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Application Note 24: AHRI 920 Performance Rating & Compairsons of DX-DOAS Unit Efficiency

This application note will present the background on the AHRI 920 typical as effectively as an overview on how to use the rating values.

Introduction

 

In 2015, the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) introduced a efficiency rating for committed outdoors air systems. All references in this document refer to the AHRI Normal 920 (I-P) document. This was a quite timely announcement as it corresponded with the escalating demands for measurable power efficiency options and procedures to minimize worldwide warming. At the similar time the ASHRAE SPC 90.1- 2016 committee was updating their typical and wished to incorporate minimum rating values for this variety of solution.

 

The AHRI 920 and 921 are sister documents in that they each present a methodology to calculate the integrated seasonal values. The distinction is in that AHRI 920 is the inch-pound typical and AHRI 921 is the metric typical. It should really be noted that due to variations in the rating points becoming rounded to the nearest entire quantity, the actual power efficiency numbers can’t be converted amongst each and every typical.

 

The most up-to-date versions of ASHRAE 90.1-2016 power typical is emphasizing the significance of off-peak circumstances. Preserve in thoughts that design and style circumstances take place significantly less than 1% of the time, so the program functions for most of its operating life at element loads. Like the chiller product’s integrated element load values (IPLV) and air-conditioners IEER and SEER, the new DOAS typical has introduced its personal version of integrated values.

 

DX-DOAS Definition

 

DX-Devoted Outside Air Systems (DX-DOAS) situation outside ventilation air independently from the creating HVAC program. This method to handling ventilation air outcomes in superior humidity handle by limiting the major supply of humidity in most buildings – ambient humidity carried in by the ventilation air – straight at its supply just before it enters the creating. The decoupling of the ventilation air’s specifications from the building’s internal sensible specifications permits each systems to be developed for maximum power efficiency. The power savings can be obtained by operating the separate, sensible cooling only, interior cooling program significantly less frequently or at a greater evaporating temperature, as a result enhancing the general power efficiency.

Additional power savings may possibly be realized by offering only the quantity of ventilation air vital and/or by working with power recovery from the creating exhaust air to pre-situation the ventilation air.

Although the notion of introducing ventilation air is straightforward adequate, in actual practice there are several diverse configurations that can be utilized to resolve this purpose. These diverse configurations have a substantial influence on the power efficiency of the DX-DOAS units, so the very first step in the evaluation is to identify what configuration will be utilised at the precise jobsite. The category selections are:

  • Power recovery pre-conditioning
    If exhaust air can be brought back to the DOAS unit, then pre-conditioning the outdoors aire with an power recovery wheel or plate heat exchanger will enhance the power efficiency of the program.
  • Condenser
    There are various condenser kinds that influence the minimum efficiency of ASHRAE 90.1-2016

    • Air cooled (either split or packaged)
    • Water cooled (water temperature will influence program efficiency)
  • Cooling Tower
    The dehumidification only program utilizes a cooling tower water loop for its condensing water supply. The temperature of the fluid typically varies from 55°F to 90°F.
  • Chilled Water
    The dehumidification only program utilizes a chilled water loop for its condensing water supply. The temperature of the fluid typically varies from 40°F to 60°F.
  • Ground supply, closed loop
    A heat pump that utilizes fluid circulated by means of a subsurface piping loop as a heat supply/heat sink. The temperature of the fluid is associated to climate and operating history circumstances and typically varies from 25°F to 100°F.
  • Ground-water supply
    A heat pump that utilizes water pumped from a effectively, lake or stream as a heat supply/heat sink. The temperature of the water is associated to climate circumstances and typically ranges from 45°F to 75°F for deep wells.
  • Water Supply
    A heat pump that utilizes fluid circulated in a frequent piping loop as a heat supply/heat sink. The temperature of the piping loop fluid is typically mechanically controlled inside a moderate temperature variety of 61°F to 89°F.
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This rating typical has been created to present design and style engineers with a relative guide to quantify the power a DX-DOAS program will consume although dehumidifying and reheating ventilation air to a neutral 70°F situation at various diverse outdoors air circumstances. In contrast to air conditioning systems, the dehumidification course of action needs the program to situation the air to the design and style dewpoint (this is established at a 55°F dewpoint in the case of this typical) in order to get rid of the moisture just before it enters the space. The program then should use 75% to 90% website recovered power, no new power, to reheat to the neutral situation as stated in ASHRAE 90.1. Use of the hot gas reheat coil is a frequent remedy utilized to realize the website recovered requirement. The ambient circumstances that the unit should be developed for differ all through the year, so the AHRI typical utilizes 4 diverse dehumidification getting into air circumstances or rating points as a approach to demonstrate the moisture removal efficiency (MRE) at element load circumstances. This is expressed as the quantity of moisture removed per kilowatt-hour of power utilised.

Refer to Figure 1 for a psychrometric plot of the calculation of the total cooling necessary to meet the 55°F dewpoint and the quantity of moisture that should be removed. In order to meet this demand, DOAS units should have a quite low sensible heat ratio and have exceptional refrigeration circuits to manage this wide range of getting into air circumstances. A detail psychrometric assessment can be discovered in Desert Aire’s Technical Bulletin #three, Dehumidification and the Psychrometric Chart.

 

Figure 1 – Calculation of Total Cooling Necessary for a DOAS Style

 

If the program is a heat pump, the typical also defines two rating points for winter heating. The heating efficiency is stated as coefficient of efficiency (COP), which is units of watt output divided by the watt input. Refer to Tables 1 and two for the input values primarily based on the configuration chosen for the precise application.

To model genuine planet circumstances, the typical also has established an external static stress for several provide air volumes and has added a pumping penalty to account for the power consumption adder of water-cooled systems.

 

Figure two – Plot of Rating Points from Tables 1 and two

 

Table 1 – Operating Circumstances for Air &amp Water Cooled Systems (Table two – AHRI 920)

Rating Situation

Outside Air Getting into Temperatures

Exhaust Air Temperatures

Inlet Fluid Temperatures

Ambient Air

 

°Fdb

°Fwb

°Fdb

°Fwb

Cooling Tower

Chilled Water

°Fdb

Dehumidification

 

 

 

 

 

 

 

A

95

78

75

62.five

85

45

95

B

80

73

75

62.five

80

45

80

C

68

66

75

62.five

68

45

68

D

60

58

75

59.six

55

45

60

Heating

 

 

 

 

 

 

 

E

35

29

70

58.five

N/A

N/A

35

F

16

12

70

58.five

N/A

N/A

16

 

Table two – Operating Circumstances for Heat Pump Systems (Table three – AHRI 920)

Rating Situation

Outside Air Getting into Temperatures

Exhaust Air Temperatures

Inlet Fluid Temperatures

 

°Fdb

°Fwb

°Fdb

°Fwb

Closed Loop

Ground Water

Water Supply

Dehumidification

 

 

 

 

 

 

 

A

95

78

75

62.five

85

70

85

B

80

73

75

62.five

80

70

85

C

68

66

75

62.five

70

50

75

D

60

58

75

59.six

70

50

75

Heating

 

 

 

 

 

 

 

E

35

29

70

58.five

41

70

75

F

16

12

70

58.five

32

50

70

Integrated Seasonal Ratings

 

The AHRI committee utilised a composite bin hour information set of several cities to present a weighted worth moisture removal efficiency and coefficient of efficiency worth. This weighted worth puts emphasis on the element load values in calculating the integrated seasonal ratings. Table three lists the weighting percentage values.

Table three – Weighting Percentages for Integrated Ratings

MRE

COP

Rating Point

Weight %

Rating Point

Weight %

Situation A

12%

Situation E

77%

Situation B

12%

Situation F

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23%

Situation C

36%

 

 

Situation D

24%

 

 

The integrated seasonal values are defined as a weighted typical of the person rating point values and are expressed in pounds of moisture per kilowatt (MRE) in the dehumidification mode and in watts input per watts output (COP) in the winter heating mode if the program is a heat pump.

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ISMRE = (MREA•0.12) + (MREB•0.28) + (MREC•0.36) + (MRED•0.24)

exactly where

MREA = Normal Rating Situation A (95 °Fdb / 78 °Fwb)

MREB = Normal Rating Situation B (80 °Fdb / 73 °Fwb)

MREC = Normal Rating Situation C (68 °Fdb / 66 °Fwb)

MRED = Normal Rating Situation D (60 °Fdb / 58 °Fwb)

ISCOP = (COPE•0.77) + (COPF•0.23)

exactly where

COPE = Normal Rating Situation E (35 °Fdb)

COPF = Normal Rating Situation F (16 °Fdb)

 

Table four – ASHRAE 90.1-2016 Minimum Values

Gear Variety

With out Power Recovery

With Power Recovery

Dehumidification Mode

ISMRE

ISMRE

Air Cooled

four.

five.two

Cooling Tower

four.9

five.three

Chilled Water

six.

six.six

Water – Ground Supply – Closed Loop

four.eight

five.two

Water – Ground Water Supply

five.

five.eight

Water Supply

four.

four.eight

Heat Pump Heating Mode
ISCOP
ISCOP

Air Cooled

two.7

three.three

Water – Ground Supply – Closed Loop

two.

three.eight

Water – Ground Water Supply

three.two

four.

Water Supply

three.five

four.eight

 

It is significant to note that the rating typical adds the requirement that if the DOAS unit can’t bring the leaving air temperature back to 70°F with recovered power, that a supplementary heat penalty be incorporated primarily based on the use of an electric heater. An option approach would be to use a bigger compressor that gives excess capacity to reheat the air to 70°F at a greater input energy.

In the dehumidification mode some systems have to have to enhance their hot gas bypass capacity at the C and D circumstances. These systems use the recovered power to stop the evaporator coil from freezing as an alternative of directing the power to the hot gas reheat coil, for that reason not possessing adequate power to meet the 70°F requirement. This typical as a result permits an suitable comparison by permitting these systems to use a lot more power to attain the necessary leaving air temperature.

Likewise, in the winter heat pump mode some systems are unable to heat the winter outside air to the necessary 70°F minimum worth listed in the typical. The penalty adds in an electric heater to make up the distinction. Such a program will have a reduced COP, but be in compliance of the 70°F leaving air temperature specification.

For systems that are not equipped with a heat-pump mode of operation, the variety and efficiency of supplementary heat sources will be published with companies information. These heat supply kinds and efficiency should really be reviewed very carefully when generating comparisons amongst diverse kinds or companies gear.

 

Comparison Dehumidification

 

The AHRI 920 typical gives a approach to evaluate a precise manufacturer’s design and style to the minimum typical as published by the AHRAE 90.1-2016. This permits the design and style engineer the capability to evaluate the diverse configurations of companies of DX-DOAS gear in order to meet the power objectives of the creating owner.

For an air cooled DOAS that has an ISMRE worth equal to eight., the comparison to the ASHRAE 90.1-2016 minimum worth of four. would present a conclusion that the program was two instances as effective (eight. / four. = two.). To estimate the correct price of power, you will have to have to convert t he ISMRE worth into a kilowatt worth and annual power price. One particular way to roughly estimate the price for dehumidifying the outside air could be as follows:

 

ISMRE Continual .020 lb/hr/cfm

The ISMRE conversion worth for the typical moisture removal of 20 lb/hr per 1,000 cfm was arrived at by calculating the person moisture removed for points A to D for 1,000 cfm of outdoors air to a 55F dew point. These person values then have been compiled into a single worth working with the weighted values from the ISMRE calculation. So the derived worth is .020 lb/hr/cfm.

 

1) Working with the weighted typical moisture removed from the integrated values, calculate the typical capacity (see sidebar for facts of the continual derivation).

two) Working with the typical moisture capacity, convert the efficiency to typical energy.

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Formula to convert the efficiency to average power

three) Multiply the typical energy use by the neighborhood utility price and the bin hours about the design and style dew point.

Formula to determine annual cost of operation

For instance, a five,000 cfm DOAS unit would get rid of a weighted moisture content material of roughly one hundred lb/hr. A four. ISMRE program would have an typical energy of 25 kW throughout dehumidification. Working with a bin hour total in the dehumidification mode of four,800 hours and a utility price of $.ten/kWh would present an annual cost of $12,000. The chosen unit at an ISMRE worth of eight. would price only $six,000, as a result the chosen unit would save the owner $six,000 per year of operation.

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Even though there are variances in loads for any unique location and the weightings of the 920 typical are a composite of several diverse location of the US, expertise has shown that the formula above offers a quite fantastic estimate of the power charges.

Outcomes can be utilised to make suitable choices concerning the initial program price verses continued power use.

ISCOP Continual .0125 kW/kW/cfm

The ISCOP conversion worth for the typical heating load of 12.five kW per 1,000 cfm was arrived at by calculating the person heating specifications for points E and F for 1,000 cfm of outdoors air to a 70F dry bulb set point. These person values then have been compiled into a single worth working with the weighted values from the ISCOP calculation. So the derived worth is .0125 kW/kW/cfm

 

Comparison Heating

 

A heat pump program can also have its heating power price compared for its annual comparison. For a ground supply, closed loop DOAS that has an ISCOP worth equal to four., the comparison to the ASHRAE 90.1-2016 minimum worth of two. would present a conclusion that the program was two instances as effective (four. / two. = two.). To estimate the correct price of power, you will have to have to convert the ISCOP worth into a kilowatt worth and annual power price. One particular way to roughly estimate the price for heating the outside air could be as follows:

1) Working with the weighted typical heating requirement removed from the integrated values, calculate the typical heating capacity (see sidebar for facts of the continual derivation).

Formula to calculate the average heating capacity

two) Working with the typical moisture capacity, convert the efficiency to typical energy.

Formula to convert the efficiency to average power

three) Multiply the typical energy use by the neighborhood utility price and the bin hours above the design and style dew point.

Formula to calculate the annual heating cost

For instance a five,000 cfm OAS heat pump unit would call for a weighted heating requirement of 62.five kW (213 MBH). A two. ISMRE program would have an typical energy of 31.25 kW throughout heating. Working with a bin hour total in the heating mode of three,950 hours and a utility price of $.ten/kWh would present an annual price of $12,344. The chosen unit at an ISMRE worth of four. would price only $six,172, as a result the chosen unit would save the owner $six,172 per year of operation.

 

Conclusion

 

DOAS units operate in several diverse climates and can be chosen with a wide selection of styles and condensing selections. This tends to make it tricky to evaluate the power efficiency of treating outdoors air from one particular design and style to a different or from one particular manufacturer to a different manufacturer. Nevertheless, this is very significant as the course of action of treating outdoors air to enhance indoor air high quality is power intensive. The AHRI 920 typical is a tool that can be utilised to evaluate the power efficiency of diverse manufacturers’ offerings to optimize the power consumption for the finish user.

In addition, ASHRAE has established minimum values for DOAS units. This gives the designer the base level to establish the specification criteria. By working with the calculations in this application note, the design and style engineer can also evaluate the payback time necessary to specify the greater power effective styles.

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