![]() Microwave rules of thumb have been handed down to new-hires by microwave old farts for the last century. Obviously, you must use some discretion when you apply these rules, exact results can vary widely depending on influences you haven't considered, such as the phase of the moon. ![]() What we mean by a microwave rule-of-thumb could be an inexact but notable relationship of one or more design parameters with performance, or it could just be an easy way to remember something that other lesser people often mix up. ![]() Sadly, in some countries today women have to deal with even worse treatment. However, in most cases a detailed analysis is necessary to make investment decisions.The origin of the phrase "rule of thumb" is debatable some say it was once a man's right to beat his wife with a stick no wider in diameter than his thumb. Sometimes when using rules of thumb and worst-case scenarios, decisions to implement projects can be made with no further evaluation. They are also used on the front end to quickly cull projects from an identified list of opportunities. These rules are used to quickly determine whether someone else’s complex analysis is likely to be accurate. Savings ~ 135,000 kWh, or roughly $8,100 at $0.06 gas Speed and PowerĪn extensive rule-of-thumb list is an energy analyst’s most useful tool. 85 (estimated drive loss at 50% speed) = 23,000 kWh Rule of thumb is that power varies with the cube of percent full speed. The average cooling load and therefore required chilled water flow is most likely less than 50% but to be conservative, that is what we will use. Although a throttled pump will use a little less energy than one that runs at full flow, this isn’t significant for a rule-of-thumb calculation. Savings ~ 5,000 therms, or roughly $6,000 at $1.20 gas Variable Frequency DriveĮxample: A 50 horsepower chilled water pump operates at a constant speed for 5,000 hours per year. Proposed case: ~ 8,400 therms x (1-60% recovery effectiveness) = 3,360 therms Unit could be installed to recovery 60% of the recoverable heat from the exhaust.īase case: 4,000 cfm x 1.08 Btu/hr/F x 6,500 degree days x 24 hr/day / 100,000 Btu/therm / 80% heating efficiency ~ 8,400 therms Since the space is fairly warm the balance point temperature will be high so assume 6,500 heating degree-days. Appropriate ventilation levels would be 1 cubic foot per minute per square foot of pool, or 4,000 cfm. The pool surface area is approximately 4,000 square feet. Savings ~ 350,000 kWh, or $21,000 per year at a estimated $0.06 kWh Exhaust Heat RecoveryĮxample: A swimming pool facility has a 100% outside air unit to provide makeup air for exhaust. Proposed case: 50% power x 200 hp x 0.75 kW/hp x 6,000 hours per year ~ 450,000 kWh If this compressor were replaced with a compressor with variable frequency drive control the savings could be quickly estimated as follows.īase case: 90% power x 200 hp x 0.75 kW/hp x 6,000 hours per year ~ 800,000 kWh The compressor has inlet valve control so the minimum power, even at almost no flow is approximately 80%. Compressed AirĮxample: A two hundred horsepower compressor operates 6,000 hours per year with an average load of 50%. ![]() Let’s demonstrate with some examples that can be completed in a couple minutes each. Savings can be quickly quantified using parameters such as square footage, typical ventilation rates, fan or pump characteristics, typical equipment efficiencies, and hours of operation. The origin of the commonly used “rule of thumb” phrase is debatable, but the principle underlying the modern rendition is invaluable to quickly screening energy-saving opportunities.
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