ISSUE Many systems revert the mixing dampers back to minimum ventilation when the outside air exceeds 70 ⁰F dry bulb temperature, attempting to reduce the cooling load at the cooling equipment. Environment Canada information records the average lowest Toronto outdoor relative humidity at 32.5% RH for June, July, August and September 2010. They record the average high at 100% RH and the average mean at 73.61% RH for those months during 2010, in Toronto. Air 70 ⁰F at 32.5% RH contains 22.32 BTU/lb of dry air while air 70 ⁰F at 100% RH contains 34.08 BTU/lb of dry air. Air 70 ⁰F at 100% RH (average high) contains 52.7% more heat energy than air 70 ⁰F at 32.5% RH (average low). Switching from free cooling to no free cooling, based only on temperature is often an incorrect decision. The relative humidity, as well as the dry bulb temperature must both be considered for both the return air as well as the outdoor air when determining the lesser cooling load. This combination of humidity and temperature is called enthalpy. The psychrometric chart on page two illustrates this relationship. EXAMPLE: Pick the point where the temperature and relative humidity of your air stream intersect and follow a line diagonally up to the left, parallel with the other lines, to the enthalpy line that presents values from 0 BTU/lb of dry air to 60 BTU/lb of dry air. The point you cross presents the BTU’s/lb of dry air your air stream contains. GOAL Use the least amount of mechanical cooling in the HVAC system, while satisfying the requirements of the occupied space. REQUIREMENTS -1-Psychrometric chart. -2- Transmitters for sensing both the return air and the outdoor air relative humidity and dry bulb temperatures. -3- Two reset receiver controllers producing two pneumatic, varying signals, ranging from 3 PSIG to 15 PSIG, representing a change in enthalpy from 21 BTU/lb of dry air to 39 BTU/lb of dry air for both the outdoor air and return air. -4- Custom make two overlays for 3# to 15# transmission gauges, ranging from 21 BTU/lb of dry air to 39 BTU/lb of dry air. (A CD/DVD labeling program can be used.) -5- A relay to subtract the return air enthalpy signal from the outdoor air enthalpy signal. -6- A snap acting air switching valve or receiver controller acting as the determining point to select the air stream with the lesser BTU content. -7- Install components as per drawing. 8-108
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