Which size orifice is required for a natural gas system with a demand of 40000 Btu at 4.0 in/wc?

To determine the required orifice size for a natural gas system with a demand of 40,000 Btu at a pressure of 4.0 inches water column (in/wc), you need to consider the relationship between gas demand, pressure, and orifice size.

Natural gas appliances are rated in Btu and require a specific orifice size to maintain proper gas flow to meet their heating demands. When dealing with natural gas, as the pressure increases, the size of the orifice required to accommodate a certain flow rate typically decreases. Therefore, a lower numbered orifice size corresponds to a larger opening, accommodating higher flow rates at the same pressure.

In this case, the demand of 40,000 Btu at 4.0 in/wc is a standard scenario that many manufacturers provide data for. Based on empirical data and flow rate tables commonly used in the industry, an orifice size of #39 is often indicated as the optimal choice for this specific demand and pressure setting.

Selecting orifice size B provides sufficient flow to meet the heating demands effectively without risking gas starvation or pressure issues, which illustrates the importance of equilibrium between the orifice size, gas demand, and pressure.