Me 421 Steam Generator and Heat Exchanger Design




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ME 421 Steam Generator and Heat Exchanger Design

Spring 2009 Design Project Report


DRAIN WATER HEAT RECOVERY SYSTEMS


GROUP 5



Emrah EMİNOĞLU

emraheminoglu@gmail.com

M.Uğur GÖKTOLGA

Ugur.goktolga@gmail.com

Bilâl KARAOĞLU

kara_oglu@yahoo.com

Ercan KİRAZ

kirazercan@gmail.com



ABSTRACT


Considerable amount of heat is wasted through drain water during shower. This heat can be recovered and used to preheat shower water by means of a suitable heat exchanger. In this project, a gasketed plate type heat exchanger is used for this purpose. After setting the plate sizes and inlet and outlet temperatures of the fluids, an initial overall heat transfer coefficient iteration method is applied to find number of plates and effective heat transfer area. The calculated pressure drop and heat transfer values proved that the design is both safe and effective and therefore available to meet the requirements. Therefore an efficient heat recovery is achieved.


INTRODUCTION AND LITERATURE SURVEY


Nowadays World faces an energy crisis. As the available fuel sources decrease year by year, new regulations considering energy consumptions are put by the governments throughout the World. It is stated by Iwamoto and Kamata [1] that in Japan it is mandatory to evaluate energy consumption of the buildings and it must be within a certain value. This demand to save energy brings the needs for new technologies. One field in which the energy saving policies can be accomplished and new technologies can be applied is recovery of heat available in waste waters.

As can be seen in Figure 1, showering, which constitutes 13% of the energy consumption in homes, has a great importance in household energy consumption.




Figure 1. Energy Consumption in Homes


According to U.S Department of Energy (DOE) approximately 80-90% of all hot water energy goes down the drain and a recent DOE study shows that energy savings of 25% to about 30% can be obtained by installing a drain waste water heat recovery system.[2]

Furthermore, in residential, the typical drain waste water heat recovery (DWHR) unit will reduce green house gas (GHG) emissions by about 200kg/person/year when displacing natural gas water heating. [3]

After searching the journals available in the web and library’s databases, it is found that many solutions regarding utilizing drain water used in shower are available in the market [4, 5]. In these works, the water drained during the shower is used to heat the tap water before heating it in the water heater of the house. Nobile [6] argued in his patent that pipe carrying drain water can be wrapped around the house’s main plumbing line such that it heats the tap water to suitable temperatures.

Gravity Film Heat Exchanger (GFX) is the most widely used heat exchanger in drain water heat recovery applications. Since it uses the gravity principle, it does not require any pump. Consequently, it is used commonly. However, due to the high initial cost (about 450$-950$ depending on size) and space occupation of GFX, Gasketed Plate Heat Exchanger (GPHE) is used instead of GFX. Gasketed Plate Heat Exchanger has high area / volume ratio, in other words, it allocates less space than GFX. Moreover, it has less cost when compared to GFX. It is easy to clean. By considering the fact that the system concerned is a shower system, cleaning of the hex will have a great importance. In addition, due to its construction, GPHE can be changed easily depending on the case which enables flexible designs.

The focus of this project is to utilize the heat in waste water in house, especially recovering the heat wasted during shower with drain water. The heat recovered will then be used to preheat the city water which is supplied to the shower at the end; therefore, less heat will be needed in order to heat the city water to the temperature needed for showering.


THEORY AND SOLUTION PROCEDURE


In order to start calculations to find the desired heat exchanger, some initial measurements and assumptions are made. First of all, the inlet temperature of cold side is taken as an average value of the city water. The hot side inlet temperature is assumed from the literature for similar applications.


The mass flow rate of the cold water is measured for an average shower. Then by neglecting the evaporation and other losses during the shower, hot water flow rate is determined to be the same with cold water flow rate.


A commonly used plate type, MIT 522 [7], is chosen initially. Therefore the plate sizes and allowable pressure drop are based on this choice.

The hot side fouling factor is chosen for waste water and cold side fouling factor is chosen for hard city water [8].


Table I Application Data


Parameter

Magnitude

Hot Side Inlet Temperature

37 °C

Cold Side Inlet Temperature

11 °C

Cold Side Outlet Temperature

31 °C

Hot Side Mass Flow Rate

0.2 kg/s

Cold Side Mass Flow Rate

0.2 kg/s

Hot Side Fouling Factor

0.00005 m.K/W

Cold Side Fouling Factor

0.0000086 m.K/W

Allowable Pressure Drop

16 bar


As an initial step, actual heat transfer is found using the inlet and outlet temperatures of cold side. Then the outlet temperature of the hot side is calculated.


Since the plate is chosen as MIT522 and the inlet and outlet temperatures are fixed, only remaining variables to calculate are number of plates, Nt, and effective heat transfer area, Ae. An iteration procedure is applied to calculate these values. Firstly, a reasonable fouled overall heat transfer coefficient is assumed and safety factor is fixed to a generally used value. Then in the following order; actual heat duty, number of plates, mass velocity, Reynolds and Nusselt numbers, hot and cold side heat transfer coefficients are found. As a final calculation, fouled overall heat transfer coefficient is found and compared with the initial assumption. Then by changing the initial assumption and observing the change in the calculated value, many iterations are performed until they become the same. Subsequently, the calculated number of plates and effective heat transfer area are checked once more using the final overall heat transfer coefficient.


Finally, pressure drops of both hot and cold side and the required pump power for the hot side is calculated. It is decided from the pressure drop for the cold side that there is no need for a pump for the cold side since city water has a higher pressure value than calculated pressure drop.


DATA, RESULTS, AND DISCUSSIONS




After some iterations, cold water outlet temperature is determined as 31 °C. Found plate numbers for different outlet temperatures can be seen from Table II.


Table II Size Change of Heat Exchanger with Parameters


Tc2 (°C)

Number of Plates

U (W/K*m^2)

ΔPc (Pa)

ΔPh (Pa)

29

11

876

215

215

30

13

783

155

155

31

17

702

119

119

32

23

536

52

52

33

27

392

21

21



A trade-off between number of plates and outlet temperature of the cold side fluid has been performed. Considering available space in the pipe system of house versus maximum possible heat transfer; 31 °C is chosen.


After fixing and inlet and outlet temperatures of the fluids and knowing the plate dimensions only variable to choose freely is chevron angle. Therefore, by changing chevron angle various characteristics of the system are observed. The first one is pressure drop of the fluids.



Figure 2. Pressure drop versus chevron angle


The pressure drop decreases with increasing chevron angle. However, since the pressure drops are very small these decreases are not so important. The pressure drops are small because heat exchanger size and mass flow rates are small. Second characteristic is pump power.





Figure 3. Pump power versus chevron angle

The pump power decreases with increasing chevron angle, too. This is predictable since pump power strongly depends on pressure drop. The pump power is calculated only for the hot side.


This is because cold side is city water and city water has much higher pressure value than pressure drop at the cold side.


However, again the changes are not so big and do not have much effect on the design. Third characteristic is net heat gain.




Figure 4. Net heat gain versus chevron angle


Net heat gain can be defined as heat added to the cold fluid for shower without the recovery system minus heat added to the system with the system minus the pump power. Since inlet and outlet temperatures are fixed, heat added to the system remains same. Heat added to the system without the recovery does not change either. Therefore the only change occurs in pump power. Since pump power changes are small, the change in the heat gain is almost negligible. Therefore it does not have an effect on the design. Fourth characteristic is actual heat duty.





Figure 5. Actual heat duty versus chevron angle


As can be seen from the figure, there occurs a considerable amount of decrease with increasing chevron angle. High actual heat duty provides high safety factor. Therefore maximum safety factor occurs at 30 degree chevron angle.


30 degree chevron angle is chosen as optimum design and calculations are performed based on this design. The results can be seen from Table III.


Table III Optimum Design Results


Parameter

Magnitude

Number of Plates

17

Cold Side Pressure Drop

119 Pa

Hot Side Pressure Drop

119 Pa

Allowable Pressure Drop

1600 kPa

Fouled Overall Heat Transfer Coefficient

702 W/K*m^2

Required Pump Work

0.04 W

Net Heat Transfer

1.672x10^-4 W



The pressure drops are very small compared to allowable pressure. Therefore the design is safe. Net heat transfer is an indication of the gain of the design. For a small system, it is a considerable amount of gain.


CONCLUSION


In this project, design of a heat exchanger for drain water recovery system is made. Throughout the project, firstly, the problem which is originated from the wasted heat during shower is determined. Secondly, a market survey is conducted and the available solutions are investigated. In this survey, advantages and disadvantages of the available solutions are indicated. Then, as an alternative and suitable solution Gasketed Plate Type Heat Exchanger is selected for the application. Finally, the required engineering calculations are made, the system is verified and the design is completed.


The main idea about the system is to use the heat energy of the water wasted during the shower and to heat the city water up to some higher temperature. By this way, the available heat energy in drain water does not go to the nature but it returns to the consumer. Consequently, less energy is consumed in the water heating unit of the houses. That results in less fuel consumption. Therefore, it can be also said that this system saves the natural sources indirectly. Moreover, it gives some help to less gas emission to the atmosphere.


At the end of the design, it is seen that the system is quite efficient. Furthermore, it is fairly desirable in economical considerations. In conclusion, the drain water heat recovery system with Gasketed Plate Heat Exchanger is a very useful and beneficial method for residential applications


REFERENCES


[1] Iwamoto, S. and Kamata, M., 2003, “The Standard for Evaluation of Energy Saving on Hot Water Supply System in Japan”, The Proceedings of CIB W62 Symposium, pp. 361-367.

[2] http://www.energy.gov/waterheating.htm, last viewed on June 5th 2009

[3] http://www.renewability.com/dhrt.htm, last viewed on June 7th 2009

[4] Holohan, D., 2008, “Products Worth Watching”, Supply House Times, 51, pp. 52-54.

[5] Karpen, D., 1999, “Gray Water Heat Recovery Device Wraps Around Drain, Unwraps Savings”, Air Conditioning Heating and Refrigeration News, 206, pp. 84.

[6] Nobile J.R, 1998, “Heat Recovery Device Showers”, US005791401A

[7] http://www.plateheatexchanger.org/plate-heat-exchanger-/standart-plate-heat-exchanger/mit-522-plate-heat-exchanger/1-standart-plate-heat-exchanger/25-mit-522-plate-heat-exchanger.html, last viewed June 6th 2009

[8] Kakaç, S. and Liu, H., 2002, Heat Exchangers: selection, rating, and thermal design, CRC Press, Florida, pp. 56-245.

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