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Enthalpy- facts & myths

Heat recovery unit with moisture recovery

First of all, what is an enthalpy exchanger - a specific type of construction, which allows the simultaneous heat and moisture recovery from the exhausted air. The basic difference between the enthalpy exchanger and the standard one is the use of the selective vapour-permeable cellulose membrane instead of typically used metal lamellas. In order to understand fully the principle of operation of the device, it is worth recalling what enthalpy is. It determines the thermodynamic potential of gas, which, to put it as simple as possible, is the measure of energy carried by the medium with the given parameters, i.e. the so-called heat content. In the case of air, which is the mixture of non-condensing (dry) gases and water vapour, a measure called the moist air enthalpy with the moisture content of x [kg/kg) is used. It is defined as the sum of enthalpy of 1 kg of dry air and x kg of water vapour. 

 Moisture heat recovery

where:
I – moist air enthalpy
ip – dry air enthalpy [J/kg]
iw – water vapour enthalpy [J/kg]
x – absolute air moisture [kg/kg]

 

High latent transfer rate

This means that the quantity of energy carried by the air pretty much depends on the water vapour content and the energy related to it (the so called latent heat). In many constructions, the concept of latent heat recovery contained in the exhausted air is used. Traditional solutions are based on the condensation of water on the walls of the exchanger, which increases the efficiency of the exchanger, but necessitates the discharge of condensate and, to a great extent, dries the air which is supplied into the building.

High efficiency

The effectiveness of temperature recovery (based on the temperature measurement) in the HRU-ERGO enthalpy exchanger (value up to 85%) is lower than the effectiveness of a similar, traditional counter-current exchanger. This is because the temperature of the supplied air does not increase at the expense of the condensed water. However, the water vapour with room temperature, which permeates the supplied air, has a certain energy, the consideration of which causes the total effectiveness exceeding 100% to be obtained. Despite the fact that the achievement of such a value seems impossible, it is worth noticing that similar calculations are made for CO furnaces using the condensation phenomenon, and the efficiency value above 100% results from using this effect, which is not normally used in typical solutions.

Moisture heat recovery exchangers

Illustration 1
Enthalpy and heat exchanger for HRU-ERGO

It is worth becoming aware of the fact the air with the temperature of even 15 degrees and moisture of 60% has a higher heat content than the dry air heated to the temperature of 19 C and with the moisture of 30% (parameters that can be obtained in traditional recuperation), which is obviously related to the varied quantity of heated water vapour contained in it. It would appear that we are more anxious about achievement of an appropriate temperature inside the room, as we can feel it directly, than about the high value of the pretty abstract parameter which is enthalpy.  It is obviously true that the temperature and humidity of air temperature and moisture are fundamental for our comfort, however it is the heat (expressed as enthalpy) that we really lose through the walls and windows of a building, and the effective recovery of this energy allows the building heating cost to be reduced. Paradoxically then, it easier and cheaper to maintain the constant temperature at home by supplying cooler moist air acting in a way as a thermal buffer, than the dry and overheated air. 

Illustration 2
Heat and moisture recovery unit HRU-ERGO with a controller

Antimicrobial 

The standard air heating causes its drying irrespective of the type of the used ventilation. Its relative moisture decreases significantly and the effects of this phenomenon are felt in particular during the winter period. The supply of the relatively hot and dry air leads to an unpleasant drying of mucous membranes, significantly reducing the comfort of the persons staying in the room. It is necessary then to use additional humidifiers, which, except generating investment and operational costs, decrease the supply temperature as a result of spraying water in air. This limits the benefits resulting from the recuperation and recovery of temperature in the standard unit. In the HRU-ERGO recuperator with the enthalpy exchanger, the air is moistened with water vapour containing heat energy from rooms, i.e. the temperature in the rooms is higher than the supplied air temperature. However, a question occurs - what if we want to decrease the moisture, for instance, in the kitchen or bathroom, and not to increase it? It is worth remembering that the air saturated with water vapour is continuously removed from the ventilated room. Because of the mechanism of moisture transfer inside the exchanger’s structure, the air supplied into the building contains less moisture than the exhaust air. In the overall energy balance, and above all, in feeling the climatic comfort, the enthalpy exchanger has an advantage over the exchanger with the standard construction. On top of this, there is no risk of fungi formation in the ventilation ducts, which exists in the case of duct humidifiers. 

Summing up

The advantages of the exchanger with heat and moisture recovery include:

  • very high efficiency of heat recovery, which translates into high energy savings.
  • inner moisture recovery, which entails the improvement of the climatic comfort and health conditions inside the building without using an additional humidifier.
  • no frosting, which results again in the higher annual mean efficiency of the recuperator and undisturbed ventilation.
  • easy control
 
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