xdxadmin
08-31-2004, 09:19 AM
Natural Humidification used to be common place when our industry used the Off-Cycle Defrost method of maintaining higher humidity in the conditioned space. Re-Humidification of the air after product has been dried cannot re-humidify product. Defrost Considerations Concerning Humidity should be weighed in efforts to reduce the drying of product.
It is commonly held that if an evaporator temperature operates below 32F, that it must build frost; if the evaporator is colder than the air that it must collect condensate or frost. It is easy to understand that the evaporator temperature can sufficiently break the melting point, and to do so for a substantially long enough duration can allow the frost to melt from the coil
More difficult to understand is the concept of sublimation. From day-to-day, children experience, the change in the surface of the frozen pond while ice skating. The ice doesn’t melt, but rather sublimates due to the evaporative work of the dry air moving across the surface.
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http://xdxusa.com/images/technology/frostsub.gif
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Consider that the above X-STREAM® Technology / XDX® equipped system, which has annular flow within the entry passes of the evaporator, and that the evaporator surface is near the Dew Point of the surrounding air. When the evaporator surface approaches or exceeds the temperature of the air, the moisture on the coil is forced into the air.
In the example shown, the same .00017 pounds of water per pound of dry at -26F falls below 50% Relative Humidity at -13F when the vapor fraction within the evaporator warms the surface of the tubing, and does so even into the refrigerating portion of the refrigeration cycle.
In our industry frost and defrost is associated with temperature, and while the air-conditioning segment of the industry deals with wet bulb and dry bulb, the service portion of the refrigeration side is usually only concerned in practice with ambient conditions and infiltration. Frost and defrost is actually a consideration of Dew Point, not temperature. Condensate or frost will only form on a surface that is colder than the dew point of the air around it, and will only build on that surface if the surface is consistently colder and if the velocity of the air allows it to collect. The greater the CFM, the less likely that frost will build.
Our industry is familiar with determination of evaporator sizing with consideration given to fin-spacing and Air-side Delta T (TD). Fin spacing directly affects sustained CFM. A 10F TD evaporator will tend to frost less than a 15F TD Evaporator, and more than a 6F TD evaporator when used in the same application, and does so through changes in coil surface area to BTU relationships, air-distribution and air-flow. The amount of usable surface-area and the air-side impact directly associate with Dew Point.
When frost is allowed to sublimate, or when melting condensate is evaporated, the humidity in the conditioned space is increased, and this has a beneficial effect upon product quality, infiltration, load, and Defrost Considerations Concerning Humidity. X-STREAM® Technology from XDX® can help keep humidity higher in your refrigerated space which keeps more moisture in your product. This not only reduces infiltration of humidity into the conditioned area, but reduces the amount of potential frost as well as the latent load that must be removed through the refrigeration process.
It is commonly held that if an evaporator temperature operates below 32F, that it must build frost; if the evaporator is colder than the air that it must collect condensate or frost. It is easy to understand that the evaporator temperature can sufficiently break the melting point, and to do so for a substantially long enough duration can allow the frost to melt from the coil
More difficult to understand is the concept of sublimation. From day-to-day, children experience, the change in the surface of the frozen pond while ice skating. The ice doesn’t melt, but rather sublimates due to the evaporative work of the dry air moving across the surface.
<CENTER>
http://xdxusa.com/images/technology/frostsub.gif
</CENTER>
Consider that the above X-STREAM® Technology / XDX® equipped system, which has annular flow within the entry passes of the evaporator, and that the evaporator surface is near the Dew Point of the surrounding air. When the evaporator surface approaches or exceeds the temperature of the air, the moisture on the coil is forced into the air.
In the example shown, the same .00017 pounds of water per pound of dry at -26F falls below 50% Relative Humidity at -13F when the vapor fraction within the evaporator warms the surface of the tubing, and does so even into the refrigerating portion of the refrigeration cycle.
In our industry frost and defrost is associated with temperature, and while the air-conditioning segment of the industry deals with wet bulb and dry bulb, the service portion of the refrigeration side is usually only concerned in practice with ambient conditions and infiltration. Frost and defrost is actually a consideration of Dew Point, not temperature. Condensate or frost will only form on a surface that is colder than the dew point of the air around it, and will only build on that surface if the surface is consistently colder and if the velocity of the air allows it to collect. The greater the CFM, the less likely that frost will build.
Our industry is familiar with determination of evaporator sizing with consideration given to fin-spacing and Air-side Delta T (TD). Fin spacing directly affects sustained CFM. A 10F TD evaporator will tend to frost less than a 15F TD Evaporator, and more than a 6F TD evaporator when used in the same application, and does so through changes in coil surface area to BTU relationships, air-distribution and air-flow. The amount of usable surface-area and the air-side impact directly associate with Dew Point.
When frost is allowed to sublimate, or when melting condensate is evaporated, the humidity in the conditioned space is increased, and this has a beneficial effect upon product quality, infiltration, load, and Defrost Considerations Concerning Humidity. X-STREAM® Technology from XDX® can help keep humidity higher in your refrigerated space which keeps more moisture in your product. This not only reduces infiltration of humidity into the conditioned area, but reduces the amount of potential frost as well as the latent load that must be removed through the refrigeration process.