The sizing starts at 0.25 mm and increases by 0.05 mm increments. These drill bit sizes have their diameters expressed in millimetres. Bigger drill sizes are available although the unit of increment is larger. Flow rate (Q) increases with the square root of the pressure increase. Altering the manifold pressure will change the flow rate through the orifice. It is normally the difference between the manifold pressure and the atmospheric pressure surrounding the orifice outlet. The pressure drop across an orifice is determined by the manifold pressure of the appliance. This K-factor alteration is the reason that some manufactures will specify that you cannot drill out the orifices when doing a fuel conversion. ![]() This helps explain why drilling out an orifice doesn’t always get the exact flow result you may expect. If the angle of approach is altered by drilling, or by changing the orifice type, the K factor will change. ![]() The orifice coefficient of flow factor (K factor) is determined by the angle of approach to the orifice and by the orifice design. The volume of gas (flow rate) that passes through an orifice can be expressed as ft3/hr or m3/hr of flow and the input as Btu/h or kW.īefore examining the sizing of gas orifices, you must be familiar with the following four factors that influence the flow of gas through an orifice (Figure 8).įigure 8 Burner showing gas flow through an orifice K Factor Figure 7 Insert type pilot orifice Sizing Orifices It gets held in place and sealed by the pilot tubing inlet fitting. It is an aluminum cone shaped insert that sits in a receptacle on the burner assembly (Figure 7). The insert type of pilot orifice has no threaded seat or gas connection. Figure 6 Spud type pilot orifice Insert type This short straight thread is used to secure the spud to the pilot burner assembly. The spud type orifice will have a threaded or flare gas supply connection on its inlet end as well as threaded seat on the orifice end (Figure 6). They come in two common designs types to accommodate their mounting to the pilot burner: Figure 5 Adjustable orifice Pilot OrificesĪs the flame is very small for a pilot burner, the pilot burner orifice opening will be smaller than the main burner orifice. The gas flow rate can be altered from zero, with the needle tight against the orifice wall, or to the full flow rate, with the needle backed away from the orifice wall (Figure 5). The needle inside the orifice spud is adjustable to increase or decrease the effective opening of the orifice. Figure 4 Cap orifice setup for natural gas Adjustable orificesĪdjustable orifices are used on some nozzle-mix and pre-mix industrial burners. For the most part the opening size in the hood becomes the fixed orifice for the required natural gas flow rate, but some flow rate adjustment is available by fine tuning the amount of gas that can squeeze through the area between the needle shoulder and the edge of the cap hole. This allows the gas to also flow both through around the needle opening and around the opening between the shoulder and the edge of the orifice in the hood (Figure 4). (Figure 3) Figure 3 Cap orifice setup for propaneįor natural gas use, having a lower heat value, a greater flow rate is required so the orifice cap or hood is unthreaded (counter-clockwise) away from the needle. For propane operation, which has a higher heat value, the cap is threaded onto the needle (clockwise), giving a fixed flow rate through the needle hole. Under the cap (or hood) of the orifice is a needle with a hole drilled through it which is sized for the propane use. Figure 1 Orifice Spud Cap (universal) orificesĬap or universal orifices are found on dual fuel appliances designed to be operated on natural gas or propane without replacing the orifices (Figure 2). The orifice spud comes with a male thread which allows it to be screwed into the manifold of the burner assembly (Figure 1). Main burner orifices come in three common types, each having particular characteristics:įixed orifices, also known as orifice spuds, are the most common orifice type and are simply a drilled opening in a brass or aluminium orifice spud. ![]() ![]() Orifices discussed in this section fall into two categories main burner orifices and pilot orifices. Types of Gas OrificesĪ gas orifice is a simple device that comes in a variety of shapes and forms. An orifice is a hole or opening used primarily in the gas industry to control the direction and amount of gas that is discharged into a burner.Ĭhanging input to the burner can only be accomplished by adjusting the manifold pressure or by changing the area of the orifice.
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