X = 9, Y = 1

Question: A new salt factory is being developed at a remote location in salt affected region of Victoria. This factory does not have access to utilities such as grid electricity, piped gas and town water supply. The factory site has a bore well that can be used to meet the water requirements of the salt factory. The average annual temperature of the bore water is 20C and the average annual ambient temperature is 20C (T₀ dead state temperature) and the average annual atmospheric pressure is 100.3kPa (P₀ dead state pressure).

The proprietors of the planned salt factory have hired an engineering consulting company to design the new salt factory facility. And you work as a trainee engineer in this company. Your team leader who is a senior engineer has given you the responsibility to develop preliminary design of stand-alone utilities supply system for this facility to meet their electrical and hot water demand. The salt factory will operate from 8am to 5pm (9 hours) every day for 5 days per week.

Electrical power is required by number of processes in this salt factory that includes Drying, Burning, Filtration and Vacuum drying along with some base demand for number of other small equipment that need continuous operation. The hot water is required for two main processes that include raw salt washing and sterilization and extraction and the temperature of the hot water must be between 80C to 85C when it reaches these facility. For this reason, it is advised that the water is heated to a maximum temperature of 85C.

During the initial briefing of the project, your team leader has suggested using Diesel generator for electrical energy supply and has asked you to explore the possibility of using waste heat from the Diesel generator to produce hot water. (Note: such systems are commonly known as co-generation system)

Another engineering team that is responsible for design of the salt factory layout and process equipment selection has provided you with the estimated plant utilities demand profile (EXCEL) and the plant layout (shown in figure below).

Demand Profile:

All design activities start with literature review to develop better understanding of the existing technologies and state of the art. This also helps with preparing introduction part of the design report. When designing a thermal-fluid system the sequence should be first energy analysis followed by exergy analysis. Following steps have been proposed to help you guide through the task of developing the preliminary design of stand-alone utilities supply system.

Find:

Energy analysis: Study the electrical and hot water demand profiles to select suitable Diesel generator and determine the size of the hot water storage tank needed.

Refer to the Figure 2 that shows the schematic of the proposed co-generation system provided by your team leader (Students must use this proposed co-generation system shown in Figure 2 for the following steps). Note that there are two heat exchangers, one for heat recovery from engine coolant and second for heat recovery from exhaust gases. Bore water first receives heat from the coolant and then it is further heated by the exhaust gas. Assume that both the heat exchangers are very well insulated so there is no heat transfer between the heat exchanger and the surroundings. This means that the total heat gain by the bore water is equal to the sum of heat lost by engine coolant and exhaust gases. Further assume the engine coolant, exhaust gas and the water experience negligible pressure losses and kinetic and potential energy changes while flowing through the heat exchangers.

Present your demand profile in a tabular and graphical format. Use following sub-steps as a guide,

• Determine the total electrical power (kW) demand every hour.
• Determine the total hot water demand in (kg/min) and in (kW) every hour. Also determine the average flow rate (kg/min) of hot water and use this average flow rate value as a constant flow rate that is supplied to the heat recovery heat exchangers. Using constant flow rate allows use of a simple water pump without the need for complicated active flow control system. After the water recovers the heat it is stored in an insulated tank and you will determine the size of this tank in the following steps.

Graphically present total hourly electrical power demand (kW) and one for total hourly hot water demand (kW and kg/min), against time.

• Now select a Diesel generator from the catalogue provided that is suitable for the electrical energy demand and the heating demand for hot water supply. Note that as per the catalogue the generator can be operated between 50% load to 110% load and the corresponding fuel consumption have been given (Note that in-between values can be determined by linear interpolation). Using this information and the electrical demand for every hour, determine the available waste heat from the Diesel generator engine coolant and exhaust. Now compare total available waste heat (per day) to the total heating demand (per day) to identify suitable Diesel generator.
• For average flow rate of hot water determine the temperature of the water after the engine coolant heat exchanger T₂ (C).
• Determine the temperature of the exhaust gas after the exhaust gas heat recovery heat exchanger (C). (NOTE: It is advised that the exhaust gases should not be cooled down below 110C. This is to prevent formation of acidic condensates when the water vapour from the exhaust condenses. This will extend the life of the heat recovery system.) If the exhaust temperature is lower than 110C, then select larger capacity generator (one level up).

Graphically present total hourly temperature fluctuation of hot water after receiving heat from coolant (i.e. T₂ ) and the exhaust gas temperature after it leaves the exhaust gas heat recovery heat exchanger, against time.

• Using the information of the hot water demand and the available waste heat with average flow rate from the Diesel generator, graphically show the over supply and under supply of the hot water every hour and determine the minimum water storage needed in terms of volume and mass of hot water storage (in Ltr and kg).

The hot water storage tank must be cylindrical in the shape this is to minimise structural stresses on the tank wall. The hot water storage tank will be of vented type having a permanent open vent with discharge capacity enough to prevent the pressure in the tank exceeding the rated working pressure under the conditions of uncontrolled maximum energy input as suggested in Australian standards (AS3498:2020). (you can access the Australian standards from RMIT library database digital access for reference). You only need to be aware of this standard for reference and will not use this for any calculations.

The tank must have a 0.12m thick insulation on all outer surfaces (i.e. cylindrical surface and the top and bottom faces). The heat losses of the insulated tank will be significantly lower as compared to a tank without insulation. But heat losses should not be ignored. Based on the specification from the supplier of the insulation material, a 0.12m thick insulation will have a heat flux rate (kW/m²) of 0.15 0.01*X/9 (where X is the last digit of your student number). Heat flux rate is based on the outer surface area of the insulation wrapped around the tank (i.e. if the outer diameter of the tank is 0.5m, then the outer diameter of the tank with insulation will be 0.5 + 2 * 0.12 = 0.74m).

• Determine the dimensions (diameter and height) of the storage tank to have minimum heat losses from the tank. Please refer to the factory layout for the maximum allowable hot water storage tank dimensions (Figure 1). (Dimensions in meters.) (for minimum storage calculated in previous step, you may choose to add a small amount ( for example 3% to 5%) of volume on top of the minimum storage, but it is not requested at this stage of the design).

To present all values (answers) in a tabular format as shown below. And at least one sample calculation for every parameter that you determine. Following is an example of how to present hourly values in tabular format.

9 Y 1 Processes within the Salt Factory Burning of organic Drying material Base electrical demand Raw salt washing and sterilization Extraction Time (duration) Filtration and Vacuum Drying Constant electrical power (kW) Average hot water flow rate @ 85 C (kg/min) Constant electrical power (kW) Constant electrical power (kW) Average hot water flow rate @ 85 C (kg/min) Constant electrical power (kW) 8 to 9 AM 6 5.3 6.15 5.8 9 to 10 AM 6 5.3 5.9 3.1 5.9 4.5 10 to 11 AM 6 6.15 5.9 7.3 11 AM to 12 PM 6 5.3 6 3.1 4.2 12 PM to 1 PM 6 5.3 6.15 5.9 5.8 1 PM to 2 PM 6 6.15 5.8 2 PM to 3 PM 6 2.65 6.4 3.1 5.9 4.4 3 PM to 4 PM 6 5.3 6.15 5.9 5.8 4 PM to 5 PM 6 7.15 3.1 5.9 3.5 NOTE: Constant electrical power (kW) values provided are for the entire hour. Average hot water flow rate @85C (kg/min) values provided are for the entire hour. So to estimate the cumulative mass for one hour you multiply the average flow rate with 60min. Salt factory layout Filtration Extraction Vacuum Drying Burning of organic material Insulated hot water pipe Electric power line Raw salt washing and sterilization Drying D Insulated hot water pipe Potential diesel generator site Potential hot water system A Insulated tank maxim um diameter must not exceed 1.6m And height must not exceed 2m Figure 1 Salt Factory Layout Texh out = ? (110C) Warm exhaust gases to atmosphere mexh Hot water to salt factory 80C to 85C Coolant to water heat exchanger Exhaust to water heat exchanger Insulated hot water storage tank T,=??C T1 =20C m water Cold water in Tz = 85 C 7 coolant out 1 - 35C T coolant in Linginc Exhaust 85C exh in =455C Diesel generator Electric Diesel and air power out Co-generation system schematic Figure 2 Proposed heat co-generation schematic Diesel Generator Rental Catalogue Radiation heat loss Generator Diesel - MP 11 output rating Exhaust system Fuel consumption Generator Combustion air (100% load) Rental cost of inlet flow rate Cooling System to the surroundings Fuel Heat rejected to from engine and Consumption the engine alternator outer (Diesel) (100% coolant @ 85C surface at 364K load) ) (100% load) temperature (100% load) Generator 50Hz /230V (100% load) Exhaust gas flow Exhaust gas Model number Power factor 110% Load 100% Load 75% Load 50% Load (100% load) temperature 0.8 kWe L/hr kW KW m/min m/min "C L/hr L/hr L/hr L/hr $/ Week MP-12 - DC3E 12 4.3 13.6 6 1.2 3.1 455 4.8 4.3 3.3 2.4 $ 125 MP-14 - DC3E 14 5.1 16.1 7.1 1.4 3.6 455 5.6 5.1 3.9 2.8 $ 144 MP-16-DC3E 16 5.8 18.3 8.1 1.6 4.1 455 6.4 5.8 4.5 3.2. $ 164 MP-18 - DC3E 18 6.5 20.6 9.1 1.8 4.7 455 7.2 6.5 5 3.6 $ 187 MP-21 - DC3E 21 7.6 24 10.6 2.1 5.4 455 8.4 7.6 5.8 4.2 $ 212 MP-24 - DC3E 24 8.7 27.5 12.1 2.4 6.2 455 9.6 8.7 6.7 4.9 $ 239 MP-27 - DC3E 27 9.8 31 13.7 2.7 7 456 10.8 9.8 7.5 5.5 $ 268 Note: L/hr stands for Litre per hour. Diesel specific gravity = 0.85; kWe stants for kilo Watts of electrical power; Air intake from atmospheric pressure and exhaust out to atmospheric pressure; Diesel calorific value 45MJ/kg; Time (duration) Sum of electrical power for all processes (kW) 8 to 9 AM 9 to 10 AM 10 to 11 AM 11 AM to 12 PM 12 PM to 1 PM 1 PM to 2 PM 2 PM to 3 PM 3 PM to 4 PM 4 PM to 5 PM Sum of average hot water Time (duration) flow rate for all processes together (kg/min) 8 to 9 AM 9 to 10 AM 10 to 11 AM 11 AM to 12 PM 12 PM to 1 PM 1 PM to 2 PM 2 PM to 3 PM 3 PM to 4 PM 4 PM to 5 PM