A New Approach in Control Valve Design With a New Hybrid Flow Characteristic

Transcription

1 A New Approach in Control Valve Design With a New Hybrid Flow Characteristic Dr. R.S. Madhusudan, ERS Mechanical Team, HCL Bangalore F e b r u a r y 2012

2 TABLE OF CONTENTS Abstract... 3 Abbreviations... 4 Market Trends/Challenges... 5 Challenges for valves in HVAC applications:... 8 Solution... 9 Best Practices Common Issues Conclusion Reference Author Info... 16

3 Abstract A control valve is often required to be designed for different kinds of flow characteristics, depending on the process to be controlled. The flow characteristics refer to the sensitivity of the valve spindle movement or opening to the increase in the flow. In this paper, a new hybrid flow characteristic is explained. Generally, flow characteristics are achieved by various valve trims or shapes of the plugs to be designed. The challenge lies in the design of the shape of the valve trim to achieve the required flow characteristic. Often, many iterations of design, manufacture and testing are done, and this cycle is repeated to achieve the flow characteristic. In this paper, a novel iterative method is demonstrated to achieve not only the above flow characteristics, but also their new S-shaped flow characteristic derived by the author. A new empirical relation for the flow coefficient K is derived, which is verified by CFD analysis. Further, after the design, the valve can also be virtually verified by CFD analysis. 3

4 Abbreviations Sl. No. Acronyms Full form 1 CV Control Valve 2 EEV Electronic Expansion Valve 3 P Pressure 4 V Velocity 5 VRF Variable Refrigeration Flow 4

5 Market Trends/Challenges Valves are used for controlling the flow in engineering production processes, for environment control in closed chambers, and in many other applications. The processes range from chemical processes, steam generation, pharmaceutical, food industry, textile industry, etc. The expectations from valve design are as follows. Accurate flow control by suitable valve design for better end product quality: In many cases, the quantity of the flow affects the quality of the end product. This is particularly seen in the chemical, pharma, food and textile industries. Reduce the power consumption: In many cases, such as HVAC applications, pumping applications, etc., the mass flow through the valve causes higher pressure loss and thus energy consumption. Control the cost of the end product: To control the cost of the end product, it is necessary to reduce energy consumption, and in the case of process, precisely control the quantity of costly reactants, the quantity of heating steam for heating, or the quantity of refrigerant for cooling and maintaining the temperature in a chamber. Valves are used in HVAC to achieve the required pressure, and thus the temperature drop for the refrigerant. The earlier trend was to use a capillary tube because the mass flow of the refrigerant was fixed. Later, thermal expansion valves were used to expand the refrigerant and reduce its temperature. 5

6 The present trend is to use an electronic expansion valve (EEV),which helps in energy saving. The advantage is that the movement of the valve plug is very precisely controlled in approximately 200 steps using a stepper motor. Introduction to proportional flow control valve The flow characteristics generally used are: 1. Quick opening 2. Linear and 3. Equal percentage characteristics 6

7 In order to achieve one of the above flow characteristics, one of the plug shapes shown below may be used. Valve trim is the physical shape of the plug and seat arrangement. The valve trim causes the difference in valve opening between these valves. Typical trim shapes for spindle operated globe valves are compared in the figure below. 7

8 As shown above, the expansion valve has a stepper motor on top of it. The temperature sensor senses the room temperature and regulates the mass flow of the refrigerant, thus when the mass flow is reduced, the work done by the compressor is reduced, saving the energy of the compressor. In a Variable Refrigeration Flow (VRF) system, an electronic expansion valve is used for energy saving with the compressor working with variable speed. Challenges for valves in HVAC applications: There is an increasing trend to use New Design Expansion Valves in HVAC, wherein valves with stepper motors are used to control and reduce the mass flow rate of the refrigerant when the cooling load required is less. Such expansion valves are called Electronic Expansion Valves (EEV). The EEV is used in case of Variable Refrigeration Flow Units, commonly called VRF units. The EEV used in VRF units helps in reducing the power consumption for running the compressor. The mass flow rate is reduced by reducing the speed of the compressor. 8

9 Solution A new mathematical solution for the design of a Proportional Flow Control Valve: Cooling capacity KW = M (Kg/sec) * (H evap. out H evap. in)/1,000 M (Kg/sec) is the mass flow rate of refrigerant H evap out: (KJ/Kg) Enthalpy at the exit of the evaporator H evap in: (KJ/Kg) Enthalpy at the inlet to the evaporator Pressure difference available for the flow, dp = Pin Pout Density of the fluid at the EEV inlet for Pinlet and Tinlet Maximum theoretical velocity m/s = SQRT(2* dp/density) V actual (velocity) = Where K empirical= N = = K * V Theoretical Max* Cos Z, Where Z is the angle between Valve plug surface and the axis (Flow Area/Orifice Area) ^ (1/N) An empirical number found by the author by correlating the results for valves of various sizes and capacities and with Orifice flow meter analogy For the initial calculations, the angle Z can be ignored. Later, after finding the valve trim dia at various openings, the valve shape can be drawn. Subsequently, the appropriate taper angle can be measured from the drawing and the value of angle Z can be introduced in the above equations and calculations can be repeated. Initially, we can assume the Velocity coefficient K as 0.1 to 1 linearly for openings from 10% to 100%. V actual = K * V max Volume Flow = Mass flow/density Flow Area = Volume Flow/V actual Initial rough estimate of Orifice Diameter = Sqrt [4*Flow Area/ pi()] Flow area = Pi/4* Orifice dia^2 - Pi/ 4* Plug dia^2 Pi/4 * Plug dia = (Pi/4*Orifice Dia^2- Flow Area) Hence, Plug diameter is found K empirical = (Flow Area/Orifice Area) ^ (1/n) 9

10 Check this with initial assumed value of K. If different, repeat the calculation with the new value of K. The plug or the valve trim dia is to be calculated for various % flow rates at the respective valve % position. Sample calculations and Results: The valve was to be designed suitable for a cooling capacity of 8 TR (Tons of Refrigeration) = 8 * (KW/TR) = KW With 30 % margin we design EEV for 37.2 KW Inlet to EEV Temperature, C Exit to EEV Temperature, C Super heat K P Inlet bar Abs Pr outlet abs bar Pr Drop Require bar The cooling capacity KW with 1 Kg of R410A refrigerant with the above temperature condition is calculated as follows. Cooling capacity KW with 1 Kg/s = 1 (Kg/s)* (H evap. out H evap. in)/1,000 = 1 Kg/s* ( )KJ/Kg = KW For the cooling capacity of KW, the mass flow of R410A flow required 1 Kg/sec For the cooling capacity of 37.2 KW, the mass flow required will be 37.2/176.05= Kg/sec S-shaped flow characteristics - A solution for energy saving: This new valve has an innovative hybrid flow characteristic for % flow increment for % stem movement. At low flows for linear valves flow increases drastically for small opening. This valve has equal percentage characteristic initially, and later a linear characteristic. 10

11 The valve characteristic at low flow rates. The valve trim shape at near zero flow has a hump to achieve the above flow characteristic. The new S-shaped Flow characteristic is derived by the author. The flow characteristics are such that the flow increases gradually in the initial opening. Further, in the midrange, the flow increases at a faster rate. In the last 90 to 100% closed condition, the flow drops to zero very gradually. 11

12 Best Practices The best practices for flow characteristics design are suggested below for the respective conditions. 1. Collect the operating conditions for which the valve has to function 2. This includes the ranges of the refrigerants, ambient temperatures at various geographic 3. From HVAC calculation, estimate the mass flow of fluid required to achieve the required cooling or heating 4. Conduct hand calculations for the valve port dia or the orifice dia 5. Conduct hand calculations for the design of the valve trim dia for a particular valve opening say 5% to meet the required flow (say 5% flow) at this position. For linear flow characteristic the flow at 5% valve opening will be 5% of the maximum flow. 6. Repeat the above calculations for other % valve openings and % flow 7. Prepare a CAD model of the valve trim with the valve seat 8. Conduct CFD analysis with the operating condition and the input operating condition at minimum flow, median flow and maximum flow 9. Specify the only the pressures and temperatures at the inlet and outlet for the CFD analysis. The mass flow is to be estimated by the CFD analysis 10. Check whether the mass flow obtained from CFD analysis matches with that estimated from the hand calculation. 11. If the resulting mass flow rates match, conduct CFD analysis for other valve openings 12. If the resulting mass flow rates do not match, repeat the design process from step After successfully attaining the required flow from the CFD analysis within the accepted tolerance, the entire valve manufacturing drawings may be completed 14. The valve may be manufactured 15. Test the valve for its flow characteristic 12

13 16. If the flow characteristic is not achieved, diagnose the problem 17. Benchmark the CFD with the new experimental result 18. Derive the new velocity coefficient 19. Repeat the design process 13

14 Common Issues The Electronic Expansion valve (EEV) in HVAC has to provide the right pressure drop, and thus the temperature drop, at all mass flow rates. The same EEV of a certain capacity has to meet: 1. The ranges of Mass flow rates from nearly 0.5 % of design flow to 130% of the design flow 2. Able to handle various refrigerants/fluids as per customer choice 3. Various operating conditions of temperatures depending on the country where it is sold for various operating conditions of pressure depending on refrigerants and country where it is sold 4. Flow varies depending on the application, whether for air conditioning or refrigeration or display cases 14

15 Conclusion The design calculations for the flow characteristic of a proportional flow control valve may go through a few iterations to match the flow obtained from the CFD at the respective valve position. It is observed by the author that the single phase liquid flow CFD analysis is good enough for the expansion valve flow verification. The single phase liquid flow matches fairly well with the known performance of a typical EEV valve, as was verified by the authors for a base case, though in the actual performance in an expansion valve, the fluid changes phase from liquid at the entry to a liquid-gas mixture at the exit. The single phase flow analysis will reduce the computational time and still be close to the actual performance of the valve. The flow characteristics of valves play a major role in energy saving. Hence, it is necessary to attain the required flow characteristic to match the overall operation of the compressor with the valve. 15

16 Reference General information on types of valves from the internet Author Info Dr. Madhusudan, R.S. (popularly known as Doc), SME, Fluid Power, ERS, Mechanical, HCL Tech, Bangalore He earned his Mechanical Engineering degree from the National Institute of Technology, Surat, India in 1984, his Master of Technology in 1986 from the Indian Institute of Technology, Madras, and his Ph.D. in Mechanical Engineering in 1993 from the Indian Institute of Technology, Madras. He has 28 years of experience in the design development of fluid power engineering aspects of pumps, valves for process, HVAC, compressors, blowers, and heat exchangers. Boilers, Burners and Flow meters. 16

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