100% Outside Air and Dehumidification Management

This document is intended to assist licensed professionals in the exercise of their professional judgment.

2011 Addison

100% Outside Air and Dehumidification

Management


2011 Addison

100% Outside Air - What is it ?

• Referred to as “Make-up Air”– Unit operates continually during occupied

mode– Replaces exhausted air– No mixture with return air– Totally changing load

• Summer design (High temperature and high humidity)

• Winter design (Cold temperature)


2011 Addison

Why 100% Outside Air?

• Ventilation standards indicating recommended air changes for a variety of space usages and occupancy have been established by ASHRAE. The ventilation requirements for a given structure or space are based either on desired number of air changes or the number of occupants.

• The amount of outside air fresh air required for each change per hour equals the amount of inside air that must be removed from the space during the same time span.

• CFM= Area of space / Minutes of air changes


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Ventilation Requirements (continued)

• College classroom 8 air changes 20-30 cfm/person• Hospitals 9-17 air changes 40-50

cfm/person• Kitchens 10-20 air changes 40-50

cfm/person• Schools 8-12 air changes 30-35

cfm/person• Theaters 5-10 air changes 10-15

cfm/person• Factory Buildings 10-15 air changes 25-30

cfm/person


2011 Addison

•Return Air Ducts –Typically will not affect space conditions– May not require Reheat– Control is not as critical

•Direct to Space–Usually applied to large areas

•(Auditoriums, Kitchens, Corridors)

– More precise control is necessary– Reheat is required for control

Applications of Make-Up Air


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100% Outside Air “Supply Air Properties”

• Unit discharge air properties– Main purpose – positively impact building

humidity• Lowest dew point possible

– Ideally close to room (neutral) conditions• Typically between 68 -72 degrees• Not to overcool or overheat space

– Below 70% RH per ASHRAE standards


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Dew Point Comparison Reheating to Neutral Air

DRY BULB TEMPERATURE - °F

70°F 72°F 75°F

DEW POINT RELATIVE HUMIDITY – RH %

45°F 40.6 37.9 34.3

50°F 49 45.8 41.4

55°F 58.9 55.1 49.8

60°F 70.6 65.9 59.6

65°F 84.2 78.6 71.1


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Air Control

• Leaving Air Control– Hot Gas Reheat

• Cycling Control• Modulating Control

• Room Control– Hot Gas Reheat

• Cycling in conjunction with a reheat thermostat• Modulating in conjunction with a reheat sensor


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100 % Outside Air - Room Reheat Control

Reheat Control - room thermostat in series with

discharge thermostat

(neutral or cold air)

T

T


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Compressors Usage

• Single Compressor -- Single Circuit – Used when low dew point in required– Dehumidifies all the air all of the time

• Dual Circuits – Single Compress each circuit– Used when leaving air conditions are not critical– Air will bypass lag circuit and then mix with lead circuit

• Dual Circuits – Two compressors (Tandem) each circuit)– Lag compressor cycles based on suction pressure.– Four (4) stage operation– Large Tonnage


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“Refrigeration” Control• Hot Gas Bypass

– Modulates to match the evaporator load– Evaporator freeze protection– Suction pressure activated

• Hot Gas Reheat – Raises leaving air temperature to room neutral conditions– Cycling with discharge air thermostat– Modulating with sensor.

• Liquid Subcool Coil– Raises leaving air temperature– Subcools liquid– Two (2) degrees of subcool equals one (1) percent increase

in evaporator capacity


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Standard Cycling Hot Gas Reheat


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Hot Gas Reheat Plus Sub-Cooling


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Hot Gas Reheat Using Digital Scroll


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100% Outside Air Example

• Summer design

– 1800 cfm– 92/75 ambient– Leaving air

• 55 deg Dew Point• 65 deg leaving air temperature

• Winter design– 0 degree ambient– Leaving air (minimum 70 deg)


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100% Outside Air Summer Design w/Reheat

Ambient92/75

Supply Temp.55.0/55.0

Leaving Air79.7/64.1 10 ton

1800 cfm


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100% Outside Air Summer Design w/Reheat Plus

Ambient92/75

Supply Temp.52.2 D.P.

Leaving Air90.5/67.7

10 ton

1800 cfm

Liq. Sub-cool66.1/59.4


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100% Outside Air Summer Part Load w/Reheat

Ambient75/75



10 ton

1800 cfm


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100% Outside AirPart Load w/Reheat Plus

Ambient75/75



10 ton

1800 cfm



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100% Outside AirSummer Part Load w/Reheat

Ambient70/70


Leaving Air79.5/61.2 10 ton

1800 cfm


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Ambient70/70



10 ton

1800 cfm



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Ambient65/65



10 ton

1800 cfm


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Ambient

65/65

Supply Temp.

47.0 D.P.


10 ton

1800 cfm



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Ambient60/60



10 ton

1800cfm


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Ambient60/60



10 ton

1800 cfm



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100% Outside AirWinter Design Conditions

Ambient0

Leaving Air72 10 ton

1800 cfm

175,000 BTUH Furnace


2011 Addison

Recap

• Do No Harm– Dehumidify to 55 0 Dew Point or less.– Reheat to Room Neutral.– Temper to Room Neutral– Possible humidify when ambient

temperatures are below 35 0


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Energy Conservation Wheel Applications

• Require exhaust air from the building

• Require defrost in most areas

• Total Unit EER can exceed 25 on some water source heat pump applications


2011 Addison

Load Reduction Example:

Dedicated Outdoor Air

Cooling Dehum. Cooling Dehum.

(db / wb) 93.9 / 74.8 82.0 / 76.4 93.9 / 74.8 82.0 / 76.4(enthalpy) 38.22 39.91 38.22 39.91(db / wb) --- --- 80.7 / 67.0 77.1 / 67.6(enthalpy) --- --- 31.50 32.02(db / wb) 52.0 / 52.0 52.0 / 52.0 52.0 / 52.0 52.0 / 52.0(enthalpy) 21.43 21.43 21.43 21.43

196.58 206.61-40.0% -42.7%136.49 119.37-31.5% -16.3%60.09 87.24

-53.2% -60.0%

Outdoor Air (db)

ECW Supply Air (db)

Unit Supply Air (db)

Cooling Mode

MBH

MBH

Outdoor Air

ECW Supply Air

Evaporator Discharge Air

Total Capacity

Sensible Capacity

Latent Capacity

142.67

128.36MBH

Heating Capacity

ASHRAE Design Conditions for Atlanta, GA

MBH97.31

---

321.30-69.7%

48.870

Heating Mode

100% OA w/ Energy Conservation Wheel

70

0

327.62 360.63

199.26

0

100% OA

217.96

Loads based on:• 4250 cfm supply and exhaust airflow• 48” Energy Conservation Wheel• 75F/ 50%RH summer exhaust air• 70F / 30%RH winter exhaust

•Source: www.novelaire.com (graphic)


2011 Addison

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100% Outside Air and Dehumidification Management

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