ΔTmin analysis when applying pinch technology to design heat recovery exchanger of tube ice machine

du.vn Abstract. In current situation, ice product will not be enough to meet higher demand in the coming years. So, the construction of ice machine suitable to production need, reasonable price with high efficiency which are always put on top by investors. Particularly, the urgent requirement of economical machine price and efficient use of energy, the application of Pinch technology for calculation and design to optimize the heat recovery component, increase system productivity is pioneering application research. The applied Pinch technology 3 tons/day tube ice machine capacity brings higher specific cooling capacity, heat recovery of heat recovery component and the coefficient of performance compare to traditional designed tube ice machine, alternate as follows: o Heat recovery: QRe-Tra = 2.01 kW, QRe-Pinch = 6.17 kW (increase 207%). o Coefficient of performance: COPTra = 3.61, COPPinch = 3.72 (increase 11%). Keywords. Tube ice machine, heat recovery pinch, efficient tube ice machine, pinch design. PHÂN TICH ΔTmin KHI ỨNG DỤNG KỸ THUẬT PINCH TÍNH TOÁN THIẾT KẾ BỘ HOÁN NHIỆT CỦA MÁY NƯỚC ĐÁ ỐNG Tóm tắt. Trong tình hình hiện nay, sản phẩm nước đá sẽ không đủ đáp ứng nhu cầu tăng cao trong những năm tới. Vì thế, cấu trúc của máy sản xuất nước đá phải phù hợp với nhu cầu sản xuất, giá cả hợp lý với hiệu suất cao luôn luôn được các nhà đầu tư đặt lên hàng đầu. Cụ thể, yêu cầu cấp thiết là cạnh tranh về giá máy và hiệu quả sử dụng năng lượng nên việc ứng dụng kỹ thuật Pinch cho việc tính toán và thiết kế để tối ưu bộ hồi nhiệt, tăng năng suất hệ thống là nghiên cứu ứng dụng tiên phong. Việc áp dụng kỹ thuật Pinch cho máy sản xuất nước đá ống có năng suất 3 tấn/ngày có năng suất lạnh riêng cao hơn, nhiệt lượng thu hồi từ bộ thu hồi nhiệt cao hơn, và hệ số hiệu quả nhiệt cũng cao hơn so với máy nước đá ống có thiết kế truyền thống lần lượt như sau: o Nhiệt thu hồi: QRe-Tra = 2.01 kW, QRe-Pinch = 6.17 kW (increase 207%). o Hệ số hiệu quả nhiệt: COPTra = 3.61, COPPinch = 3.72 (increase 11%). Keywords. Máy nước đá ống, pinch thu hồi nhiệt, máy nước đá ống hiệu quả, pinch thiết kế. 1 INTRODUCTION The burgeoning global food and beverage industry is supporting the adoption of ice machine systems. The ice maker segment dominates the global ice production industry as the use of these machines increases in the residential and commercial segments. The need to use ice cubes for alcoholic beverages consumption and refreshments at home, offices, universities, hotels, bars, ... are supporting the increase in sales of the machines. The size of the global ice making market in 2018 was more than US $ 1.5 billion and is estimated to grow by 6% from 2019 to 2025. When assessing the market's import / export countries, it was found that the United States was the largest importer of ice machines, followed by Germany, France, Great Britain HNKH-17 Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -165- and Canada. On the other hand, China represents the next largest exporter, followed by Mexico, Italy, the United States and South Korea [1]. In recent years, global demand for ice machines has increased, with a growth rate of around 5% -20%, depending on the economic, industrial and environmental conditions of each country and area [2]. The major global competitors in ice production are KTI in Germany, Manitowoc in the US, Scotsman in Italy, Iceman in Japan, Hoshizaki in Japan, Northstar in the United States, Geneglace in France and Snowman in China. The ever-increasing demand for ice has pushed ice production facilities to always run at full capacity and expand production. According to statistics on yellowpages.vnn.vn. In Ho Chi Minh City, there are over 200 production facilities, are distributing ice. Companies and production facilities of ice machines are constantly improving to meet the needs of customers. Research activities are largely focused on refrigerant, treatment of ice making water, improvement of ice machine performance. In particular, in the urgent requirement of economical and efficient use of energy, the application of Pinch technology to the calculation and design to optimize the heat recovery component, increase system productivity is a pioneering application research. On the Vietnamese market today, there are many ice making machines of different brands with many different capacities (output from 300 ÷ 1800 kg/day for small households and small businesses; 5 ÷ 10 tons/day for medium facilities; and over 25 tons/day for industrial factories) [3]. In order to serve small and medium households business, limited space, an tube ice machine with a capacity of 3 tons/day is reasonable on building basis formulae system for calculation, detailed design of tube ice machine parts. The principle diagram of 3 tons/day tube ice machine is shown in figure 1. In order to minimize the energy consumption of this tube ice machine, maximizing heat recovery of the heat recovery component is considered by the Pinch technology. 2 APPLY PINCH TECHNOLOGY TO THE CALCULATION AND DESIGN 2.1 Using Pinch technology to calculate heat transfer of heat recovery component The parameters at the heat recovery component: Figure 1. Principle diagram of the tube ice machine Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -166- o Liquid refrigerant temperature go into the heat recovery component: t4 = 39 oC o Liquid refrigerant temperature go out from the heat recovery component: t5 = 34 oC o Vapor refrigerant temperature go into the heat recovery component: t8 = -15 oC o Vapor refrigerant temperature go out from the heat recovery component: t1 = -5.4 oC o Mass flow rate of liquid and vapor refrigerant moving in the tube: ml = 0.083 kg/s o Mass flow rate of vapor refrigerant moving outside the tube: mv = 0.083 kg/s o Specific heat of liquid refrigerant: Cp-l = 4.845 kJ/kg.K o Specific heat of overheating vapor refrigerant: Cp-v = 2.517 kJ/kg.K Cool and hot flow determination Cooling effect and heating effect: o Qheat = 0.209 x |−10 ± −5.4|= 2.01 kW o Qcool = 0.402 x |34 − 39| = 2.01 kW In table 1, the heating effect and cooling effect under steady state condition (constant heat capacities and temperatures) which must be supplied by external heater and cooler such as steam water and cold water. The idea is now to find the lowest possible Qcool for this particular problem. Now, the inlet and outlet temperatures can be divided into temperature interval numbers where the first temperature interval is between the maximum and the second largest. The next interval is between the second and third largest temperature, ... Results between such temperature intervals are shown in table 2. One can calculate the amount of heat to be supplied to a plant (called Qheat) and how much heat must be Figure 2. Heat recovery component Table 1. Process stream in the heat recovery component Process stream Inlet temp. [oC] Outlet temp. [oC] Heat capacity rate, m.Cp [kW/K] Q [kW] 1. Cold -15 -5.4 0.209 2.01 2. Hot 39 34 0.402 2.01 Table 2. Temperature interval and external heating and cooling effects Interval Number Temperature interval [oC] Stream Numbers Qcood [kW] Qheat [kW] ΔQ [kW] 1 39 – 34 2 2.01 0 2.01 2 34 – -5.4 0 0 0 0 3 -5.4 – -15 1 0 2.01 -2.01 Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -167- taken from the plant (called Qcool). The relationship between external cooling and heating can be written: ΔQ = Qcool-1 – Qheat-1 = 2,01 – 0 = 2,01 kW The ΔTmin value The best heat exchanger design must satisfy the technical and economic considerations. This depends directly on the choice of mean temperature difference ΔTmin. The smaller the Tmin, the larger the heat exchange area, resulting in lower energy costs but higher investment costs. Therefore, the total annual cost is the function of ΔTmin is the combination of energy cost (effectiveness) and investment cost. These objective functions are presented as follows [4]: TAC ($year−1) = aCin + Cop) Cin ($) = b1 x Atot b2 Cop ($year −1) = (kel τ ∆PVt ηis ) c + (kel τ ∆PVt ηis ) h Here, Cin, Cop, kel, Ꞇ, ΔP, Vt, and ηis are the investment cost, operational cost, electricity unit cost, operational hours in a year, pressure drop, volumetric flow rate, and isentropic efficiency of the pump, respectively. Also, b1 and b2 are considered to be constant, and a is the annualized factor presented below: a = i i − (1 + i)−n Where i and n are the interest rate and system lifetime, respectively. For calculating the heat recovery component in the tube ice machine system, the low temperature process with ΔTmin = (3 ÷ 5 oC). We choose the temperature ΔTmin = 4 oC. ΔTmin = 4 oC is reduced from hot streams, meaning the hot stream will be cooled by 4 oC. Therefore, a new temperature range that can be calculated is indicated in Table 3. Figure 3. Cost distribution of ΔTmin Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -168- The Problem Table Algorithm (PTA) Where Ti and Ti+1 corresponds to upper and lower temperatures in the arbitrary temperature interval i. The important following condition: o Di < 0 need for cooling o Di > 0 need for heating The energy balance for the arbitrary block i can be calculated by Qi,i+1 = Qi-1,1 - Di Where Qi-1,i and Qi,i+1 are the supplied heat and removed heat respectively for each block. Initially the supplied heat Q0, for block 1 is set to Q0,1 = 0. Table 4 shows the calculated supplied and removed heat for each temperature interval (block): Table 3. Temperature interval for Tmin = 4 oC. Interval Number Temperature interval [oC] Stream Numbers Qcool [kW] Qheat [kW] ΔQ [kW] 1 35 – 30 2 2.01 0 2.01 2 30 – -5.4 0 0 0 0 3 -5.4 – -15 1 0 2.01 -2.01 Table 4. Sequential balance problem with ΔTmin = 4 oC Sequential balance Max table Interval Temp. limits Di • Qi−1,i • Qi,i+1 • Qi−1,i • Qi,i+1 1 35 – 30 2.01 0 -2.01 2.01 0 2 30 – -5.4 0 -2.01 -2.01 0 0 3 -5.4 – -15 -2,01 -2.01 0 0 2.01 Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -169- The combination of heat flows and temperature is shown in Figure 4 from the results of Table 4. From the Grand composite curve of heat recovery component curve inform about the received heat flow and the supplied heat flow is obtained through a pinch point temperature of 18.5 oC. That is, the cold stream has the potential to receive more heat to further increase from -5.4 oC to 14.5 oC and the hot stream has the potential to remove more heat to reduce the temperature from 30 oC to 18.5 oC with ΔTmin = 4 oC. Design of Heat Exchanger Network (HEN) In order to design the HEN for the example above it is useful to create tables in which the streams specifications above and below the pinch temperature are shown, see tables 5. Note that the pinch temperature was 18.5 oC, therefore, above the pinch temperature the cold streams shall be heated from 14.5 oC (if ΔTmin = 4 oC) while the hot streams must be cooled to 18.5 oC. For stream above the pinch point, the hot stream 2 need 2.35 kW (8.24 – 6.17 = 2.07 kW) can be provided by external cooling (Table 5). Figure 4. Grand composite curve of heat recovery component Table 5. Process streams above and below the pinch with ΔTmin = 4 oC Process stream Inlet Temp. (oC) Outlet Temp. (oC) Heat capacity rate m.Cp (kW/K) Q (kW) 1. Cold - 15 14.5 0,209 6.17 2. Hot 39 18.5 0.402 8.24 Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -170- After determining the heat flows above and below the pinch temperature, the designer calculates all heat exchangers to recover heat from all the hot and cold streams of the plant. In this tube ice machine, the heat needed to recover to increase the cooling capacity is only in the heat recovery component. So, there is only 1 flow below and one flow above the pinch temperature. When the liquid line is sub cooling (hot stream 2) to maximize heat, the difference in heat between the two flows is ΔQ = 2.07 kW. Here, the average temperature of cold stream 1 is 14.5 oC and the average temperature of hot stream 2 is 18.5 oC. The temperatures of both flows are now lower than the ambient temperature in Ho Chi Minh City (31.5 oC). So, the heat from the environment will penetrate into the system if it is not well insulated. Solving the problem of the number of heat flows and choosing the number of heat exchangers, designer must consider whether the combination of these heat flows is suitable and the minimum number of heat exchangers is required to save the investment costs. In this study, the heat transfer area of the heat recovery component only needs to be increased to exchange the heat from 2.01 kW to 6.17 kW. 2.2 Recalculate the tube ice machine cycle when applying Pinch technology The working mode of the machine is characterized by the following parameters: o Refrigerant: R22 o Evaporating temperature: t8 = -15 oC o Condensing temperature tk: t4 = 39 oC o Sub-cooling liquid temperature before the expansion valve tql: t5 = 18.5 oC o Refrigerant vapor temperature drawn to the compressor th: t1 = 14.5 oC Figure 5. Grid diagram of heat exchanger network Table 6. Compare the applied Pinch technology and non-applied Pinch technology to the system Specification The system not applied Pinch technology The system applied Pinch technology Heat recovery in heat recovery component Qr (kW) 2.01 6.17 Coefficient of Performance COP 3.61 3.72 Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -171- 3 ECONOMIC TECHNICAL ASSESSMENT The economic and technical calculations for investing in a tube ice machine with a capacity of 3 tons/day. The capital cost is calculated on the basis that the machine is manufactured in Vietnam. The profit is from the selling price of ice minus the cost of production: o Selling price of ice is 8000 VND for each bag of 21 kg of ice. o Production cost include: cost of water, cost of electricity and cost of labor. Capital cost: o Materials and equipment cost of the tube ice machine: 226.589.025 VND. o Labor cost for the construction of the tube ice machine: 57.000.000 VND. o Capital cost of the tube ice machine C = 283.589.025 VND. Payback time calculation With the production time, each day producing 2 shifts (16 hours), each year is 317 days, the payback period of the traditional design tube ice machine is 3.75 years and that of the pinch design tube ice machine is 3.65 years, 1.2 months down (decrease 2.7% payback period). The inflation rate and discount rate are 8% and 12% respectively. The profitability of the tube ice machine according to the pinch design is higher than that of the traditional design as shown in Table 6. Figure 6. Payback period of pinch design and traditional design Hội thảo CÁC NGHIÊN CỨU TIÊN TIẾN TRONG KHOA HỌC NHIỆT VÀ LƯU CHẤT Khoa Công nghệ Nhiệt Lạnh -172- 4 CONCLUSION Research shows that 3 tons/day capacity tube ice machine with Pinch technology to the calculation and design to optimize the heat recovery component bringing higher heat recover from heat recovery component and coefficient of performance compare to traditional designed tube ice machine, alternate as follows: 1. Heat recovery: QRe-Tra = 2.01 kW, QRe-Pinch = 6.17 kW (increase 207%) 2. Coefficient of performance: COPTra = 3.61, COPPinch = 3.72 (increase 11%) 3. Profit of 1 batch (VND): 7.204 VND/batch vs 7.405 VND/batch (increase 2.8%) 4. Payback time (year): 3.75 years vs 3.65 years (decrease 2.7%) REFERENCES [1] A. Alpher, Frobnication, Journal of Foo, vol. 12, no. 1, pp. 234-778, 2002. [2] A. Alpher and J. P. N. Fotheringham-Smythe, Frobnication revisited, Journal of Foo, vol. 13, no. 1, pp. 234-778, 2003. [3] https://www.vogtice.com/products/. [4] Zahra Hajabdollahi, Hassan Hajabdollahi, Kyung Chun Kim; Heat transfer enhancement and optimization of a tube fitted with twisted tape in a fin-and-tube heat exchanger; Journal of Thermal Analysis and Calorimetry; Akade miai Kiado, Budapest, Hungary 2019 [5] Sepehr Sanaye*, Hassan Hajabdollahi; Multi-objective optimization of shell and tube heat exchangers; Applied Thermal Engineering 30 (2010) 1937e1945 [6] A. Alpher, J. P. N. Fotheringham-Smythe, and G. Gamow, Can a machine frobnicate?, Journal of Foo, vol. 14, no. 1, pp. 234-778, 2004. [7] V. Arnold, K. Vogtmann, and A. Weinstein, Mathematical Methods of Classical Mechanics, ser. Graduate Texts in Mathematics. Springer, 1989. [8] FLEXChip Signal Processor (MC68175/D), Motorola, 1996. [9] M.-T. Pham, O. J. Woodford, F. Perbet, A. Maki, and B. Stenger. (2012) Toshiba CAD model point clouds dataset. [Online]. Available: Points. [10] M.-T. Pham, O. J. Woodford, F. Perbet, A. Maki, B. Stenger, and R. Cipolla, A new distance for scale-invariant 3D shape recognition and registration, in Proc. Int. Conf. on Computer Vision, 2011, pp. 145-152. [11] M.-T. Pham, O. J. Woodford, F. Perbet, A. Maki, B. Stenger, and R. Cipolla, An image processing method and system, US Patent 20130016913 (A1), 2013. [12] L. A. Santalo, Integral geometry and geometric probability, in Encyclopedia of Mathematics and its Applications, G. C. Rota, Ed. Addison-Wesley, 1976, vol. 1. Table 6. Compare the obtained Interest of pinch design and traditional design Specification The machine of traditional design The machine of pinch design Profit of 1 batch (VNĐ) 7.204 7.405 Profit of 1 day (VNĐ) 230.523 236.954 Profit of 1 year (VNĐ) 73.076.000 75.115.000 Payback time (year) 3.75 3.65