Lecture 6: Closed Water (Hydronic) Systems

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Lecture 6 Closed Water (Hydronic) Systems
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Basic System

• From the text

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Thermal Aspects Simplified Equations (again)

• q heat transfer rate, Btu/h
• Qa airflow rate, cfm
• Qw water flow rate, gpm
• ?t temperature increase/decrease, ?
• ?h enthalpy difference, Btu/lb
• Sensible heat in air, q 1.1 Qa ?t
• Total heat in air, q 4.5 Qa ?h
• Heat transferred to/from water q 500 Qw ?t
• These are for air and water at standard
  conditions

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Commonly Used Temperatures and Ranges

• For comfort cooling systems 75 ? with 50 RH is a
  common target, this has a dewpoint of 55?, so the
  maximum practical return water temperature is
  around 60?. (Need some counterflow heat exchange
  for this). To avoid freezing in the chiller 40?
  supply water is practical.
• For heating systems 250? is the maximum for ASME
  low pressure code. A range of 20? has come into
  practice for residential work, and extended into
  much commercial systems. Fin tube radiators will
  economically use higher water temperatures,
  forced air heating coils a middle range, and
  radiant floor heating with plastic tubing in a
  lower range 90-130?.

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Expansion and Air Control
For type A or B they are typically located at the
main air removal device. Type C has a fixed
amount of air under the bladder. The main purpose
is to provide room for expansion of the fluid, so
for types A and C it can be important to
precharge the tank. Type C, bladder expansion
tanks, were developed to prevent waterlogging of
plain steel tanks as air was slowly dissolved
into the system water and removed at other parts
of the system.
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Expansion tank location

Once upon a time a designer was selecting a fill
pressure for Herter Hall. The building is 8
stories above the mechanical room and so the
static head is around 106 feet or 46 psig.
Assuming the tank was in the conventional
location at the pump suction I told the boys 56
psig would be fine. They told me they had always
needed over 70 psig to keep pressure at the top
of the loop. Where was the expansion tank really
located?
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Airbound

Water level
Water level
If no air vent at top and pump has insufficient
head to overcome static head difference
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Airbound even worse

Water level
Now theres differential pressure across the
terminal unit but no flow. If only we could see
into the pipes (Just make sure the air vents are
working)
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Air removal devices

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Location of air removal devices

As you can see from the charts and data in the
text concerning solubility of air and gases in
water, the optimal location for air removal
devices are at points of highest temperature and
lowest pressure.
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Other hydronic system components

• Strainers, to prevent device damage or clogs
• Check valves
• Manual valves
• Balancing valves
• Flow elements
• Pressure reducing valves (makeup connections)
• Relief valves (to prevent overpressurization)

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Valves

• Shutoff or isolation Gate, ball, butterfly.
• Throttling Globe, ball, butterfly.
• Valves are rated for temperature, static
  pressure, closeoff pressure, and resistance to
  flow (Cv)
• Cv is defined as the flow in gpm which causes a 1
  psi pressure drop across the valve (typically
  when wide open).

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Gate valve

Old time favorite, still in use for high pressure
applications. Excellent Cv since flow is
unobstructed when gate is up.
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Globe valve

Another old time favorite, good for throttling
since its so restrictive, bad for pressure drop
since its so restrictive. Often with resilient
disc (the faucet washer). The traditional
control valve.
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Ball valve

With the development of polymer seating materials
(teflon is common) ball valves have become
immensely popular, particularly in smaller sizes
(up to 2 ½). Excellent for tight shutoff, ball
can have a characterized opening to aid in
throttling or control applications.
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Plug valve

Same general operation as a ball valve, can
withstand higher pressures, has seen use for
balancing valves.
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Butterfly Valve

Resilient seated for low pressure applications
with good shutoff capability (as long as the seat
is clean!) and can have metal seating for high
pressure or steam applications. Very compact,
relatively inexpensive. Can be used as control
valve.
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Controlling thermal output with water flow

Do note that output is definitely very non linear
with flow. This makes control a little difficult
but balancing somewhat forgiving.
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Constant flow controls

Flow is constant in system regardless of load,
and chillers each see constant flow. Typical of
all our buildings built around 1970. Has
disadvantage of high pumping costs and dilution
of chilled water when only one chiller operates.
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Primary Secondary Pumping

Useful when the secondary loop has specific
differential pressure or temperature requirements
which are not ideally met with the primary flow.
A long string of perimeter heating units for
example where that large pump head need not be
imposed on the primary system flow. Primary flow
(and pipe size) could be saved if the primary
heating water temperature is run higher than the
secondaries. Also, where constant flow is
desired through one loop and variable through
another, as in a chiller system. NOTE THE
DIRECTION OF FLOW CAN CHANGE IN PIPE A-B IT
DECOUPLES THE LOOPS.
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Variable Flow Chilled Water System

Note that as flow increases in the secondary
circuit (load side) beyond the flow of the first
chiller, the flow in the common pipe reverses,
diluting the supplied chilled water to the
system, and its time to start the second chiller.
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Testing and Balancing of Hydronic Systems

• Although flow can be measured with a turbine
  meter, dp meter, ultrasonic meter, magnetic flow
  meter, most common today are dp inline flow
  elements or portable ultrasonic meters.
• DP (differential pressure elements) create their
  differential by an obstruction, which could be an
  orifice or a venturi. (A venturi will have less
  unrecovered pressure drop).
• A clamp on ultrasonic meter is the standard tool
  of the balancing contractor, but their 6000
  price make them unsuitable for small inline
  metering.
• Flow can also be calculated by known dp/flow
  characteristics of terminal devices (coils,
  chiller barrels, etc.)
• Flow can also be calculated by temperature
  rise/drop compared to know characteristics of
  terminal units. For example, if a piece of fin
  tube radiator is sized for a 20F temperature
  drop, when it is running at design conditions the
  flow can be adjusted so a 20F drop is obtained.

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Balancing reasons for

• To assure all terminal units receive adequate
  flow.
• To maintain total system flow requirements within
  the design capacity of the supply equipment.
• Classic chilled water problem low return water
  temperature (low system dT). This causes higher
  pumping costs and can rapidly exceed the flow
  capacity of a chiller even though its
  refrigeration capacity is not exceeded.

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Flow control (limiting) valves

At the operating expense of a constant
differential pressure at full flow these valves
prevent terminal units from using too much flow.
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Pipe Sizing Copper Pipe

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Pipe Sizing Steel Pipe

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Friction loss through elbows

Do remember these correlations are for decently
well developed flow. If you have any such
devices in close proximity the velocity
disruptions of the first will cause greater
pressure drops in the second. Elbows and such
dont care so much if theres real low velocity
in part of the pipe, the fact the there is high
velocity in another part will increase pressure
drops. (Pressure drop as the square of velocity)
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How Not to Pipe a Chiller