Calculating natural gas load involves determining the total volume or energy of natural gas required by appliances or a system over a specific period, most commonly expressed in cubic feet per hour (CFH) or British Thermal Units per hour (Btu/hr). This calculation is essential for proper gas pipe sizing, ensuring adequate supply, and managing energy consumption.
The fundamental step for determining the gas load for individual appliances is to divide the appliance's input rating by the heating value of the natural gas.
1. Determining Individual Appliance Gas Load (CFH)
To find out how many cubic feet of gas an appliance requires per hour, you use its rated energy input and the average heating value of natural gas.
The core formula is:
$$ \text{Cubic Feet per Hour (CFH)} = \frac{\text{Appliance Input (Btu/hr)}}{\text{Heating Value of Natural Gas (Btu/cubic foot)}} $$
Key Components:
- Appliance Input (Btu/hr): This is the maximum energy consumption rate of the appliance, usually listed on its nameplate or in its specifications. It represents the amount of heat energy the appliance consumes per hour.
- Heating Value of Natural Gas (Btu/cubic foot): This value represents the amount of heat energy contained in one cubic foot of natural gas. While it can vary slightly by region and gas composition, a widely accepted average for natural gas in many areas is 1,100 Btu per cubic foot.
Example:
Imagine you have a natural gas furnace with a rated input of 100,000 Btu/hr.
Using the formula:
$$ \text{CFH} = \frac{100,000 \text{ Btu/hr}}{1,100 \text{ Btu/cubic foot}} = \textbf{90.9 CFH} $$
This means the furnace will consume approximately 90.9 cubic feet of natural gas per hour when operating at its maximum capacity.
2. Calculating Total System Natural Gas Load
For a building or an entire system, the total natural gas load is the sum of the loads of all individual gas-consuming appliances. However, for practical pipe sizing and demand forecasting, it's also important to consider diversity factors and peak demand.
Steps for Total System Load Calculation:
- List All Gas Appliances: Identify every appliance in the system that consumes natural gas (e.g., furnace, water heater, stove, dryer, fireplace).
- Determine Input Rating for Each: Find the Btu/hr input rating for each appliance from its data plate or manufacturer's specifications.
- Calculate Individual CFH: Convert each appliance's Btu/hr input to CFH using the 1,100 Btu/cubic foot average heating value.
Individual Appliance CFH = Appliance Input (Btu/hr) / 1,100 Btu/cu. ft.
- Sum for Connected Load: Add up the CFH values for all appliances to get the total connected load or maximum possible demand. This represents the load if every appliance were running at full capacity simultaneously.
- Apply Diversity Factor (for larger systems): In many larger residential, commercial, or industrial settings, it's unlikely that all appliances will operate at their maximum capacity at the same time. A diversity factor (less than 1) is often applied to the total connected load to estimate the more realistic peak demand or design load. This factor accounts for non-simultaneous use.
Peak Demand (CFH) = Total Connected Load (CFH) × Diversity Factor- Note: For smaller residential systems, a diversity factor might not be used, and pipes are sized for the total connected load to ensure adequate pressure.
Common Appliance Natural Gas Loads (Examples)
The following table provides typical input ratings and calculated CFH for common natural gas appliances. Actual values may vary by model and manufacturer.
| Appliance Type | Typical Input (Btu/hr) | Calculated CFH (at 1,100 Btu/cu. ft.) |
|---|---|---|
| Furnace | 40,000 – 150,000 | 36 – 136 |
| Water Heater (Tank) | 30,000 – 75,000 | 27 – 68 |
| Water Heater (Tankless) | 140,000 – 200,000 | 127 – 182 |
| Gas Range/Stove | 65,000 | 59 |
| Gas Dryer | 22,000 | 20 |
| Gas Fireplace | 25,000 – 50,000 | 23 – 45 |
| Outdoor Grill (built-in) | 40,000 | 36 |
3. Understanding Units and Conversions
While CFH and Btu/hr are primary units, other units are often used when discussing natural gas:
- Therm: A common unit for billing natural gas. 1 Therm is equivalent to 100,000 Btu.
- CCF (Hundred Cubic Feet): A billing unit, where 1 CCF = 100 cubic feet.
- MCF (Thousand Cubic Feet): Another billing unit, where 1 MCF = 1,000 cubic feet.
Conversions:
- To convert CFH to Therms per hour:
Therms/hr = CFH × (1,100 Btu/cu. ft.) / (100,000 Btu/Therm) - To convert Btu/hr to Therms/hr:
Therms/hr = Btu/hr / 100,000
4. Factors Influencing Natural Gas Load
Beyond appliance input, several factors can influence actual natural gas load and its calculation:
- Geographic Location and Climate: Colder climates will have higher heating loads in winter.
- Building Insulation and Efficiency: Well-insulated buildings require less energy for heating.
- Occupancy Patterns: How many people are in the building and when they use appliances.
- Appliance Efficiency: More efficient appliances may have lower input ratings for the same output.
- Peak Demand vs. Average Demand: Load calculations typically focus on peak demand for sizing, but average demand is important for consumption forecasting and billing.
- Gas Pressure: The delivery pressure of natural gas affects how much gas can flow through a pipe, which is critical for system design.
By meticulously calculating the individual and total natural gas loads, engineers and homeowners can ensure that gas systems are safely and efficiently designed to meet demand. For further detailed information on natural gas properties and energy units, you can refer to resources from the U.S. Energy Information Administration (EIA).
