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

8/20/2019 A New Approach in Control Valve Design With a New Hybrid Flow Characteristic

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A New Approach in Control Valve DesignWith a New Hybrid Flow Characteristic

Dr. R.S. Madhusudan, ERS Mechanical Team, HCL Bangalore

F e b r u a r y 2 0 1 2


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A New Approach in Control Valve Design With a New Hybrid Flow Characteristic | February 2012

© 2011, HCL Technologies, Ltd. Reproduction prohibited. This document is protected under copyright by the author. All rights reserved.

TABLE OF CONTENTS

Abstract ............................................................................................. 3

Abbreviations .................................................................................... 4

Market Trends/Challenges ................................................................ 5

Challenges for valves in HVAC applications: .................................... 8

Solution ............................................................................................. 9

Best Practices ................................................................................. 12

Common Issues .............................................................................. 14

Conclusion....................................................................................... 15

Reference ........................................................................................ 16

Author Info ....................................................................................... 16


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Abstract

A control valve is often required to be designed for different

kinds of flow characteristics, depending on the process to becontrolled. 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 therequired 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 flowcharacteristics, 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.


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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


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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 processesrange 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 forbetter 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., themass flow through the valve causes higher pressure

loss and thus energy consumption.

Control the cost of the end product: To control thecost 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 ofheating 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 capillarytube because the mass flow of the refrigerant was

fixed.

Later, thermal expansion valves were used to expand

the refrigerant and reduce its temperature.


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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


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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 thefigure below.


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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 isreduced, 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 workingwith variable speed.

Challenges for valves in HVAC applications:

There is an increasing trend to use New Design ExpansionValves in HVAC, wherein valves with stepper motors areused 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.


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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)/1,000M (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 – PoutDensity of the fluid at the EEV inlet for Pinlet and Tinlet

Maximum theoretical velocity m/s = SQRT(2* dP/Density)

V actual

(velocity) =

= K * V Theoretical Max* Cos Z,

Where Z is the angle between Valveplug surface and the axis

Where K

empirical=(Flow Area/Orifice Area) ^ (1/N)

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 valveshape 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^2Pi/4 * Plug dia = (Pi/4*Orifice Dia^2- Flow Area)

Hence, Plug diameter is found

K empirical = (Flow Area/Orifice Area) ^ (1/n)


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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 * 3.516 (KW/TR) = 28.128 KW

With 30 % margin we design EEV for 37.2 KW

Inlet to EEVTemperature,

C

Exit to EEVTemperature,

C

Super

heat K

P Inlet bar

Abs

Pr

outlet

abs bar

Pr DropRequire

bar

32 5 5 19.676 9.32 13.5

The cooling capacity KW with 1 Kg of R410A refrigerantwith 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* (430.186- 254.136)KJ/Kg

= 176.05 KW

For the cooling capacity of 176.05KW, the mass flow of

R410A flow required 1 Kg/secFor the cooling capacity of 37.2 KW, the mass flow required

will be 37.2/176.05= 0.2113 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.


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The valve characteristic at low flow rates.

The valve trim shape at near zero flow has a hump to achievethe 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 mid-

range, the flow increases at a faster rate. In the last 90 to 100%closed condition, the flow drops to zero very gradually.


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Best Practices

The best practices for flow characteristics design are

suggested below for the respective conditions.

1. Collect the operating conditions for which the valvehas 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 flow9. 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 4

13. 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


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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 coefficient19. Repeat the design process


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Common Issues

The Electronic Expansion valve (EEV) in HVAC has toprovide 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


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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 therespective 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 liquidflow 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-gasmixture 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 energysaving. Hence, it is necessary to attain the required flow

characteristic to match the overall operation of the compressor

with the valve.


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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 degreefrom 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.


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

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