HCLT Whitepaper: A New Approach in Control Valve Design with a New Hybrid Flow Characteristic

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

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

February 2012

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

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

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


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


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


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


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


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


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


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)

= K * V Theoretical Max* Cos Z,
V actual
Where Z is the angle between Valve
(velocity) =
plug surface and the axis
Where K
(Flow Area/Orifice Area) ^ (1/N)
empirical=
An empirical number found by the
author by correlating the results for
N =
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


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

Pr
Inlet to EEV Exit to EEV P Inlet Pr Drop
Super outlet
Temperature, Temperature, bar Require
heat K abs
C C Abs bar
bar

32 5 5 19.676 9.32 13.5

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* (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/sec
For 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.

10


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

11


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 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

12


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


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


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


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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