Hot Water Mixing Valve Control
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 Most radiant hot water heating systems in greenhouses
use mixing valves to control the temperature of heating pipes. Both 3-way
and 4-way valves are commonly used. Mixing valves provide the ability to:
- Regulate
the temperature of the heating pipes in a climate by controlling the
flow of hot water from the central supply system to the local heating
zone.
- Proportion and ration the available
system heat over several independent zones, when boiler capacity is
temporarily exceeded.
By carefully adjusting the temperature of the water in
the heating pipes, the Argus system is able to closely balance the heating
energy applied with the heat loss in each compartment. This results in
very even temperature management. The computer constantly monitors the
compartment and outdoor conditions and compares them to target values. It
then calculates the desired pipe temperature required to sustain the
current air target setpoint and positions the mixing valve to achieve this
pipe temperature in the local heating zone. Pipe temperature sensors are
used to provide feedback correction for valve positioning.Types of Mixing Valves
3-Way Mixing Valves
3-way valves are typically used in situations where the central supply and
return pipes are not recirculated by a 'ring main' transport pump. Zone
recirculation pumps and mixing valves are used to circulate and
recirculate the contents of each zone's heating pipes. When no heat is
required, this is a closed loop (see figure 1 below).
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- (Click on thumbnail image for a
larger view)
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- Figure 1- A Typical 3-Way Mixing
Valve Installation
When heat is required, the Argus system opens the
mixing valve, allowing hot water from the heating supply to be drawn into
the zone loop by the zone pump (see figure 3). When fully opened, no
recirculation occurs within the compartment and full heating occurs (see
figure 4).
(Click on thumbnail images for a
larger view)
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Figure 2
3-Way Mixing Valve
Full Recirculation,
No Heat |
Figure 3
3-Way Mixing Valve
Some Recirculation,
Some Heat |
Figure 4
3-Way Mixing Valve
No Recirculation,
Full Heat |
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3-Way valves must be used when heat/cold storage is
used with CO2 extraction systems as the design does not 'contaminate' the
main return water temperature with supply water. This is a good design
choice for operations up to a few acres in size with supply and return
pipes up to a few hundred feet in length.
4-Way Mixing Valves
4-way valves are used when the zone is supplied by a transport or 'ring
main' system that has its own circulation pump to create a pressure
differential between the supply and return sides of the system. In these
systems, both the ring main and the individual zones can be continuously
circulated independent of one another. When no heat is required by any of
the zones, the ring main recirculates it's hot water via a transport pump
and each zone recirculates its water by means of a zone recirculating pump
(see figure 5).
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- (Click on thumbnail image for a
larger view)
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- Figure 5 - A Typical 4-Way Mixing
Valve Installation
When fully closed, both the supply and return pipes to
the zone are isolated from the ring main. When heat is required, the 4-way
valve opens to allow supply water to enter the zone and return water to
exit the zone (see figure 7). When full opened, supply water flows
directly through the zone with no recirculation (see figure 8).
(Click on thumbnail images for a larger view)
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Figure 6
4-Way Mixing Valve
Full Recirculation, No Heat
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Figure 7
4-Way Mixing Valve
Some Recirculation,
Some Heat
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Figure 8
4-Way Mixing Valve
No Recirculation, Full Heat
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4-Way Valves are often used with transport
systems. They provide alternate return pathways from the supply to the
return valves for any 'unused' hot water pushed through by the transport
pump.Mixing Valve Control
The Argus control system is supplied with several specialized programs for
managing hot water heating systems. By using pipe temperature control for
operating mixing valves, the system is extremely responsive to changes in
the amount of heating energy delivered to the climate zones. This helps to
precisely match the current heating demand, avoiding costly overshoots.
Mixing valve movements can be controlled by either time or an optional
position sensor. The control system can provide either direct digital
control (ON/OFF or Tri State Floating) or analog (4-20ma) control output
depending on the type of valve motor actuators you are using.Special
Applications
Several specialized mixing applications are supported
including:
Heat Zone Mixing Valves
- Heat Zone Mixing Valves
request specific pipe temperatures based directly on their own current
climate requirements.
- Minimum pipe temperatures
can be specified for each heating period as well as during CO2
application.
- You can also can specify
the minimum pipe temperature to use for each valve when snow-loading
conditions are present.
- Maximum pipe temperature
settings are provided to protect heat distribution equipment such as
plastic piping.
- Crop activation settings
enable you to configure time and light based pipe temperature
increases.
- Boiler return temperature
limit settings can be used to protect your central heating equipment
and to allocate the available heat to specified priority zones during
peak demand periods.
- Mixing valve operation can
be grouped with circulating pumps. Many lead/lag pump options are
provided including alternating or fixed cycling, and pipe temperature
based secondary circ pump operation
- Calibration settings to allow for loose linkages
and non linear valve opening position/output proportional response
ratios.
Heat /Cool Applications
In addition to the standard heat zone features, you can manage mixing
valves on systems that provide both heating and cooling through the same
distribution equipment. Linearization tables are provided to correct for
non uniform output in relation to percent opening.
Stand Alone Mixing Valves
These types of valves operate similar to heat zone mixing valves except
that they are optimized for non climate uses such as bench or soil
heating. Direct sensor based values can be used to control operation or
multiple external values can be mapped and calibrated to govern valve
operation.
Transport Mixing Valve Settings
A transport mixing valve controls the pipe temperature of a hot water
heating system's distribution piping (commonly known as a transport loop
or a ring main). Usually, several heating zones draw their hot water from
this common transport loop. The pipe temperature of the transport loop is
controlled by the requirements of the heating zones connected to it.
Floating temperature offset above the highest zone request values. This
enables the transport line to always attempt to have hotter water
available (within the system wide constraints) than the highest
temperature requested by the zones it supplies. |
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