1 Introduction to Economizer tx2 - Siemens Building Technologies

DESIGO -
Energy-efficient applications:
h,x-controlled Economizer tx2
Application data sheet
Answers for infrastructure. s

Table of contents
1 Introduction to Economizer tx2.............................................................5
2 Basics.......................................................................................................6
3 Economizer tx2........................................................................................6
3.1 Overview ...................................................................................................6
3.2 Principles of the tx2 Economizer strategy.................................................7
3.3 The principle functions ..............................................................................8
3.3.1 Comfort area .............................................................................................8
3.3.2 tx2 control..................................................................................................9
3.3.3 ERC strategy........................................................................................... 11
3.3.4 Weighted processes................................................................................12
3.3.5 Energy comparison with other strategies................................................13
3.3.6 Additional functions.................................................................................14
4 Energy savings......................................................................................15
5 Advantages and customer benefits ....................................................16
5.1 Advantages .............................................................................................16
5.2 Customer benefits...................................................................................16
6 Field of use ............................................................................................16
7 Display and operation ..........................................................................17
8 System hardware ..................................................................................17
9 Field devices..........................................................................................17
10 Versioning..............................................................................................17
11 Appendix................................................................................................18
11.1 Plant components ...................................................................................18
11.2 h,x-diagram .............................................................................................19
11.3 ERC control behavior..............................................................................20
12 About this document ............................................................................22
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1 Introduction to Economizer tx2
The Economizer tx2 application offers energy-optimized air conditioning plant
control by conditioning the air supplied to the rooms using the most favorable type
of energy. Our patented procedure continuously calculates air conditioning costs,
allowing for targeted selection of the most favorable method. Economizer tx2
optimizes the room state within a setpoint field resulting in optimum setpoint
settings for ventilation while at the same time optimizing energy recovery resulting
in the best possible air aftertreatment. Optimization can be based on energy, costs,
or CO2. As a result, Economizer tx2 helps lower energy costs and offers an
excellent basis for highly efficient operation. In fact, up to 40% energy can be
saved compared to conventional air conditioning. At the same time, comfort control
ensures that limit values are maintained for temperature and humidity. The
innovative application is modular in design and includes a number of plant variants
to control air handling units.
Definition of terms
ERC: Energy recovery
Economizer tx2
2-dimensional optimization
Absolute humidity
Temperature
Figure 1-1
Basic diagram for an air conditioning plant with Economizer tx2
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2 Basics
Air conditioning plants do not always enjoy the best of reputations. The reasons
mainly concern plant hygiene, partly incorrect plant design, often high energy
consumption as well, especially with older plants or certain designs. Nevertheless,
air conditioning plants perform important tasks and have an established place in
modern building control. A great deal of progress has been made in recent times,
especially with regard to plant hygiene, energy management, and plant equipment.
However, strategies for controlling air conditioning plants have hardly changed over
the past few years. For example, energy recovery is typically controlled according
to enthalpy, i.e. separate temperature and humidity control loops with their own
energy-neutral zones but without inter-harmonization. This method does not always
guarantee optimal behavior and generally is deficient with regard to dynamics.
This is where our Economizer tx2 comes in: Systematic exploitation of comfort
limits inter-harmonization of setpoints, process control, and energy recovery mode.
3 Economizer tx2
3.1 Overview
An air conditioning plant ready for operation with energy recovery serves as our
starting point. This air conditioning plant is tasked with providing adequate comfort
conditions (use-adjusted room conditions) at the lowest possible operating costs.
Minimizing operating costs, however, may not be at the expense of comfort.
The following aggregates can be used as air handling units:
– Hot water heating coil
– Chilled water cooling coil
– Steam humidifier, washer, spray humidifier
– Plate heat exchanger, rotating ERC, or mixed air dampers

3.2 Principles of the tx2 Economizer strategy
Figure 3-1
Economizer tx2 principle. – Comfort area control (tx2 control) and energy recovery control (tx2 ERC
strategy) together contribute to optimizing air conditioning plant operating costs through interharmonization
and coupling.
The Economizer tx2 corresponds to h,x-controlled temperature and humidity
control. The tx2-strategy controls the heat recovery system so that the air handling
process is cost-optimized considering predominant air states and specific costs of
air handling aggregates.
The principle functions are explained in greater detail below.
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3.3 The principle functions
3.3.1 Comfort area
Advantages:
– Better comfort sensation
– Greater potential energy savings
Economizer tx2 uses the so-called comfort area as a setpoint (see Figure 3-2).
There are two reasons why a field is used as a setpoint.
Comfort sensation:
Optimal working environment conditions exist not just at a specific setpoint for
room temperature and room humidity, but within a particular range.
A temperature setpoint of 22°C and a setpoint for relative humidity of 45% with a
deviation of e.g. ±2°C and ±15%r.h. are typical for office work.
Additionally, it is meaningful to limit absolute humidity at high temperatures; or else,
the air become too muggy. This limit value typically is ca. 11g/kg.
Energy savings:
The greater the comfort area area, the greater the energy savings potential.
Figure 3-2
Comfort area in the h,x diagram

3.3.2 tx2 control
Figure 3-3
tx2 control – Determination of the room setpoint and cascade control for temperature and absolute
humidity.
Both room temperature and humidity are controlled as follows using the
Economizer tx2:
The comfort area determines the room air setpoint for temparature and humidity
based on the present room air state and the defined comfort area. The comfort
area is not actively controlled. When elements are activated, control is exercised to
the field's limit.
The temperature and humidity controller are used as room air supply sequence
cascade controllers. The reference controller calculates the reference variable for
supply air from the room-side control deviation. The supply air controllers calculate
the positioning signals for heating, cooling, humidification and dehumidification
from supply air-side control deviations.
Defining the room setpoint
Economizer tx2 (Figure 3-3) identifies the best room setpoint at the border or within
the comfort area and applies optimized ERC (Figure 3-4).
Demand for heating, cooling, humidification, and dehumidification or a combination
thereof is based on the preconditioned air state and supply air setpoint.
– For "heating and humidification", "cooling and dehumidification" and
humidification and possible reheating", the related setpoint is identical to the
most energy-efficient corner point within the comfort area.
– For "cooling only", the setpoint is achieved by projecting the actual value to the
upper limit of the comfort area.
– For "heating only", projection is to the lower limit. The range with relative
humidity forming the limit represents a special case. Here, the temperature
setpoint is shifted along the relative humidity line. This results in greater heating
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demand. But, no cooling and reheating is required to achieve the relative room
humidity in this range. This lowers costs thanks to lower cooling energy
requirements.
– For "humidification only", the relative humidity actual value is projected to the
relative humidity line. This results in greater humidification demand. But, no
cooling is required in this range. This in turn lowers costs, as humidification
normally costs less than cooling.
Figure 3-4
Possible state changes in the h,x diagram for the comfort area.
See Section 11-2 for additional information on the h,x diagram.
Cascade controller
Advantages:
– Better dynamic behavior
The dynamic controller (Figure 3-3) comprises a cascade controller for temperature
and humidity control. Temperature and relative humidity represent measured
variables, temperature and absolute humidity, however, represent reference
variables. The absolute humidity is calculated from temperature and relative
humidity. This conversion decouples the two reference variables, resulting in
improved dynamic behavior.
The cascade contains two important components:
– Room controller
– Supply air controller

3.3.3 ERC strategy
Advantages:
– Optimal use of ERC
The position signals are converted into demand signals for the ERC strategy
(Figure 3-1) and weighted using specific costs for heating, cooling, humidification
and dehumidification. Weighting can be set for each plant to allow for costoptimized
operation of the plant. The tx2-algorithm then calculates the positioning
signal for heat recovery from the weighted demand signals so that the mixed air or
air state after ERC continues to be conditioned by the follow-on air handling
aggregates at minimum energy expense and costs.
Energy recovery is controlled so that the sum of all weighted demand signals for
heating, cooling, humidification, and dehumidification is minimized. The ERC
strategy is based on the t,x diagram. Each process is assigned a vector at the t,x
level (Figure 3-5) and weighting (Section 3.3.6). The vectors reflect the theoretical
impact of each process. During operation, only two vectors can be active at any
given time in addition to the ERC vector.
The supply air setpoint is at the center of the diagram. In addition, the model shows
the actual values for room and outside air. The ERC vector results from this,
depending on the degree of recovery for temperature and humidity, based on the
actual value for outside air.
The aim is to identify the point on the ERC vector that minimizes the sum of the
theoretical impact of both active processes.
Example:
The outside air is cooler than the room air. ERC also should condition cooler air,
thereby increasing the amount of outside air, if cooling demand is high. This action
reduces the required cooling energy. If humidification is required at the same time,
an additional dimension is added (Section 3.3.5).
Figure 3-5: t,x diagram
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3.3.4 Weighted processes
Advantages:
– Minimize energy costs
– Reduce energy consumption
– Reduce CO2 emissions
Relative weightings are used to optimize ERC. They result from the energy
provisioning processes for heating, cooling, humidification, and dehumidification.
The weighting factors can be used to optimize:
– Energy costs
– Energy consumption
– CO2 emissions
Example: Energy costs
Energy recovery optimizes energy costs by preconditioning outside air. To do this,
the supply air controller's (Figure 3-3) demand signals are weighted by the specific
costs of the corresponding processes (Table 3-6) and energy recovery is
controlled.
Process Specific costs Relative weighting
Heating 0.09 € 1
Cooling 0.16 € 2
Humidification 0.08 € 1
Dehumidification 0.23 € 3
Table 3-6
Example for specific costs and relative weighting.
Specific costs are in € relative to a 1K temperature difference and a one-hour operating period.

3.3.5 Energy comparison with other strategies
The DESIGO INSIGHT management station allows for displaying savings by
means of an energy comparison table. The most important characteristic values
are calculated and displayed online based on known plant variables.
Table 3-7
Energy comparison table with conventional ERC control and Economizer tx2 in DESIGO INSIGHT
A Present power in [kW]
B Provided energy (cumulated power) in [kW]
C Present cost
D Total cost (cumulated cost)
The energy comparison table (Table 3-7) displays conventional ERC strategies:
– Strategy Max ERC recovery ERC is fully controlled
– Strategy Temp. ERC is integrated in the temperature sequence
– Strategy Hum. ERC is integrated in the humidity sequence
– Strategy Enthalpy. ERC is controlled to the supply air setpoint
enthalpy
The last column contains the Economizer tx2 strategy.
The table visualizes the following variables:
– Output and energy consumption for thermal air handling, provided by the
processes heating, cooling, humidification and dehumidification.
– Water costs as a function of the humidity increase.
– Momentary and added costs as a function using cost factors for weighted output
and energy consumption.
– Fan energy and temperature increase caused by the fan. Electrical output and
energy is calculated based on volume flow and overall pressure difference via
the ventilator.
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Additional information is available in the appendix (Section 11.3).
3.3.6 Additional functions
– Modular designed standard library applications including graphics for plant
operation using DESIGO INSIGHT to operate air conditioning plants
– Control of air handling unit for multiple plant variants
– Summer/winter compensation for greater comfort
– Summer compensation for additional energy savings
– Fault shutdown for maximum safety
– Data logging and monitoring

4 Energy savings
Comprehensive building simulations have been examined for energy consumption,
comfort, and control accuracy. The Economizer tx2 algorithm generates the corner
values to compare energy consumption to a conventional air conditioning plant
control with ERC control included in the temperature sequence.
Building:
Museum
Primary plant:
Heating: Gas burner,
centralized ventilation plant with heating coils
Cooling and dehumidification: Refrigeration machine,
centralized ventilation plant with cooling coils
for humidification and dehumidification
Humidification: Central ventilation plant with steam humidifier
ERC control: ERC is integrated in the temperature sequence
with comfort band
Result:
Simulation results show that the Economizer tx2 application results in typical
energy savings of 5 to 10% annually. Savings of up to 40% are possible depending
on room use and outside air conditions.
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5 Advantages and customer benefits
5.1 Advantages
– Comfort control to maintain temperature and humidity limits
– Cost-optimized use of available energies
– Monetary display of plant efficiency
– Modular designed standard library applications including graphics for plant
operation using DESIGO INSIGHT to simplify engineering and commissioning
5.2 Customer benefits
– Suitable for existing plants, since optimization is achieved using purely controltechnical
measures without costly modifications to plant hardware
– Reduced air conditioning costs
– Optimized costs by considering energy tariffs
– Concrete means of sustainably reducing CO2 thanks to energy savings
– Time and costs savings during engineering, commissioning and occupancy
phases as well as lower service costs thanks to tested applications and detailed
documentation
– Meets the highest energy class in EN 15232 and increases the value of the plant
as well as the potential resale value of the building
Figure 5-1
6 Field of use
The following list outlines applications where it makes sense to use Economizer
tx2:
– Museums
– Printing shops
– Food industry
– Production or assembly halls with special requirements for ambient conditions
– Computer centers
– Pharmaceutical industry
– Labs
– Operating rooms
The application can be used on both new and existing plants.

7 Display and operation
DESIGO INSIGHT offers predefined plant pictures for air handling units. A table
shows a comparison of the most common energy consumption means (Table 3-7).
Figure 7-1
Overview picture with operating elements for air handling unit
8 System hardware
The Economizer tx2 application is approved for installation on the primary
automation station PXC.
9 Field devices
No special requirements are placed on field devices with regard to measuring
precision, quality, etc.
Siemens field devices should be used whenever possible.
10 Versioning
Economizer tx2 can be used from DESIGO V4.0.
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11 Appendix
11.1 Plant components
The Economizer tx2 application offers a modular design allowing for adaptation to
different types of air conditioning plants. It contains various variants and options.
Resulting in broad coverage of typical plants.
Components supported by Economizer tx2:
Components
ERC types
Economizer tx2 Comment
Mixing air X
Rotary heat exchanger X
Plate heat exchanger
Heating coil
X
Preheater X
Reheating coil
Humidifier types
X
Steam humidifier X
Washer X
Spray humidifier X
Cold water steam
humidifier
Cooling coil
X
Cooling
Fan
X
Single-speed X
Two-speed X
Modulating
Table 11-1
X
X Supported

11.2 h,x-diagram
Simplifying calculation of air state changes graphically was attempted a long time
ago. Diagrams for psychrometric calculations (e.g. enthalpy, dewpoint etc.) exist in
various forms. The Mollier diagram normally is used in Europe, and Carrier in the
U.S. Both diagrams use the same principal approach, only the orientation of the
axes varies. The temperature axis in the Mollier diagram is vertical, and horizontal
in Carrier. Water content in Mollier is horizontal, and vertical in Carrier.
Figure 11-1
h,x diagram by Mollier
Name Unit Description
A Temperature T Designates the heating condition of air. Indicated in degrees °C or
[°C] absolute in Kelvin K.
B Absolute x Absolute humidity x is the amount of water per kg in dry air,
humidity [g/kg] expressed in g.
C Relative % Dry air can only absorb a specific maximum amount of steam at a
humidity [r.h.] particular temperature. The higher the temperature, the greater the
possible amount of water in the air.
D Heat unit or h One of the most important calculations in terms of ventilation is
enthalpy [kJ/kg] determining the amount of heat needed to achieve an air state
predefined by temperature and humidity. In this case, air, whose state
is unknown, must either be mixed, heated, cooled, humidified, or
dehumidified to achieve the required air state. The heat contents h (in
kJ/kg) plays a significant role.
E Steam Pw Overheated steam mixed in air has a particular steam pressure Pw.
pressure [mbar] This steam pressure is part of the entire air pressure and, for this
reason, is also referred to as partial pressure for steam. This partial
pressure depends on the mixing ratio of steam to dry air. The higher
the steam content of air, the greater the partial pressure of steam Pw.
As a result, steam partial pressure Pw can be displayed in the mbar
parallel, horizontal to water content x, thus allowing for determining
the partial pressure Pw corresponding to a particular water content x
[g/kg].
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11.3 ERC control behavior
The best known, conventional control procedures for ERC are list below.
– Stg 1: Strategy Max ERC recovery ERC is fully controlled
– Stg 2: Strategy Temp. ERC is integrated in the temperature
sequence
– Stg 3: Strategy Hum. ERC is integrated in the humidity sequence
– Stg 4: Strategy Enthalpy. ERC is controlled to the supply air setpoint
enthalpy.
The air state after ERC depends on the selected strategy.
Figure 11-2
h,x diagram with schematic of the 4 strategies.
On the ERC lines, i.e. in the case of recirculated air on the line between the outside air and extract air,
all possible states for the mixed air are entered after mixed air damper control.
4 points on the line are special; they are the starting points for energy calculations.
Ex Extract air SpSu Supply air setpoint
x Steam content absolute T Temperature
Oa Outside air
Energy consumption is calculated based on air states 1 and 2 as per Figure 11-3
and the weighting factors to evaluate the energy of the strategies per strategy:
– State 2 is given by the supply air temperature and supply air humidity.
– State 1 can be determined for temperature and humidity based on the ERC
strategy and the effectiveness of the ERC.
– Calculation of energy costs considers the related air handling aggregates and
their specific costs. Involved aggregates result from the required change of state
and aggregate-specific, possible changes of state.
The strategies Stg 1 to Stg 4 (Figure 11-2) are displayed in DESIGO INSIGHT. For
inactive strategies, the setpoint for Sta 2 is used for calculation, see Table 3-7.

Figure 11-3:
Diagram for an air conditioning plant with entered, principal air states 1 (Sta1) and 2 (Sta2) for energy
calculation:
- Air state 1 corresponds to the state of the air after ERC
- Air state 2 corresponds to the state of the air after air handling
Ex Exhaust air Sta2 Air state 2
Oa Outside air Su Supply air
Sta1 Air state 1; depends on ERC strategy
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12 About this document
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© Siemens Switzerland Ltd, 2011 • Order no. CM110745en-TX2