Control Valve Flow Characteristics- انواع صمام التحكم في دوائر التكييف

Control Valve Flow Characteristics

اختيار  الكنترول فالف  المناسب  في  دائرة   chilled  water    واحده من اهم مهام مصمم التكييف  او   شبكات الغاز

لتحميل  المقاله بصيغة  pdf
او  قرائتها  

Control Valve Flow Characteristics

تنقسم ثلاث  انواع

 

1- Quick Opening  ( on / off applications )

2-Linear. ( steam coil terminals - by pass port in three way valve )3-  Equal Percentage ( control valve in hvac coil

كما بالشكل  التالي    flow curve  للانواع الثلاثه 

الرسمه  التاليه امثله  علي انواع المحابس  حيث يكون  

equal percentage included ( ball valve - butter fly ) 
quick opening  included ( gate and globe valve )
linear include ( plug valve )

في الرسمه التاليه  يوضح لماذا  يستخدم equal percentage  مع  hvac coil

بسبب شكل heat output curve  من ال  coil   مع equal percentage curve  يعطوا  علاقه  خطيه

بمعني عند فتح المحبس ( equal percentage )   بنسبة  50 %  فأنها  تعطي 10 %  تدفق 

وعند مرور 10 %  تدفق  علي ال coil   يعطي 50 %  heat output  

فيكون التكامل بين نسبة فتح المحبس وكمية الحراره من الكويل هي علاقه  خطيه 

اي  عند فتح 50 %  من المحبس  يعطي  الكويل  50 % 

وده مبيتحققش  في نوع من انواع المحابس التانيه

نص الكود  

Quick Opening. When started from the closed position, a quickopening

valve allows a considerable amount of flow to pass for

small stem travel. As the stem moves toward the open position,

the rate at which the flow is increased per movement of the stem

is reduced in a nonlinear fashion. This characteristic is used in

two-position or on/off applications.

• Linear. Linear valves produce equal flow increments per equal

stem travel throughout the travel range of the stem. This characteristic

is used on steam coil terminals and in the bypass port of

three-way valves.

• Equal Percentage. This type of valve produces an exponential

flow increase as the stem moves from the closed position to the

open. The term equal percentage means that for equal increments

of stem travel, the flow increases by an equal percentage. For

example, in Figure 16, if the valve is moved from 50 to 70% of full

stroke, the percentage of full flow changes from 10 to 25%, an

increase of 150%. Then, if the valve is moved from 80 to 100% of

full stroke, the percentage of full flow changes from 40 to 100%,

again, an increase of 150%. This characteristic is recommended

for control on hot and chilled water terminals.

Control valves are commonly used in combination with a coil

and another valve within a circuit to be controlled. The designer

should combine the valve flow characteristics with coil performance

curves (heating or cooling) because the resulting energy output

profile of the circuit versus the stem travel improves (Figure 17).

For a typical hydronic heating or cooling coil, the equal percentage

results in the closest to a linear change and provides the most efficient

control (Figure 17).

The three flow patterns are obtained by imposing a constant pressure

drop across the modulating valve, but in actual conditions, the

pressure drop across the valve varies between a maximum (when it

is controlling) and a minimum (when the valve is near full open).

The ratio of these two pressure drops is known as authority. Figures

18 and 19 show how linear and equal-percentage valve flow characteristic

are distorted as the control valve authority is reduced

because of a reduction in valve pressure drop. The quick-opening

characteristic, not shown, is distorted to the point that it approaches

two-position or on/off control. The selection of the control valve

pressure drop directly affects the valve authority and should be at

least 25 to 50% of the system loop pressure drop (i.e., the pressure

drop from the pump discharge flange, supply main, supply riser,

supply branch, heat transfer coil, return branch, fittings, balancing

valve, and return main to the pump suction flange). The location of

the control valve in the system results in unique pressure drop selections

for each control valve. A higher valve pressure drop allows a

smaller valve pipe size and better control.

ref

2012 ASHRAE" HANDBOOK

Heating, Ventilating,

and

Air-conditioning

SYSTEMS AND EQUIPMENT

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