6.4 Density
Density is a measurement of mass per volume that analyzes the compactness of a substance [2]. In this project, the density of each sauce would be determined to be used for Reynold’s number and NIPA calculations as described in Section 8.1. As exemplified in Equation 3, the density (ρ) values for each type of sauce would be calculated by using its mass (M) and volume (V).
ρ = M/V (3)
Before COVID-19, the plan was to measure the mass and the volume of each sauce sample by utilizing a sensitive scale and a volumetric flask respectively. Both of the needed equipment was available at the University of Rochester Chemical Engineering Department. In addition, the viscometer in Gavet 109, collects both the viscosity and the density samples. Therefore, for the sake of saving time, the team decided to compare the results of two different density experiments: using the viscometer and calculating density by measuring mass and volume of each sample. Depending on the comparison of these results, the goal was to decide which density measurements would be more efficient in order to save time. However, due to the mandatory project closure, this data collection process could not be completed.
For this density analysis, the samples from the cook kettles would be used for testing purposes. Additionally, tomato sauces tested in this project contain different levels of sugar, salt, protein, amino acids, and alcohol. This difference in ingredients will likely impact the physical properties of each sauce at different temperatures. Therefore, in order to avoid any physical variations due to temperature, the samples from the cook kettles would be tested at their operating temperatures. A hot plate and a thermometer available from the University of Rochester CHE department would be used to test the packaged samples at their operation temperatures.
Before the decision of only testing the samples from the cook kettles, the advantages of testing the packaged samples were also considered. Having density results for both the cook kettles and the packaged samples would help determining whether the physical properties of sauces change as the samples travel from the front part of the line to the end. In the plant, the ingredients are added to cook kettles where the sauce is precooked. Once the samples pass the quality tests, they are released into the pumping system. This system is used to pump the samples into the heat exchanger where high-temperature levels are utilized to finalize the cooking process. The cooked samples then travel to the packaging center where the sauces are placed into their proper packaging. Therefore, the study would show any variation in the physical properties of different sauces when the operation temperature is adjusted. This study could also be used to explore if the precook temperature can be changed in order to fix the cavitation problem. However, due the shortage of time in a single semester, Team Manticore decided to only examine the sauces from the cook kettles since this data would provide meaningful results for Reynold’s number and NIPA calculations.
8 Recommendations for the Practical Investigation
8.1 Net Positive Suction Head
To determine when cavitation occurs, it is necessary to characterize pumps. One common method of pump characterization is Net Positive Suction Head (NPSH) curves, depicted below in Figure 7.
NPSH is broken down into two subcomponents: Net Inlet Pressure Required (NIPR) and Net Inlet Pressure Available (NIPA). NIPR is the pressure required at the inlet of the pump for the pump to operate normally and draw in fluid. NIPR is most strongly influenced by pump speed as well as viscosity of the fluid. Typically, flow rate is plotted versus NIPR. A variety of graphs are made for fluids of different viscosities such that each system can be characterized accurately. NIPR curves can generally be obtained from the pump manufacturer.
On the other hand, NIPA is the pressure available at the inlet of the pump minus the vapor pressure of the fluid. NIPA is largely dependent on the piping leading up to the pump inlet, as depicted in Figure 8.
The part of the sauce delivery process concerning NIPA begins with the addition of ingredients to the open top of the cook kettles. The cook kettles are initially closed off from the pump, as the cooking procedure begins as a batch operation. The sauce is then mixed in the kettle while a jacket filled with steam provides heat for the duration of the cooking time. Once satisfactory conditions are met according to the recipe, the sauce is then pulled out of the cook kettles into the pumps, where it is moved to other parts of the plant for further processing. Now focusing on a singular kettle, NIPA analysis begins at the top of the cook kettle, which is open to atmospheric pressure. Since the kettle is partially filled with sauce, the sauce on top presses down on the sauce at the bottom, subjecting it to greater pressures at depth which assists in forcing the fluid through the piping to the pump. However, as sauce drains from the kettle, the volume and surface height of the sauce left in the cook kettle drops which in turn drops the pressure acting on the sauce in the piping beneath the kettle. 15 As sauce flows down the pipe towards the pump, it loses pressure due to friction as well. There are many formulas available to quantify these friction losses based on the piping and type of flow. To determine the fluid friction losses and the correct formula required to do so, piping must be characterized (composition, elbows, diameter etc.) and the flow regime must be determined using the Reynolds number (Equation 4).
Re = (ρvD)/µ (4)
In short, fluid density (ρ), velocity (v), viscosity (µ), and pipe diameter (D) must be known, in addition to the vapor pressure of the sauce such that it can be subtracted from the estimated available pressure, in order to determine NIPA. Once both curves are determined, they can be compared as shown in Figure 7. As long as NIPA is greater than NIPR, the pump will function correctly as enough pressure will be available to run counter to the vacuum produced at the pump inlet. However, if NIPA dips below NIPR, then there will not be enough pressure available and the pump will cease to function properly, resulting in cavitation.