1. 100% Outside Air and Dehumidification Management

2. 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)

3. 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

4. Ventilation Requirements (continued)
College classroom 8 air changes cfm/person
Hospitals air changes cfm/person
Kitchens air changes cfm/person
Schools air changes cfm/person
Theaters air changes cfm/person
Factory Buildings air changes cfm/person

5. 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

6. 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 degrees
Not to overcool or overheat space
Below 70% RH per ASHRAE standards

7. 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

8. 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

9. 100 % Outside Air - Room Reheat Control
Reheat Control - room thermostat in series with discharge thermostat
(neutral or cold air) T

10. 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

11. “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

12. 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.

13. 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.

14. 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%.

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 100% Outside Air Winter Design Conditions
175,000 BTUH Furnace
Ambient
Heating to neutral air
Leaving Air 72
10 ton
1800 cfm

27. 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

28. 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

29. 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: (graphic)

30. for your attention