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RT-PRB018-E4 VoyagerTM

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2. Fresh air options

2.2. Economizer

2.2.1. Application

Trane differential (or comparative ) enthalpy economizer is provided to take advantage of cooler outdoor air tosatisfy a cooling load in a conditioned space minimizing the need for mechanical cooling (with compressors).When fresh air is needed in the building.When the volume of fresh air needed is between 0% and 50% of the nominal airflow of the rooftop.When that volume is needed permanently and needs to be changed remotely.To save energy via free-cooling.

2.2.2. Description

An economizer consists of a fresh air damper, a return air damper, linkage to maintain an inverse relationship betweenthe two, and an actuator to control the damper position.

One hood for downflow, three hoods for horizontal flow.Filters (mist eliminator).Motorized fresh air dampers.One electronic board (ECA: Economizer Control Actuator, see illustrations 8 & 9) fitted with a potentiometer.RA (Return Air) damper(s).RA humidity sensor.RA temperature sensor.OA (Outdoor Air) humidity sensor.MA (Mixed Air) temperature sensor.

Note: There is no dedicated Outdoor Air temperature sensor for the economizer simply because the ECA uses the standardOutdoor Air temperature sensor of unit.

Illustration 5. Economizer front-view: downflow version

Illustration 6. Economizer front-view: horizontal flow version


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Illustration 7. Economizer dampers: horizontal flow version

Illustration 8. ECA with module Illustration 9. ECA with module layout

2.2.3. Operation

The potentiometer controls the fresh air damper.

The amount of fresh air introduced is set between 0% and 50% of the nominal air flow during the first potentiometer set-up and then remains constant afterwards.The fresh air dampers close in unoccupied mode when the SA - Supply Air - fan stops.Free-cooling by comparing OA and RA enthalpy levels.The OA volume will modulate accordingly, between a permanent value & 100% (see details below).

2.2.3.1. Main features

Fresh air dampers positions are limited so that the mixed air temperature does not fall below 12C ( 1,5C) and causeexcessively cool air from being discharged from the unit.

In order to maximize the use of the economizer and further reduce the need for more expensive mechanical cooling:When used with a zone sensor, an economizer setpoint that is below the cooling setpoint is used to allow sub-cooling essentially for free.Compressors are delayed from operating until the economizer has opened to 100% for 5 minutes and it has beendetermined that the economizer alone cannot satisfy the load.

Any time the supply fan is On and the building (unit) is occupied, the economizer damper will be maintained at orabove minimum position. The economizer dampers are held closed when the supply fan is Off to prevent waterfrom getting into the economizer section of the unit.

2.2.3.3. Economizer operation in cooling mode

Start-up: When outdoor conditions are suitable, the economizer OA dampers open 100% (fresh air) during 5 minutes,then a 3-minute cyclic delay evaluates if the room temperature is dropping (see illustration 10) :

If the room temperature has dropped by more than approximately 0,3C (rate of 7C/h), the fresh air dampers startto modulate.If the room temperature has not dropped by more than approximately 0,3C (rate of 7C/h), compressor n1 starts.If SA temperature is below 12C, the fresh air dampers close progressively (whenever the compressor is on or off).


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Usage with a Trane zone sensor (THS/P 03) or Integrated Control System (ICS: TZS+TrackerTM

, etc.):Economizer dampers are modulated between their minimum position and 100% to maintain the zone temperatureat the economizer setpoint.Economizer setpoint (ESP) is derived from the Cooling and Heating setpoints (CSP and HSP) of theprogrammable zone sensor or of the ICS, so that ESP is the higher of:

- 1) CSP - 1C or- 2) HSP + 1C.

Usage with a conventional thermostat (THS/P 01, THS/P 02, others):

Economizer setpoint feature, relative to the cooling and heating setpoints, is lost.Economizer enable/disable function is determined by the OA sensor through the RTRM.Economizer dampers are modulated between minimum position and 100% to maintain mixed air temperature at12C ( 1.5C) in response to a call for stage 1 of cooling (Y1 active), assuming economizing is enabled. At 10Cthe dampers will be at minimum position. If the economizer is enabled, a Y2 call for 2nd stage cooling will start thefirst compressor if required.If the economizer is disabled, Y1 will be the first compressor. If the unit has two compressors, Y2 will start thesecond compressor. When using a conventional thermostat, or other binary input, the ReliaTel

TMcontrols will only

allow two stages of cooling.If a single-stage thermostat is used, only the economizer (if enabled) or the compressor (if economizer is disabled)will operate on a call for cooling. A two-stage thermostat is required to achieve economizer operation andcompressor operation at the same time.

Illustration 10. Economizing control process

Room temperature (C)

Time (min)23,7

23,8

23,9

24

1 2 3 4 5

23,4

23,5

23,6

Temperature

evaluation

Temperature

evaluation

OA damper100% opened

0 8

24,1

24,2

24,3

OA dampermodulates

9

Compressoroperation

76

Room temperature (C)

Time (min)23,7

23,8

23,9

24

1 2 3 4 5

23,4

23,5

23,6

Temperature

evaluation

Temperature

evaluation

OA damper100% opened

0 8

24,1

24,2

24,3

OA dampermodulates

9

Compressoroperation

76

2.2.3.5. When are conditions suitable for economizing?

Three different methods can be used to determine if outdoor air contains more cooling capacity than the return air. Thesedifferent methods are suited for different applications and environments, as described below:

Comparative enthalpy: Outdoor air enthalpy is compared with return air enthalpy. This method is best suited for highhumidity climates and applications in which humidity can affect the cooling capacity of the outdoor air or return air. Traneeconomizer used this method.Reference enthalpy: Outdoor air enthalpy is compared with a reference enthalpy point. This method is best suited for highhumidity climated in which humidity can affect the cooling capacity of the outdoor air, but not necessarily the return air.

Reference dry bulb: Outdoor air temperature is compared with a user set reference temperature. This method is bestsuited for low humidity climates and applications in which humidity does not strongly affect cooling capacity of the outdoorair or return air.


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Trane Economizer works according to the comparative enthalpy method, as described below (see section 2.2.6.1.Economizer set-up for more details about the Reference enthalpy and Reference dry bulb methods).

We need to consider 4 different situations (see illustration 11):

1. OA enthalpy RA enthalpy is decreasing and above -6,98 kJ/kg The economizer is disabled (mechanicalcooling is used).

2. OA enthalpy RA enthalpy is decreasing and below -6,98 kJ/kg The economizer is enabled.

3. OA enthalpy RA enthalpy is increasing and below 0 kJ/kg The economizer is not disabled if it was inoperation, but not enabled if it was not in operation.4. OA enthalpy RA enthalpy is increasing and above 0 kJ/kg The economizer is disabled and mechanical cooling

is enabled.

Illustration 11. Economizing conditions

Time

[Outside air enthalpy] [Return air enthalpy](kJ/kg)

OA-RA enthalpy

isdecreasing

OA-RA enthalpy

isdecreasing

OA-RA enthalpy

isincreasing

OA-RA enthalpy

isincreasing

OA-RA enthalpy

>-6.98 kJ/kg

OA-RA enthalpy

[Return air enthalpy]

[Outside air enthalpy]-6.98 kJ/kg

OA-RA enthalpy

[Return air enthalpy]

[Outside air enthalpy]


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Explanation :

Compared to a dry bulb changeover economizer, the time of economizing will be greater as it will admit highertemperature fresh air when the relative humidity allows it. Illustration 12 shows the additional working area (pink) of thedifferential enthalpy economizer. It will also avoid letting unsuitable OA enter due to an aggressive setpoint (see blackarea).Compared to a reference enthalpy economizer, RA and OA enthalpies will be measured and the most suitable one(lowest value) will be chosen. A single-sensor enthalpy or temperature changeover strategy is based on a simple control

decision whether the outdoor air is above or below an arbitrary setpoint (see blue line). That setpoint may beconservative, to safeguard occupant comfort, or aggressive, to maximize energy savings. This strategy ignores the factthat the return air may be more suitable for occupant comfort and less expensive to cool.

Illustration 12. Economizing capabilities

Note: the building balance point is the ambient temperature at which neither heating nor cooling is required to maintain theroom setpoint. In other words, the internal load matches exactly the external load.

2 4 7 10 13 16 18 21 24 27 29 32 35 38 41 43C

13

10

7

4

2

16

18

21

24

27

ENERGY SAVEDDRY BULB

ENERGY WASTED

DRY BULB

ADDITIONALENERGY SAVED

COMPARATIVEENTHALPY

RA: 23C, 50% relative humidity

building

balance pointeconomizer dry bulb

changeover setpoint

economizer maximum

operating limit

EXAMPLE OF REFERENCE

ENTHALPY SETPOINT

2.2.5. Incompatibilities

0-25% manual fresh air hood.

2.2.6. More details

2.2.6.1. Economizer set-up

On VoyagerTM

III, Trane economizer always comes configured as a comparative enthalpy one. However, it can beconverted into a dry bulb change over or a reference enthalpy change over depending on sensors connections. If the RAhumidity sensor is disconnected, reference enthalpy method will be used. If the RA temperature sensor is disconnected,reference dry bulb method will be used.

Dry bulb/Reference Point Selections: The Dry Bulb or Reference Enthalpy Point is user-selectable, according to thechoices below. This selection is made on the ECA and is only functional on units with a reference or comparative enthalpyoption.


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Reference enthalpy method:OA enthalpy is compared with a reference enthalpy point (REP).Economizer enabled if: OA enthalpy < [REP 1,16 kJ/kg]Economizer disabled if: OA enthalpy > [REP + 1,16 kJ/kg]Economizer status not changed if: [REP 1,16 kJ/kg] < OA enthalpy < [REP + 1,16 kJ/kg]

Reference dry bulb method:OA temperature is compared with a reference dry bulb point (RDBP).Economizer enabled if: OA temperature < RDBP

Economizer disabled if: OA temperature > [RDBP + 3.0C]Economizer status not changed if: RDBP < OA temperature < [RDBP + 3.0C]

llustration 13. Comparative / Dry bulb / Reference enthalpy economizing requirements

ustration 14. Dry bulb / Reference enthalpy change over point choices

Potentiometersetting points

Dry bulb changeover point

Referenceenthalpy

A * 22.8C ** 62.8 kJ/kg

B 21.1C 58.1 kJ/kg

C 19.4C 53.5 kJ/kgD 17.2C 51.1 kJ/kg

* factory setting** dry bulb is fixed at 22.8C and is not adjustable

2.2.6.2. Detailed example of the comparative enthalpy economizer benefits

This study was made in North American cities -The weather description can help us to find similar applications in Europe

1) Building type

A digital computer was used to vary the seasonal and geographical weather conditions on a model of a retail store. Sixdifferent geographical regions were used. The retail store was modeled with the following characteristics:

single-storey 418 m2

building on a concrete slabconstruction materials consisting of a brick exterior over concrete block walls with plaster on the interior walls and withceiling insulationstand-alone building exposed on all exterior wallsglass windows covering 25% of the wall areaoccupancy of 7 days per week for 12 hours per day

false ceiling with a fluorescent lighting load of 13.5 W/m2

rooftop HVAC unit with natural gas heat and with air conditioning sized at 110 W/m2 of floor spacesingle zone 2-stage cooling, l-stage heating thermostat with a 24C daytime cooling setpoint, a 21C daytime heatingsetpoint, a 13C night time heating setpoint setback, and with fan motor and cooling shutdown at night

ventilation set at 10% outdoor air during occupancy (one air change per hour) and with the outdoor air damper closedwhen building is unoccupiedoccupancy which can be changed dynamically between 14 and 27 personsno exhaust or make-up air systembuilding balance point of 10C, the temperature at which neither heating nor cooling is required

2) Weather conditions

The weather simulation for each site was based on humidity and temperature data from the Typical Meteorological Year(TMY) developed at the Sandia Laboratories from meteorological data compiled by the National Climatic Center. The siteswere selected on the basis of providing a variety of regional humidity, length of cooling season, and temperature conditionsas follows :


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no economizer

13C dry bulb

enthalpy

differential

energy usage in kWh for mechanical cooling

(1) Lake Charles, LAa hot humid southern regionan area usually regarded as unfavorable for producingsignificant savings from economizers(2) Los Angeles, CAa mild Pacific coastal area with long cooling seasons and an area which is usually very favorable forproducing economizer savings(3) Madison, WIa northern Midwest region with cold winters and relatively short cooling seasons(4) New York, NYa Northeast region with cold winters, short cooling seasons, and moderate to high utility rates(5) Albuquerque, NMa hot, dry Southwest region with a long cooling seasons

(6) Seattle, WAa north Pacific coastal region with mild seasons and with moderate to high humidity

3) Procedure

The computer simulation was used to determine the compressor and fan kilowatt hour consumptions for the differenteconomizer systems at each location. The systems studied were :

one with no economizeran economizer with a dry bulb temperature changeover control set at 13Can economizer with a single enthalpy changeover controlan economizer with differential enthalpy changeover control

4) Results

The conclusion of this study is that there is a substantial energy saving potential from installing economizers with differentialenthalpy changeover, and that differential enthalpy savings can nearly double the savings expected from those of a single

enthalpy changeover. It was also concluded that economizers with dry bulb changeover produce less savings thaneconomizers with enthalpy changeover and that, in practice, many economizers with dry bulb changeover are set tooconservatively to produce any savings at all.

13C13C dry bulb

enthalpy

differential

% of energy saved

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