FOR BABER MAKAYLA ONLY

The goal of this laboratory was to compare the mass flow measurements made by several different devices. Two rotameters, an orifice plate, and a positive displacement meter were used to measure the flow rate of air through a flow loop. Different volumetric flow rates and regulated pressures were used to evaluate the change in error associated with each. Errors from 25 to 55% were found for the different flow meters when compared to the positive displacement meter. The error in the rotameter flow rates seemed to increase with increased flow rate and regulated pressure. The opposite trend was found for the orifice plate.

Volumetric and mass flow rates are an important aspect of fluid mechanics. Most industrial, commercial, and residential facilities have a need to measure mass and volume flow of various fluids. In industrial application the flow rate of fluids are used to monitor and control processes. In commercial and residential applications flow rates may be used to measure the total amount of different fluids for billing purposes. In this laboratory flow rate measurements of air were taken using four separate devices. Two rotameters an orifice plate and a positive displacement meter. The positive displacement pump (PDM) was used as the excepted value for mass flow rate. All of the other flow measurement devices were compared to the PDM based on the conservation of mass principle. Flow measurements were taken at different flow rates and also at different regulated pressures.

The different flow measurement devices were connected in a series configuration using 1.065 inch pipe. A pressure regulator is installed upstream of the flow measurement devices. The pressure regulator is connected to a shop air supply and is used to adjust the pressure entering the flow measurement rig. A diagram of the flow measurement rig can be seen in Figure 21. The air flow first enters rotameter 1 where the volumetric flow rate can be observed directly off of the meter in standard cubic feet per minute. A thermocouple and pressure gauge are placed at the exit of rotameter 1. The air then passes through the orifice plate where the differential pressure is measured with a pressure gauge. Flow measurement is then measured once again with a rotameter labeled as rotameter 2. An additional thermocouple and pressure gauge is found at the exit of rotameter 2. Lastly the air passes through a PDM before the air exits into the atmosphere.

The rotameters and orifice plate are both indirect methods of measuring flow rate. The rotameter uses a float inside of a tube with a varying area [2]. An example of a rotameter can be seen in Figure 22. The forces acting on the float are weight, drag, and buoyancy. The weight and buoyancy forces are constant for any flow rate. The drag force is proportional to velocity. As flow increase the velocity at a given location in the tube will also increase. The float will experience an unbalanced drag force and will rise in the tube until the velocity decreases to a point where the drag force is balance by the weight and buoyancy force. The scale on the rotameter is normally calibrated for flow rate. The orifice plate is an abrupt reduction in the flow area. As the flow area decreases the velocity must increase and the pressure will decrease. The drop in pressure is measures and can be used to calculate the flow rate through the meter using Bernoulli’s Equation.

The PDM is a direct flow measurement method. There are many different types of PDM’s but all operate on the same basic theory. Internal components of the meter are in the path of the flow to be measured and cycle periodically [3]. Each cycle of the components represents a certain volume of flow. Figure 23 shows a two lobed rotor type of PDM. As the flow passes through the PDM the rotors rotate and the number of rotations can be counted. The number of rotations over time can be used to calculate flow rate.

Atmospheric pressure and temperature of the laboratory were taken and recorded. The initial sensor readings were taken with a regulated pressure of 10psi. The flow adjustment on rotameter 1 was used to adjust the flow rate to 5 scfm. Pressure and temperature readings were taken at the exit of rotameter 1 and 2. A flow measurement was also taken at rotameter 2. The differential pressure of the orifice plate and PDM were observed and recorded. Finally the amount of time needed to pass 1000 cubic feet of air through the PDM was measured and recorded. The same readings were then taken for 10 and 15 scfm. The regulated pressure was then adjusted to 20psi were an additional set of readings were taken at 15scfm.

The mass flow rate of each device was calculated and reported in pounds mass per second. The volumetric flow rate for the PDM was calculated by dividing the known volume of 1000ft3 by the time recorded for each flow rate. The mass flow rate was then calculated by multiplying the volumetric flow rate by the density at the exit of rotameter 2. The mass flow rate for each of the rotameters was calculated by multiplying the actual volumetric flow rate by the density indicated at the exit of the rotameter. The actual volumetric flow rate was calculated using Equation 1. The orifice plate mass flow rate was calculated by converting the collected data to metric units and using Equation 2. The density of the fluid was inferred by the pressure and temperature data at the exit of rotameter 1. Each of the flow rates where compared to the flow rate obtained from the PDM using Equation 3.