100% Outside Air and Dehumidification Management

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)

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

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

Applications of Make-Up Air 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

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

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

Air Control Leaving Air Control Room Control Hot Gas Reheat Cycling Control Modulating Control Room Control Cycling in conjunction with a reheat thermostat Modulating in conjunction with a reheat sensor

100 % Outside Air - Room Reheat Control Reheat Control - room thermostat in series with discharge thermostat (neutral or cold air) T

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

“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

Standard Cycling Hot Gas Reheat Show hot gas reheat being fed from the top discharge line from the compressor. The reheat solenoid is controlled by a reheat thermostat located in the blower discharge. The Varispeed head pressure control maintains the head pressure when the unit goes in and out of reheat. The hot gas bypass is activated by low suction pressure. K is the Addison drain for liquid refrigerant in the reheat coil. The liquid line pump down solenoid is only used on long refrigerant line applications.

Hot Gas Reheat Plus Sub-Cooling Follow the liquid leaving the condenser coil. The normal sub-cooling for an air conditioner is 15 degrees. By adding the liquid to the sub-cooling coil and then running 55 degree air across this coil, sub-cooing improves to around 45 degrees. This allows you to select an 8 ton unit where the competition would need to use a 10 ton unit to accomplish the same performance.

Hot Gas Reheat Using Digital Scroll When using a digital scroll, the hot gas bypass requirement is not needed. The unit will maintain neutral air by means of capacity reduction- down to 10%.

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)

100% Outside Air Summer Design w/Reheat Supply Temp. 55.0/55.0 Ambient 92/75 These are design conditions. The unit is operating at 100% Capacity. Leaving Air 79.7/64.1 10 ton 1800 cfm

100% Outside Air Summer Design w/Reheat Plus Liq. Sub-cool 66.1/59.4 Supply Temp. 52.2 D.P. Ambient 92/75 This shows liquid sub cooling added. Note the capacity of the unit has increased so the supply air dewpoint is lower. 10 ton 1800 cfm Leaving Air 90.5/67.7

100% Outside Air Summer Part Load w/Reheat Supply Temp. 53.9 D.P. Ambient 75/75 75 degrees is right at the temperature where the lag compressor will stage off and the unit will operate at 50% capacity. There is more than enough reheat available at this temperature. Leaving Air 86.1/64.3 10 ton 1800 cfm

100% Outside Air Part Load w/Reheat Plus Liq. Sub-cool 65.7/58.8 Supply Temp. 50.8 D.P. Ambient 75/75 Same condition with reheat. Leaving Air 94.0/67.1 10 ton 1800 cfm

100% Outside Air Summer Part Load w/Reheat Supply Temp. 48.8/48.8 Ambient 70/70 One compressor operating Leaving Air 79.5/61.2 10 ton 1800 cfm

100% Outside Air Part Load w/Reheat Plus Liq. Sub-cool 59.9/54.3 Supply Temp. 48.1 D.P. Ambient 70/70 subcooling Leaving Air 83.1/64.3 10 ton 1800 cfm

100% Outside Air Summer Part Load w/Reheat Supply Temp. 46.8/46.8 Ambient 65/65 Leaving Air 73.7/63.8 10 ton 1800 cfm

100% Outside Air Part Load w/Reheat Plus Liq. Sub-cool 55.8/51.5 Supply Temp. 47.0 D.P. Ambient 65/65 Plenty of reheat is available- 7,000 BTUH of reheat available per ton(12,000 BTUH). 10 ton 1800 cfm Leaving Air 79.4/61.0

100% Outside Air Summer Part Load w/Reheat Supply Temp. 51.4/51.4 Ambient 60/60 Just short of neutral air on a 60 degree rainy day. 25% -50% capacity reduction by means of hot gas bypass. Leaving Air 64.3/56.2 10 ton 1800cfm

100% Outside Air Part Load w/Reheat Plus Liq. Sub-cool 55.2/52.1 Supply Temp. 50.4 D.P. Ambient 60/60 25% -50% capacity reduction by means of hot gas bypass. Leaving Air 72.6/57.4 10 ton 1800 cfm

100% Outside Air Winter Design Conditions 175,000 BTUH Furnace Ambient Heating to neutral air Leaving Air 72 10 ton 1800 cfm

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

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

Dedicated Outdoor Air Load Reduction Example: Loads based on: 4250 cfm supply and exhaust airflow 48” Energy Conservation Wheel 75F/ 50%RH summer exhaust air 70F / 30%RH winter exhaust The table compares the required capacity for a DOAS unit without an energy recovery wheel to one that has an energy recovery wheel. Make note of the different entering air conditions and resulting loads between the “Cooling” design day and “Dehumidification” design day. In many location the enthalpy of the dehumidification design condition will be greater than the cooling design day conditions, resulting in a great unit capacity requirement. This is very important for DOAS units since controlling humidity is a primary concern. Also note here the significant reduction in the winter heating requirement. Source: www.novelaire.com (graphic)

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