL HEAT EXCHANGERS

The ground source heat exchanger ComfoFond-L is designed to temper incoming air to the CA 350, CA 550, Paul by Zehnder Novus 300 and Paul by Zehnder Focus 200. During cold winter weather, the incoming air is warmed to help prevent frost and increase efficiency. During hot, humid summer periods the incoming air is cooled, and some moisture is removed. Controls are integrated for the ComfoAir units while the Novus and Focus units utilize an external controller and power connection allowing for easy installation and flawless operation.

HEAT EXCHANGERS: Why Efficiency Matters 888.778.6701

In this eBook we will cover… 1.1 Heat recovery 1.2 The principle of heat exchange 1.3 Cross-flow heat exchanger 1.4 Counter-flowheat exchanger 1.5 Rotary heat exchanger 1.6 Cross-counter-flowheat exchanger

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.1 Heat Recovery Ventilation Heat Recovery Ventilationand Energy Recovery Ventilation reduceboth the heating/cooling costs and loads of the building. In a leaky home, during a 0° C/32° F cold winter day, incoming air comes in the building through cracks and openings at the same cold outside temperature. Therefore, an inhabitant needs to operate heating equipment frequently to warm spaces to a comfortable room temperature. The unconditioned, uncontrolled outside air can produce uncomfortable drafts and variations in temperature throughout the building depending on the sources of leaks/infiltration. With a high-efficiency Heat Recovery Ventilator in an energy-efficient building, the fresh filtered incoming outdoor air is pre-heated and mechanically ventilated throughout the house within several degrees of the room temperature. Therefore, heating loads can be greatly reduced and uncomfortable cold drafts eliminated.

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.2 The Principle of Heat Exchange The core of heat recovery is the heat exchanger.Some heat exchangers are so efficient that up to 95% of the heat from the outgoing stale air is recovered and used to temper the incoming air from the outside. There are various types of heat exchangers available including cross-flow, counter-flow (includes rotary/wheel) and cross-counter-flow. Through a heat exchanger, fresh-filtered air flows in the house continuously and an equal volume of stale air simultaneously flows out of the home. These airflows are allowed to pass by each other- separated only by a thin membrane. The longer the two streams flow past each other, the higher the efficiency. Two distinct processes are at work here: The air flows are moved against each other by a fan in the return air flow and a fan in the supply air flow. The yellow return air becomes the brown expelled air and the green outside air becomes the red supply air. There is a temperature difference between the two air flows which pass by each other. We can see this in our previous mentioned example. The air flow emerging from the house is 20° C/36° F warmer than the incoming air, which is at 0° C/32° F. Now the heat is exchanged. Heat passes from the warm air flow to the membrane, and through the membrane; the warm air flow cools little by little. The heat passes through the membrane and into the colder air stream, and the cold air gets ever warmer. 1 2

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 In the example above with a Zehnder HRV, it can clearly be seen that the temperature difference between the incoming pre-heated air and the comfortable existing room temperature air is only about 2°C/4° F. This eliminates cold unpleasant drafts in the home. The incoming air temperature is warmed to about 18° C/64° F within 2° C/4° F of the 20° C/68° F existing room temperature. This greatly reduces both the heating costs and loads. To reach 20° C/68° F there only needs to be an energy source in the house to add the minimal extra warmth.

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.2.1 Efficiency The efficiency can be defined as follows: Using the general equation for energy transfer by convection E = mass flow × specific heat capacity c × temperaturedifference (Tintake - Toutlet) the following applies: Because Vout A = Vret A and the specific heat capacity for the two air flows is the same, so, as long as the return air is dry:

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.3 Cross-flow heat exchanger How a Cross-flow Heat ExchangerWorks Cross-flow heat exchangers can be constructed to be very compact. A significant drawback though is the heat efficiency. The supply air does not reach as high a temperature as with the counter-flow heat exchanger. In an optimal scenario it reaches about 70% efficiency. The crossing of flows means that at two corners the air streams with the greatest temperature difference meet each other. It can be seen that the upper and lower air streams with larger temperature differences encounter each other: that is the green and the yellow flows, and the brown and the red ones. At the sides however the situation is rather the counter flow principle (green flows past brown and yellow flows past red). The potential, to achieve a high temperature in the supply air using the heat from the outlet air is lost in cross-flow heat exchange. Going back to the original example, even at its optimal performance of 70% efficiency, the make-up air temperature = 32°F + (68°F - 32°F)*(0.70) or 0°C + (20°C-0°C)*(0.70) =14°C/57°F within 6° C/11° F of the 20° C/68° F existing room temperature. Therefore, one will have to spend more on monthly heating to bring the rooms to a comfortable temperature in comparison to a greater efficiency system.

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.4 Counter-flow heat exchanger How a Counter-flowHeat Exchanger Works These can be distinguished in that the counter-flow zone comprises the largest part of the device. At the beginning and end of the device there are very small zones with crossed air streams, and here too the problem of joining streams must be resolved. The efficiency in the case of very long dimensions is entirely dependent on the available surface area and in practice reaches 95%.

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.5 Rotary/Wheel heat exchanger How a Rotary/Wheel Heat ExchangerWorks A rotary/wheel heat exchanger is considered a counter-flow heat exchanger. The advantage of the rotary heat exchanger is that the air distribution is more straight-forward than in some other counter-flow heat exchangers. In the latter device there is the issue of the complex guiding of air from one side into the counter-flow zone and then on the other leading out from the counter-flow zone. The counter-flow heat exchanger also involves relatively complex membranes and complex problems of maintaining air tightness between the membranes, as the two air flows are not to mix. However it's on exactly this point that a drawback to the rotary/wheel exchanger arises. While the other devices presented above take pains to ensure the air flows do not come in contact, the rotary exchanger design accepts this will happen. The cell through which the return air flows will have outside air flowing through it just a short time later. Therefore the airtightness is not good due to the moving wheel and it's more susceptible to leakage between thefresh air and stale airstream. The air flows through a honeycomb matrix constructed in the form of a wheel. The wheel turns and the outlet air blows through one half of the wheel and as it rotates the intake air flows through. The warm air (return air) heats up the honeycomb matrix. The wheel turns and moments later the cold outside air passes from the opposite direction through the same section of honeycomb. The air takes in heat from the warmed honeycomb walls.

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701 1.6Cross-counter-flow heat exchanger (Zehnder HRV Exchanger) How a Cross-counter-flowHeat ExchangerWorks This design has several benefits: a relatively small size can be achieved; the crossing of the air streams at the front and behind resolves the problem of joining the multiple streams; the counter-flow area in the middle gives high efficiency The thermally wasteful corners are omitted thanks to the counter-flow zone. The remaining cross-flow zones do not play as crucial a role if the counter-flow zone has sufficient surface area. With this geometry too, an efficiency of up to 95% can be achieved. This is the best option for both reducing heating/cooling loads and general comfort of the occupants. 1 2 3

Zehnder America, Inc. 540 Portsmouth Ave. Greenland, NH 03840 888.778.6701

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