Something is hissing, hammering,
frozen, roasted, or short cycling. Another service call is dispatched
to that building. If it's not noise, it's complaints about being
hot or cold, but the controls check out! What on earth is the
problem? Many times it's the simplest device in the system: the
control valve.
While simple, the water control valve is one of the main components used in HVAC control. Therefore, valve sizing is one of the most important tasks when designing a control system. Properly performing this task involves using tables or calculations to determine the best size valve for the specific application. We are going to begin exploring this process in the next few 20/20 Insights, starting with water valve sizing.
Capacity is one of the major
considerations when sizing a valve. The valve must be large enough
so that when open, full flow is delivered to the coil. The valve
must also be small enough so that at low load conditions, the
valve has enough modulated stroke to afford good control.
Valves are sized by flow capacity, which is measured by the flow coefficient (Cv). The goal is to select a valve with a Cv which will offer good control and capacity. Once the correct Cv is found, the appropriate valve may be chosen from the Kele catalog.
A table is often used to get the correct Cv (see the Technical Reference section in our catalog). This table is derived from the following formula.
Cv = GPM / square root of "delta P"
GPM is gallons per minute through the coil and "delta P"
is differential pressure across the valve at full flow. Using
either the table or the formula is acceptable.
1. The maximum GPM is a quantity that is given and can be found in the mechanical drawings for the loads to be controlled. If they are not given then there are coil capacity formulas to determine GPM which your Kele Sales Associate can send to you on request.
2. The valve pressure drop (?P) is often given in the specifications. If the pressure drop is not given, then the rules of thumb in Table 1 should help.
3. Calculate the Cv using the formula above and select the valve Cv closest to the calculated Cv. Caution: Sometimes there are wide gaps between Cv values. If your calculated Cv is not close to the smaller size valve, round up to the next larger size valve.
It's always good to double-check your valve choice with the following Rule of Thumb: The valve you pick will never be larger than the line size and will often be one pipe size smaller. If your valve falls outside this rule, double check your calculations.
The following additional criteria should be considered when selecting the correct valve for a specific application:
A) What close off pressure is required?
The safest close off is the pump head pressure in PSIG. Most often
the pump head is specified in feet of water. The conversion from
feet of water to psig is: PSIG = Feet of Water x 0.433.
Make sure your actuator can close off against this pressure.
B) What flow characteristics are needed:
Equal Percentage, Linear, or Quick Opening?
a)Control valves used for modulating flow through coils should
use equal percentage.
b)Control valves used in modulating mixing applications use linear
or equal percentage.
c)Control valves used in two position applications can havequick
opening, linear, or equal percentage characteristics.
In general an equal percentage type valve works well with any
HVAC control application.
C) What
body mounting style is required?
Most smaller valves (less than 3'') use styles such as threaded,
union, sweat or flared type fittings. However, threaded is the
most common. The larger valves use a flanged fitting.
D) The maximum temperature and pressure
are also considerations for steam and high temperature hot water
systems.
E) The next consideration is the actuator
for your valve.
Pneumatic actuators offer less cost if air is available
in large jobs, always have spring return and may be configured
with a positive positioner in the field if needed. It is important
to select the appropriate spring range for your application and
sequence. If you plan to operate from a transducer, (such as a
UCP-522) a high spring range such as 8-13# is best so you
have better return force. The drawback is that you must have a
pneumatic air system. The pneumatic actuator also invites less
precise control due to back pressure from the water causing a
spring range shift, thus changing the control pressure required
to close the valve. A positive positioner may be used to solve
this problem.
Electric actuators have a different selection criteria
including type of power, signal type and whether or not spring
return is required.
Power: 24 VAC/DC or 120 VAC
Signal: 2 position, tri state, or modulating control signals such
as (0-5V, 1-5V, 0-10V, 2-10V, 0-20 mA, 4-20 mA, or
pneumatic signals with a PVI-1 transducer)
The main advantage is very precise control without the need for
a positioner. The unit can also be ordered with multiple control
signals and power selections. The drawback is generally higher
unit cost, but may actually be less expensive if control air is
not available, which is typical on medium and small jobs.
Look for the next 20/20 Insights which will focus on another valve sizing topic. In the meantime, call Kele for solutions to all your control valve needs.
