Importance of CFM Calculation:
Ensures safety, enhances energy efficiency, extends equipment lifespan, and contributes to better indoor air quality.
Differences in CFM Calculation:
Gas Furnace: Heat loss through the flue pipe reduces efficiency.
Electric Furnace: 1:1 input-output ratio (BTU in = BTU out).
Temperature Rise Method:
Used to calculate airflow (CFM) in gas furnaces.
Formula involves dividing the furnace’s heat output (BTUs/hour) by the temperature difference (T) and multiplying by a constant for standard air conditions.
Process for Gas Furnace:
Step 1: Clock the gas meter to measure cubic feet per hour (from previous lesson: 75 cubic feet/hour).
Step 2: Convert cubic feet to BTUs (e.g., 75 cubic feet/hour × 1000 BTUs = 75,000 BTUs input).
Step 3: Adjust for efficiency using a combustion analyzer (e.g., 85% efficiency = 0.85).
Step 4: Multiply input BTUs by efficiency (e.g., 75,000 × 0.85 = 63,750 BTUs output).
Step 5: Use a two-channel thermometer to measure the temperature difference (T) between supply air and return air (e.g., supply = 120°F, return = 70°F, difference = 50°F).
Step 6: Multiply the temperature difference by the constant for standard air (1.08).
Step 7: Divide BTUs output by the result of the previous step to get CFM (e.g., 63,750 ÷ 54 = 1181 CFM).
Example:
Using the example of 75,000 BTUs input, 85% efficiency, and a 50°F temperature difference:
CFM = 63,750 ÷ (1.08 × 50) = 1181 CFM.
No comments:
Post a Comment