MEC 422 Design of Thermal

Systems Project 28

Jackie ChenMatthew Stevens

Luis Lituma

Problem 64: Energy Recovery from an Electrical Generating Facility

This problem was taken from Design of Thermal Systems, 3rd Edition.Four 185 kW diesel engines are used to generate electricity at a facility that is located where power from the utility company is very expensive. It may be cost effective for the facility to generate its own power, and so it is desirable to have as little waste as possible. The proposed layout of the system is shown in figure 4. As indicated, the engine-generator unit are spaced 2 m apart, although this spacing can be changed if necessary. All are fed from a common heard that brings fuel from a tank located at an unknown (at this time) distance L away.The exhaust gases from these engines are to be used to heat water to as high a temperature as possible. The water flow rate is that typically supplied by the city, although a pump can be used if a higher flow rate is desirable. Any type of heat exchanger can be used.It has also been suggested to recover the waste heat at the radiators as well. The water that circulates through the four radiators would be used to heat water in one or more heat exchangers, again to as high a temperature as possible. The water being heated by the exhaust gases.

Figure 1 - Layout of diesel engines set up for heat recovery project

Product Design CriteriaThe cost of the project should be minimized, and at least

must be justified by economic gains from efficiency boosts.Energy recovery system should maximize harvested energy

from exhausted heat.The system should be adjustable and require minimum

technical expertise to improve its potential for retrofitting.System should be durable and robust.System should be easily maintained.Frictional losses in the piping system and thermal losses in

the exchanger should be minimized.

Product Design Specifications Tubing and manifold should be made of easily fitted materials. Cooling fluid reservoir will come from municipal sources, and be pumped

through the system. Product should be able to transfer heat available from the exhaust into

water in parallel or series flow arrangements. Payback period for system must be below 20 years. System should be resistant to corrosion from water, diesel fuel, and waste

gases produced by diesel engines. System should use the water heated from radiator jacket as the cooling

fluid of the main gas-liquid heat exchanger. System should be simple to install, clean and maintain. The outlet temperature of the cooling fluid must be as high as possible

without phase changing.

To be designed, selected, or determined:1. What typically is the fuel requirement for a 185-KW diesel engine?2. What is the power delivered by a 185-kW engine? Design the manifold

and fuel inlet line to each engine. Specify the material to be used.3. How much of the energy in the fuel is used to generate electricity? How

much energy is exhausted from these engines, and at what temperatures? How much energy is rejected at the radiator and at what temperature?

4. The heat available in the exhaust from four engines can be transferred to water in a parallel or series flow arrangement within one or more heat exchanger. The same can be said of the heat rejected by the engines at their radiators. Select flow arrangements for increased heat recovery and determine an optimum water heating scheme. Keep in mind that the objective is to obtain the highest possible outlet temperature of the water. City water is to be used with or without a pump. What is the typical pressure and flow rate delivered by the city and at what cost?

5. Calculate the cost of the fuel piping system and the cost of the heat exchangers. Devise a calculation for estimating the savings involved in the heat recovery system.

Typical 185kW diesel engine : Chosen for calculations Cummins QZ230

What typically is the fuel requirement for a 185-KW diesel engine?Typical fuel requirement for a

185-kw diesel engine is an average of 13.38 (gal/hr) at full load. This number was obtained using the data from engine spec sheet and the interpolation technique.

What is the power delivered by a 185-kW engine? Nominally, the mechanical power delivered by a

185 kW engine is 185 kW, and is reduced by engine, generator, and power transformation inefficiencies.

Design of manifold and fuel inlet line to each engine. In order to feed the four Cumming engines with

diesel from the fuel tank we need to design an intake manifold. The diesel will be transported though a single point injection manifold. A pump will be added to feed the diesel into the engines.

It was found by assigning the pipe sizes to each port and applying the modified Bernoulli that :

Due to the exceptionally low flow rate of the diesel in the fuel inlet line, frictional effects and inertia forces are low enough to be neglected.

As a result, volume flowrates in each of the manifold ports are equal to one another. That is, split between the four ports, the fuel flowrate of the diesel in each port is expressed by:

Materials and Design The distance travelled by the diesel fuel from the tank to

the engine is no more than 30 feet, and prospective problems involving interrupted or inadequate flowrates due to distance are avoided. The pipes are 4 nominal schedule 40 galvanized “black iron” (mild steel) tubes, and are joined together by welding. Black iron was chosen over copper, zinc, and iron for its resistance to corrosion in the presence of sulphur particulates in the diesel fuel, and over stainless steel for its lower cost and ease of joining by welding.

The main manifold pipe and the four ports will be joined by three T-intersections and a 90 degree elbow. The ports are all also 4 nominal schedule 40 galvanized black iron.

The manifolds for the outlet lines are the same make, material, and sizing as those for the inlet lines.

How much of the energy in the fuel is used to generate electricity? To calculate how much energy in the fuel is used

to generate electricity per engine/generator, the following formula is used:

That is, 32% of the total Lower Heating Value of the diesel is used to generate electricity.

How much energy is exhausted from these engines, and at what temperatures? This applies to one engine. From reference 3 heat

combustion is 43.4 MJ/kg and the fuel consumption rate is 0.0126 kg/s. The formula to obtain the energy rejected at the radiator is:

The energy rejected at the radiator is recovered with a water flow before being sent to the cooling fluid inlet line in the heat exchanger.

The percentage of heat not used for work and exhausted by the radiator are taken from typical values found for diesel engines.

What is the typical pressure and flow rate delivered by the city and at what cost?For this case the assumption of the power

generating plan is made to be on Suffolk County Long Island. According to reference 5 the flow rate is 21ft3/s. According to the Suffolk County water authority the average water pressure is 50 psi and the price is $20.73 service charge per quarter .

These values were obtained from the Suffolk County Water Authority website.

Market Research To analyze the market for existing solutions, three

different models of heat exchanger capable of accommodating gas to liquid heat transfer were taken under consideration.

Model1 Spirax-Sarco Heat Pipe Heat Exchanger

The Spirax-Sarco model is our choice for best design, as it can be retrofitted to a wide variety of industrial applications, using a number of materials and both water and gas as working fluids. It has consistent documented success in both cooling processes as well as recovering energy. It is designed specifically for hazardous environments, and is easy to clean and maintain. While Spirax solutions have been developed for an aforementioned wide range of applications, Spirax also vows to engineer the correct solution for an application should it not exist. The Spirax Heat Pipe heat exchanger can also be easily modified post installation, with the ability to easily change heat pipe configurations and add additional modules as required.

Model 2Bowman Exhaust Gas Heat Exchanger

The Bowman Exhaust Heat exchanger is our choice for worst design, as it is limited to small industrial applications. While we were initially intrigued by the small and simple stature of the design, ultimately this will not prove useful in recovering heat in industrial applications handling large volumes of exhaust gases.

Model 3 Cain Industries Heat Recovery Heat Exchanger The HRS Radial waste heat recovery silencer is a module configuration packet. It has 176 standard modules available. It has a full exhaust bypass, full heating surface access, factory insulation, hard shell exterior. This product is designed to receive the total exhaust liquid flow from a single source and control exit temperatures.

Modifications To adapt the chosen heat exchanger to fit the requirements

outlined in our PDS and in the project problem statement, we propose the following modifications:

A redesigned shell, cylindrical and tall rather than rectangular and wide, to increase exposure of internal vertical cooling fluid pipes to exhaust gases and permit the addition of baffles.

The addition of multiple baffles to force the exhaust gases into repeated crossflow with the cooling fluid tubes and thereby improving heat transfer.

The addition of a maintenance port in the shell wall, to retain the original design’s advantage of easy access to internal heat transfer tubes for cleaning and inspection.

The consolidation of multiple heat transfer “cassettes” into a single group of cooling tubes to reflect the need for maximized heat transfer rate and the lesser importance of modularity in our case scenario.

Assumptions In order to model the whole system and

proceed with calculations it was necessary to base our calculations on a simple case scenario. To emulate the simple case scenario the following assumptions were made.

The heat exchanger operates under steady-state conditions.Heat losses to or from the surroundings are negligible.There are no thermal energy sources or sinks in the exchanger

walls or fluids.Over the cross section in counterflow the temperature of the

each fluid is uniform.For the entire exchanger the wall thermal resistance is

distributed uniformly. There is no phase change in the fluid streams.On the walls longitudinal heat conduction in the fluids is

negligible.The individual and overall heat transfer coefficients are

constant.

The thermophysical properties of each fluid can be represented by averages throughout the length of the heat exchanger.

For large surfaces the overall extended surface efficiency is considered uniform and constant.

The heat transfer surface area A is uniformly distributed on each fluid side.

For the plate-baffles the temperature rise is considered to be small compared to the total temperature rise in the exchanger.

The temperature and velocity at the inlet of the heat exchanger on each fluid side are uniform over the flow cross section.

The working fluid rate is uniformly distributed through the exchanger on each fluid side in each pass.

Flowrate differences in manifold ports are near zero when frictional forces are negligible.

The individual and overall heat transfer coefficients are constant.

The specific heat of each fluid is constant throughout the exchanger.

For large surfaces the overall extended surface efficiency n_o is considered uniform and constant.

The heat transfer surface area A is uniformly distributed on each fluid side.

For the plate- baffled the temperature rise is consider to be small compared to the total temperature rise in the exchanger.

The temperature and velocity at the inlet of the heat exchanger on each fluid side are uniform over the flow cross section.

The working fluid rate is uniformly distributed through the exchanger on each fluid side in each pass.

Fluid Properties

Tubing Properties and Flow Areas

CAD model with modifications

CAD model Isometric View

Sectional View and Baffles

Final Schematic of the System

Calculating the cost of the fuel piping system and the cost of the heat exchanger. For the Spirax-Sarco heat exchanger there is a

lack of information on pricing schema, and its cost changes depending on customer needs. The price is given on a project demand and complexity basis. In this case, we have taken the price of 30,000 USD as an estimate for the cost of a standard heat exchanger. The price is based from comparisons with a similar product, the CDTM-40 Heat Exchanger manufactured by China Thermal Heat.

Payback Period Total Project Cost=Heat Exchanger + Materials &

LaborTotal Project Cost=30,000+3,711.39=

33,711.39

Years to payback exchanger = Total Project cost /cost savings

Years to payback exchanger = $33,711.39 purchase cost /$2,793.57 yearly savings = 12 years

References Janna, William S. Design of Fluid Thermal Systems. 3nd ed. Boston: PWS Pub.,

1998. Print. American Institute of Chemical Engineers, Effectively Design Shell-and-Tube Heat

Exchanger. Chemical Engineering Progress, February 1998. "Generator Set Data Sheet." Cummins Power, 1 Jan. 2007. Web. 20 Nov. 2014.

<http://www.cumminspower.com/www/common/templatehtml/technicaldocument/SpecSheets/Diesel/na/d-3221.pdf>.

"Approximate Diesel Fuel Consumption Chart." Industrial Diesel Generators: New & Used Generator Sets â“ We Buy/Sell� . Diesel Service & Supply, 1 Jan. 2013. Web. 20 Nov. 2014. <http://www.dieselserviceandsupply.com/>.

Water Flow Rate. National Water Authority. Web. 20 Nov. 2014. <http://www.nwda.gov.in/>.

"SCWA." Water Pressure Facts. Suffolk County Water Authority, 1 Jan. 2013. Web. 20 Nov. 2014. <http://www.scwa.com/search/?keywords=pressure>.

"Heat Exchanger For Power Plant /heat Exchanger Production Bases In China - Buy Heat Exchanger For Power Plant,First Brand Heat Exchanger For Power Plant Product on Alibaba.com." Www.alibaba.com. Web. 20 Nov. 2014. <http://www.alibaba.com/product-detail/china-first-brand-heat-exchanger-for_1337739734.html>.