REMOTE MONITORING, FLOW LIMITATION AND CONTROL –
Pressure
Independent Control Valves [PICVs] have become the
go-to solution in variable load systems with their easy selection and 100%
authority.
The
addition of smart actuators is beneficial in flow rate specified applications
where remote setting is preferred, whether looking to reduce site commissioning
time and costs, accessibility limitations or changing maximum load conditions.
FlowCon PICVs
used in conjunction with a FlowCon BUS actuator allow
for remotely programmed flow settings and feedback. Direct communication with
the BMS is achieved through BACnet or Modbus with no requirement for a separate
controller.
Remote
monitoring using the BUS actuator allows for valve blocking protection, a
communication failure mode in the event of an invalid control signal, leak
detection when the ΔT is above 8 °K when the valve is closed for more than 6
hours and the ability to flush remotely on a schedule flush time.
As part
of the commissioning procedure each control valve is programmed with valve type
and the design flow settings. Each of the design flow settings is calculated on
a 0-100% input signal, based on the valve’s maximum capacity. Flow feedback can
be configured as percentage value or as an estimated flowrate [l/h].
DELTA
T MONITORING AND CONTROL –
The
purpose of a hydronic system is to transfer the thermal load into the
controlled zone.
The
transfer of this thermal load output can be lowered by a poor differential
temperature (ΔT). The main cause of a low ΔT is unit overflows. This results in
hot and cold spots in the building, causing comfort issues for the occupants.
This key problem is termed as the ‘low delta-T syndrome’.
A main
concern in heating systems, especially with the use of condensing boilers as a
high return temperature will not allow the boiler to condense and in turn then
it becomes an expensive conventional boiler. In cooling systems, chillers are
designed with a certain ΔT and flowrate. A low ΔT with a constant flow rate
reduces the chiller capacity, causing inefficiency as the chillers must remain
on to satisfy the load [1].
In
addition to the previously mentioned comfort issues, overflows and a low ΔT can
cause higher operational costs, such as increased expenditure for a higher pump
demand than required, reduced lifespan of equipment and the cost of corrective
design actions.
PICVs are
most suitable for terminal unit applications by only delivering the designed
amount of water, reducing the risk of overflow and maintaining a higher ΔT
through the coil.
However,
issues with low ΔT can occur in forced convection [fan] heat exchanger
applications at low fan speeds when changing space temperatures command the
valves to be opened, either in cooling or heating modes. The ΔT can be diminished
through the lack of air volume being unable to remove more energy from the heat
exchanger.
The
diminished temperature difference can be recognised by using a ΔT system as a
diagnostic tool to measure the actual ΔT and collecting the data points for
improvement of the control strategy. Optimisation would be through utilising
the ΔT control system to position the valve accordingly, to achieve the set ΔT
design.
Installation
of the supply and return temperature sensors on the FlowCon BUS actuator allows
for differential temperature measurement, which can be used for the above
mentioned ΔT monitoring and control strategy. As a control strategy the
actuator will override the input signal and maintain the minimum ΔT by starting
to close the valve when the specified ΔT is not achieved. The return set-point
temperature will adjust on increases/decreases to the supply temperature,
achieving optimal thermal output.
THERMAL
POWER (ENERGY) CONTROL –
Flow rate
and differential temperature control can be used independently as a form of
load control. Although a reduction in thermal power consumption can be obtained
by controlling both variables through a differential temperature (ΔT)
management system [2].
Measuring
a ΔT with a known flow rate can be used to calculate a power output using the
following equation [Eq.1]:
Eq.1: Q
= mc ΔT
Thermal
Power = Mass Flow * Specific Heat Capacity * Temperature Difference
Kw
= kg/s * kJ/kg/°C * °C
Should
the thermal power be limited under a control strategy it is important to
consider the power saturation point of the coil – the point in which the coil
cannot produce additional power regardless of increased flow rate, as shown by
the waste zone in Figure 1. Designing a strategy with the power output value
close to/higher than the saturation zone will lead to unnecessary overflows and
a low ΔT. And so, implementing an efficient strategy based on thermal power
control is achievable with the understanding that the coil saturation point is
known. Monitoring, collecting and graphing data for the coil’s heat
transfer behaviour is beneficial before solely using the power output as a
limiting variable.
Thermal
power is limited as a control strategy when the power output is the only known
design variable. Unit control can be achieved as a power output without knowing
water flow rates and ΔT. This is a controllable factor for comparing unit
efficiency against the design load for the space.
The
FlowCon BUS actuator with the two PT1000 sensors attached enables readings to
the supply and the return temperature. Along with this an estimated flow
reading and the heat capacity constant allow for an estimated power output
reading by using the controller’s built in PI-controller.
RETURN
TEMPERATURE CONTROL –
For a
heating circuit the return temperature is a key indicator of system efficiency.
A low return temperature results in a larger ΔT, meaning that lower flow rates
are required for the same kW delivered. A lower return temperature means
designers can reduce pipe sizes. Going down one pipe size reduces capacity by
36% and heat loss on average by 10%, going down two pipe sizes reduces the
capacity by 62% and heat loss reduction by 19% [4]. Thereby reducing capital
costs and power usage. Lower controlled return temperatures improve the
efficiency of boilers, heat pumps and CHPs. System temperatures can be
controlled through the individual return temperatures of space heating, HIUs
and heat exchangers on the same network.
A general
issue with standard minimum flow by-pass’ situated at the end of line is that
they contribute to raising the temperature of the network. Investigations have
shown that it is possible to insert temperature-controlled valves instead of
manual valves, resulting is significant cost reductions [4].
The
project discussed in the referenced article [5] recommends installing
temperature sensors in a fixed by-pass and monitor the system in periods of
changing supply temperature, making periodical comparisons between changing
conditions and a controlled fixed return system temperature.
The
FlowCon BUS actuator with 1 PT1000 for return temperature measurement in
combination to the controller’s built-in PI-controller allows for control by
return temperature.
SUMMARY
–
The
addition of the BUS actuator range effectively transforms the FlowCon Green
PICV range into Smart Valves,
retaining the well-known benefits, such as the compact size and removable
inserts, which allow for most accurate set point selection and flexibility.
The BUS
actuator can easily be retro-fitted in existing systems, enabling data
gathering and finer control, thereby upgrading the existing valve population
into energy
valves and obviously, can be fitted in new
installations, contributing to an enhanced control strategy, that optimises
occupant comfort as well as energy consumption.
Contact us
for more detail on our energy and smart valve solutions engineering@flocontrol.ltd.uk
