Dynamics of Hydronic Systems - JCI
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Transcript of Dynamics of Hydronic Systems - JCI
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Valve and Actuator Manual 977
Hydronic System Basics Section
Engineering Bulletin H112
Issue Date 0989
1989 Johnson Controls, Inc. 1Code No. LIT-351H112
A hydronic system can be configured in many different ways. This report
will discuss the following types:
1. Reverse Return
2. Direct Return
3. Primary/Secondary
The variant configurations of the hydronic systems noted above will be
analyzed as well as the relative advantages and disadvantages of each.
Part-load characteristics will also be considered.
Any of the three types of hydronic systems could be utilized in either a
heating or cooling system. To simplify the remainder of this report only
the case of a cooling system will be considered. Most of the concepts
covered however, would be applicable for either heating or cooling
systems.
It is strongly recommended that the concepts discussed in Engineering
Reports H111 and H110 be understood before preceding with this report.
Engineering Report H111 includes a detailed discussion of the
performance of control valves. Engineering Report H110 discusses the
relationship between fluid flow and pressure drop as it pertains to typicalhydronic systems.
Accurate, stable control is dependent upon two factors. The first is the
ability of the controller and control valve to compensate for process
nonlinearites. A coil is a good example of a nonlinear process. If the
capacity of a coil is plotted for each flow rate through the coil, the result is
a logarithmic relationship. Secondly, the physical limitations of the
control valve must be considered. As discussed in Engineering Report
H111, all control valves have some amount of uncontrollable flow when
the valve plug is initially lifted form its seat. The magnitude of this
uncontrollable flow is related to the valve rangeability and the pressure
differential across the valve. Rangeability values are limited by machining
process tolerances. Therefore, it becomes important to minimize the
variation in pressure differential across the valve to minimize the
uncontrollable flow through it.
Dynamics of Hydronic Systems
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2 H112 Engineering Bulletin
Even small amounts of uncontrollable flow can compromise the ability of
a hydronic system to be properly controlled. This is the result of the
extremely high coil gain at low flow rates. It is important to properly size
the control valves. It is also necessary for the consultant to design a
hydronic system which is able to maintain a relatively constant branch
differential pressure, Pb, regardless of building load. This reportdiscusses how well the standard types of distribution systems are able to
maintain a constant Pb.
In a reverse return system the pressure drops in the distribution
piping between the circulating pump and each air handling unit are
equal. This is true because the system is designed so that the
length of the water circuit is the same for each air handling unit
regardless of its location with respect to the circulating pump. As
a result the Pbfor every branch is equal regardless of the
magnitude of the building cooling load. If the air handling unit
cooling coils are all selected for the same pressure drop this type
of system is self-balancing. Unfortunately the installed cost of a
reverse return piping system is higher than that of the other types
of hydronic systems. This is due primarily to the cost of the
additional chilled water return piping (See Figure 1).
Reverse ReturnDistributionSystems
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H112 Engineering Bulletin 3
When 3-way valves are installed in a reverse return system, each valve can
be optimally selected based upon the same criteria. Changes in the
magnitude of the cooling load will have little effect on the chilled water
flow rate. As a result, the value of Pbfor every branch will remain
constant regardless of changes in the magnitude of the building cooling
load.In the case of the reverse return system shown in Figure 1 the Pbfor
every branch will alwaysbe 50ft H2O. Since Pbis always
50ft H2O, every control valve can be optimally selected, based upon the
above criterion. If a valve authority of 50% is desired, all of the control
valves can be selected for a 25ft H2O pressure drop at design flow. Since
the branch pressure drop does not deviate from 50ft H2O, there should be
no increase in uncontrollable flow nor will there be any loss in effective
valve travel as the building cooling load decreases.
As with any system which utilizes 3-way valves, the flow rate through the
chiller(s) can always be maintained at a constant level without therequirement for a large central plant bypass with its associated controls.
Because the 3-way control valves shown in Figure 1 are mixing valves
piped in a bypass configuration, problems associated with control valve
actuator spring range shift or the inability to shut off flow through the
cooling coil will be minimized.
The disadvantages of utilizing 3-way valves are increased installation and
operating costs; specifically, a more complex (costly) coil piping
arrangement, a high continuous building flow requirement and a lower
overall chiller coefficient of performance (C.O.P). When 3-way control
valves are installed, the building demand for chilled water does not vary
with diversity in the building thermal load. Remember, the flow rate
through each branch does not change with the cooling load requirements
of the space served by the air handling unit. Only the chilled water flow
rate through the cooling coil changes with the load. Consequently, chilled
water pumps must often be operated without their associated chiller to
insure each branch will have chilled water available to meet the space
cooling load. Otherwise the branches nearest the chilled water pumps will
consume all of the available chilled water.
In the case of a chilled water system the operation of extra chilled water
pumps not only increases pumping horsepower requirements but also
reduces the overall chiller plant C.O.P. The lower overall chiller plantC.O.P. is the result of mixing relatively warm water passing through a
nonoperating chiller with exceptionally cold water leaving a operating
chiller. This is required to meet both the building flow and chilled water
supply temperature requirements. The C.O.P. of the operating chiller
decreases since it is forced to provide a lower chilled water supply
temperature for a given building thermal load.
This problem is illustrated in Example 1.
Reverse ReturnSystem with3-way Valves
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4 H112 Engineering Bulletin
Refer to Figure 1. This distribution system was designed for a total
cooling capacity of 400 tons. There are four air handling units (AHU),
each with a design capacity of 100 tons. Each AHU requires 150 gpm of
chilled water at a 16F rise (45-61F) when fully loaded.
Assume that the AHU is Branch #4 serves a computer facility and has a
constant cooling load of 100 tons regardless of outdoor conditions. Alsoassume the AHUs in branches #1 through #3 serve exterior zones in the
building. Finally, assume it is a temperate spring day, such that the
cooling load on the AHUs located in branches #1 through #3 is 20 tons per
AHU.
The total building cooling load is 160 tons. It would seem, since each
chiller has a capacity of 200 tons only one chiller will need to operate.
Unfortunately things arent that simple. Because this distribution system
has 3-way control valves, both chilled water pumps must operate to
maintain flow in all branches.
The cooling coil in branch #4 is fully loaded. Therefore 150 gpm ispassing through this coil and is being heated from 45F to 61F.
Meanwhile in branches #1 though #3 only 30 gpm of the chilled water
actually passes through the cooling coil. The other 120 gpm of 45F
chilled water bypasses the coil. These two streams then mix resulting in a
branch chilled water return temperature of 48.2F. The temperature of the
chilled water returning to the central plant will be 51.4F after the water
returning from all four branches is mixed.
Here is the problem. A single operating chiller must produce 38.6F
chilled water if 45F water is to be supplied to the building. This
extremely low chilled water temperature requirement is dictated by thefact an equal volume of 51.4F water is simply passing through the
nonoperating chiller and the AHU in branch #4 requires 45F to meet its
cooling load.
When a chiller designed to produce 45F chilled water is forced to produce
lower temperature chilled water its C.O.P. drops in a nonlinear manner.
For each additional degree drop in chilled water supply temperature
increasing amounts of compressor power is required. In this case it is
unlikely that the one operating chiller could even provide 38.6F chilled
water. In this extreme case both chillers would be required to operate even
though the actual cooling load is less than the capacity of one chiller. Thisis very inefficient.
Example 1
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H112 Engineering Bulletin 5
When 2-way valves are installed in a reverse return system the Pbfor
every branch will the same regardless of load, but the magnitude of the
Pbwill vary with changes in the building cooling load. This occurs
because the pipe-friction factor changes with the variable chilled water
flow rate (See Figure 2).
A reduction in cooling load will cause one or more of the 2-way control
valves to reduce the system chilled water flow rate. The hydronic system
depicted in Figure 2 does not have a central plant bypass. Without a
bypass the pressure developed by the chilled water pump(s) will increase
with the reduction in the cooling load. The pumps operating point will
ride up its pump curve as the chilled water flow rate decreases. Thisincrease in pressure developed by the pump, as well as the reduced pipe-
friction factor, will increase the differential pressure across the branches.
This is undesirable. The numbers shown in Figure 2 without parenthesis
represent the system pressures at maximum design cooling load. The
numbers shown in parenthesis represent system pressures when each
branch is at (1/2) of its design load. Notice how each Pbincreased from
50ft to 120ft H2O.
Reverse ReturnSystems With2-way Valves
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6 H112 Engineering Bulletin
If the distribution system had a central plant bypass, the magnitude of the
Pbcould have been held constant (See
Figure 3).
The control valve located in the central plant bypass will be controlled by
a differential pressure controller. The high and low reference sensing lines
for the controller will be connected across one of the branches in the
distribution system. If the sensed differential pressure deviates from the
controller set-point, the control valve in the bypass piping will modulate as
required to maintain the desired Pb.
Since each branch has the same Pbin a reverse return system, common
sense would dictate that the sensing location for the differential pressure
controller would be across the branch closest to the bypass. This would
reduce the length of the controller sensing and/or output lines, thereforereducing installation costs. This system has all of the advantages of a
reverse return system with 3-way valves, but does not have its
disadvantages.
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H112 Engineering Bulletin 7
As an alternative to using a central plant bypass to maintain a constant
Pb, a variable frequency drive could be installed on the chilled water
pump motor. The differential pressure controller would then vary the
speed of the chilled water pump to maintain the desired Pb. The main
problem with using a variable speed drive in lieu of a central plant bypass
is that the flow through the chiller(s) will now vary with changes in thecooling load. This potential problem must be carefully considered, and it
would be wise to consult the chiller manufacturer to obtain acceptable
guidelines.
The major advantage in using a variable speed drive in lieu of a central
plant bypass is the potential for energy savings. The variable speed drive
will save energy in two areas. First, any energy losses which normally
occur within the bypass valve will be avoided. In addition, the reduction
in the chilled water flow rate will proportionally decrease the chilled water
pump brake-horsepower requirement. Remember, in a system with a
central plant bypass the flow rate to the building is directly proportional to
the cooling load, but the flow rate through the chilled water pumps
actually increases with reductions in the cooling load. In a system
utilizing variable speed drives the flow rate through the pumps will vary
directly with the cooling load.
Keep in mind however, that in a properly sequenced multiple chiller
central plant the flow rate through the bypass line will be less than the
water flow rate associated with only one chiller. In other words, the
excess amount of chilled water produced in the central plant above and
beyond the demand required by the building would be less than the
amount of water provided by one chilled water pump. This assumes there
is one chilled water pump for each chiller. Therefore, the savingsprovided by using a variable speed drive compared to a central plant
bypass is not a function of the total plant chilled water pumping capacity.
Instead the potential savings is a function of the capacity of only one, not
all, of the chilled water pumps. If the central plant has a large number
(greater than four) of chillers and chilled water pumps, variable speed
drives will save very little energy over a properly operated constant speed
system utilizing a bypass valve.
In a direct return distribution system the length of the distribution piping
between the chilled water pump and each air handling unit will vary (See
Figure 4). As result the Pbfor every branch will be different. The closer
an air handling unit is located to the chilled water pump, the larger the
Pb. The magnitude of the Pbwill also be affected by the chilled water
flow rate. Remember the pipe friction-factor changes with the square of
the flow rate through the pipe.
DirectReturnDistributionSystems
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8 H112 Engineering Bulletin
The following four items can be utilized to minimize the variation in Pb:
1. Properly sizing the distribution piping can help to maintain a
constant Pb. This piping should be sized for small (less than 4 ft
head loss per 100 ft of pipe) friction-factors. The change in
pressure drop of the distribution piping, as flow is varied from
maximum to minimum, then should not exceed the design pressuredrop across a properly sized control valve. This is true for all but
the largest systems. Some central campus systems, and the like,
would require booster pumps in remote locations.
2. Utilize a central plant bypass or variable speed pump. The Pb
across one of the branches can then be maintained at a constant
level. This will prevent unwanted changes in the pressure
developed by the chilled water pump from being shifted out to the
branches.
3. Utilize control valves with large design pressure drops.
4. In some cases, utilize balancing valves.
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H112 Engineering Bulletin 9
When 3-way valves are installed in a direct return hydronic system the
magnitude of the Pbfor each branch will be constant. This is true
because the chilled water flow rate is always constant and is not affected
by the load. However the magnitude of the Pbwill be different for each
branch. This occurs because the length of water circuit between the
branch and chilled water pump is different for each branch (See Figure 4).The control valves for each branch can, therefore, be optimally sized for
any cooling load condition. Unfortunately each branch will have a
different sizing criterion since the Pbis different for each branch. In
comparison, the 3-way valves in the reverse return systemcan all be
sized for the same pressure drop.
As discussed earlier, the 3-way valves will insure a constant flow rate
through the chiller(s). Problems with actuator spring range shift, and
cooling coil shutoff will also be minimal. Unfortunately, the same
disadvantages are still apparent. The installed cost of a system with 3-way
valves is higher due to more complex coil piping arrangements. Also asdiscussed previously the chiller plant C.O.P. will be lower when compared
to a system with 2-way valves. This results from the high continuous
building demand for chilled water dictating operation of chilled water
pumps without their associated chiller.
When 2-way valves are installed in a direct return system, the Pbwill be
dependent upon the location of the branch relative to the chilled water
pump and the magnitude of the cooling load (See Figure 5). The numbers
in parenthesis indicate the system pressures when each branch is at (1/2) of
its design cooling load. The numbers without parenthesis indicate the
system pressures when the system is at maximum design cooling load.
If the system does not have a central plant bypass the value of Pbwill be
extremely dynamic. These extreme changes in differential pressure occur
for three reasons: the chilled water pump will ride up its curve, the pipe-
friction factor in the distribution piping will change with load, and the
length of the water circuit is different for each branch. This system is as
far from optimal as any can get.
Direct ReturnSystem with3-way Valves
Direct ReturnSystems with2-way Valves
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10 H112 Engineering Bulletin
If the system does have a central plant bypass, the pressure fluctuations
across each branch are reduced (See Figure 6). The numbers in
parenthesis indicate the system pressures when each branch is at (1/2) of
design cooling load.
Adding the central plant bypass keeps changes in the pressure developed
by the chilled water pump from being shifted out to the branches.
However, since the pipe-friction factor in the distribution piping will
change with load, the value of the Pbwill be affected.
Whenever the load in the building decreases and the bypass valve
opens, the flow rate in the central plant will increase. This occurs
because the pressure drop across the bypass will decrease as it is
opened. In turn the pressure developed by the pump will decrease.
As the pump operating point moves further out on its pump curve,
its flow rate will increase. This increased flow rate will, in turn,
increase the pipe-friction factor of the central plant piping.Eventually a balance point between the increased central plant
piping friction losses and the decreased pressure across the bypass
will be found. This is graphically shown in Figures 7, 8 and 9.
Notice how the slope (pipe-friction factor) of the line representing
1/2 design load changes within the central plant portion of the
graph.
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H112 Engineering Bulletin 11
Assuming the branches are uniformly spaced in a distribution
system, the best differential pressure sensing location for the
bypass controller is in the middle of the system. Figure 7 relates
the change in Pbwith location as a function of cooling load. The
graph illustrates the possible variation in Pbas the building load
changes from no load to maximum design load. The slope of each
of the different load lines is equal to the negative of the respective
piping friction factors. The area underneath the lowest load line
represents the pressure available for the branch piping, coil and
control valve. Depending on the location of the branch in the
system the lowest load line could be either the design or no loadline. In this case the control valves in the first half of the branches
which are closest to the pump should be selected for a higher
design pressure drop than those in the second half of the system.
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12 H112 Engineering Bulletin
By sensing differential pressure in the middle of the distribution system
the magnitude of the maximum variation in branch differential pressure is
minimized when compared to other sensing locations. Compare the
maximum variation of Pbin the building portion of Figure 7 to that
shown in Figures 8 or 9.
The bypass control scheme shown in Figure 8 will provide good pressure
control at the end of the system. Unfortunately the variation in Pbat thebeginning of the system may be excessive.
The bypass control scheme shown in Figure 9 can provide good pressure
control at the beginning of the system. Unfortunately the variation in Pb
at the end of the system may be excessive.
In spite of some inherent problems, direct return hydronic systems with 2-
way control valves are very popular. This is likely due to the fact it has a
lower initial cost relative to other types of hydronic systems and it is a
very energy efficient pumping system. Lastly, a direct return system
incorporating either a central plant bypass or variable speed pumping can
provide acceptable pressure control when distribution piping is properlysized. Proper distribution pipe sizing is critical to insure trouble free
control. If the distribution piping is undersized the Pbcan vary
significantly with the load.
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H112 Engineering Bulletin 13
As a rule, the potential changein Pbmust be less than the designpressure drop of the control valve, or problems can occur. These problems
can manifest themselves in the following ways: unacceptable
uncontrollable valve flow, problems with valve shutoff and spring range
shift, valve cavitation, and overflowing marginally sized cooling coils.
A primary secondary distribution system utilizes two sets of pumps. The
first set is used to pump water through the chillers. These pumps are
called the primary pumps. The second set of pumps are used to pump
water through the building. These pumps are called the secondary pumps.
The primary and secondary pumps are hydraulically isolated from oneanother. This isolation is accomplished by installing a bypass line
between the primary and secondary pumping systems as shown in
Figure 10.
Primary/SecondaryDistributionSystems
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14 H112 Engineering Bulletin
The bypass line is also often called a decoupler line since it isolates or
decouples the primary and secondary systems. The direction of flow in
the decoupler piping can be in either direction. The direction of flow will
depend upon the amount of chilled water produced in the central plant
(primary flow) and the amount consumed by the building (secondary
flow). If the primary loop produces a greater volume of chilled water than
the secondary system consumes, flow in the decoupler will be from supply
to return. If then secondary system consumes a greater volume of water
than the primary system produces the water in the decoupler will flow
from return to supply. Normally flow within the decoupler should be from
supply to return. Otherwise the chilled water supply temperature in the
secondary loop will rise to unacceptable levels.
The hydraulic isolation allows both the primary and secondary pumping
systems to function as if the other was not present. The primary pumpsare, therefore, able to maintain a constant flow rate through the chiller
regardless of the building load.
The secondary system can be configured as either a direct or
reverse return distribution system. The same advantages and
disadvantages discussed earlier will still apply. However, in
either case only two-way control valves should be installed.
The chilled water flow rate in the secondary system will then
vary in relation to the building load. This is desirable for two
reasons.
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H112 Engineering Bulletin 15
First, a variable flow secondary system will require less pumping energy
than other types of systems. The secondary flow rate, in a primary
secondary system, can be reduced to extremely low flow rates. The
minimum secondary flow rate is not fixed by flow limitations imposed on
the system by the chillers. If a variable speed secondary chilled water
pumps is installed, the amount of pumping power required will beminimized. In a variable speed pumping system, the pumping power
requirement will vary with the cube of the flow rate as dictated by the
pump affinity laws. The sensing location for the differential pressure
transmitter should be near the middle of the secondary system. Once
again, this location is the best compromise for minimizing the variation in
the magnitude of the Pb.
Secondly, if both the magnitude and direction of the flow rate in the
decoupler are known, this information can be utilized in various building
automation schemes. Particularly for chiller sequencing. Chiller
sequencing is discussed in Engineering Report H324.
It is important for the decoupler line which separates the primary and
secondary systems to be properly sized. It must be sized so that its
pressure drop at full flow is kept very small (i.e., less than 1 psig).
Normally full flow is about 115% of the flow rate associated with the
largest chiller. Flow rates in excess of this value would indicate an
operating chiller should be stopped. The decoupler should be a straight
length of pipe with no restrictions. It is very important for the pressure
drop in the decoupler to be kept very small to provide the hydraulic
isolation between the primary and secondary systems. If the size of the
decoupler is too small, pressure changes in the secondary system will betransmitted into the primary system. This interaction can cause stability
problems and undesired flow variations through the chillers.
Occasionally, a primary secondary distribution system with a check valve
installed in the decoupler line is encountered (See Figure 11).
Decoupler LineSizingConsiderations
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H112 Engineering Bulletin 17
Note: Do not use square law type flow meters to measure the flow rate in
a decoupler line. Square law type flow meters include annubars,
orifice plates, flow nozzles, and venturi tubes. The output signal
cannot be accurately measured with these flow meters at
turndowns greater than 3:1. A bidirectional turbine flow meter
should be utilized because it can indicate both direction and
amount of flow accurately at much lower flow rates. Turbine
meters with turndown ratios of 10:1 are typical, but are available
with ratios up to 30:1. Remember water must be able to flow bothways through the decoupler line.
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Notes
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H112 Engineering Bulletin 19
Notes
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Notes
Controls Group
507 E. Michigan Street
P.O. Box 423
Milwaukee, WI 53201 Printed in U.S.A.