Heatloss Calculations
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Account for high blower temperatures.
The function of the stainless steel portion of a dropleg is to cool the process air before it enters the lower PVC dropleg.
The Function of the Stainless Steel Dropleg
While looking at the top of an empty aeration tank you may have observed one piece of stainless steel pipe in the system. Perhaps you have wondered why that pipe is stainless steel while everything else is PVC. The short answer is that the stainless steel portion of the dropleg is there to cool the process air before it enters the lower PVC dropleg.
Oftentimes the air coming from the blower can be 180°F (82°C) or higher. The recommended maximum mean wall temperature for PVC pipe is 130°F (55°C). If uncooled, the pipe may begin to melt and deform, compromising the integrity of the system and ultimately resulting in leaks and possible failure at the joints.
Hot process air can also result in discolored and brittle pipes making it more susceptible to damage from the natural vibrations of the pipe within the aeration basin.
If the air coming off of the blower is too hot for the run of the stainless steel pipe to cool, additional stainless steel pipe, called a cooling loop, may be provided to allow for additional cooling of the process air before reaching the PVC pipe.
When are cooling loops needed and how to specify them?
While looking at the top of an empty aeration tank you may have observed one piece of stainless steel pipe in the system. Perhaps you have wondered why that pipe is stainless steel while everything else is PVC. The short answer is that the stainless steel portion of the dropleg is there to cool the process air before it enters the lower PVC dropleg.
Oftentimes the air coming from the blower can be 180°F (82°C) or higher. The recommended maximum mean wall temperature for PVC pipe is 130°F (55°C). If uncooled, the pipe may begin to melt and deform, compromising the integrity of the system and ultimately resulting in leaks and possible failure at the joints.
Hot process air can also result in discolored and brittle pipes making it more susceptible to damage from the natural vibrations of the pipe within the aeration basin.
If the air coming off of the blower is too hot for the run of the stainless steel pipe to cool, additional stainless steel pipe, called a cooling loop, may be provided to allow for additional cooling of the process air before reaching the PVC pipe.
There are several variables that should be analyzed to determine if cooling loops are needed including:
• High ambient air temperatures (>105°F, 40°C)
• High process water temperatures (>85°F, 30°C)
• Deep tanks (>20 feet, 6 m)
• High peaking factors (which creates high air rates and may result in a high top of drop pressure)
If there is a concern that cooling loops may be required, manufactures like Sanitare can perform heatloss calculations for each grid. The length of cooling required will depend on the quantity of air going to the grid and the size/thickness of the dropleg. Therefore, each grid may have a different amount of additional cooling required.
When specifying grids that require additional cooling, it is important to state how many feet of submerged stainless steel pipe are required for each grid. Doing so ensures that, regardless of manufacturer, the aeration system provided will function during all anticipated design conditions.
