Air Conditioning 101
STN Presentation AC101
•What is Refrigeration?
–Refrigeration is Cooling by the Removal of Heat
•Heat is Measured In BTU’s
–A BTU is a British Thermal Unit
–It is the Amount of Heat to Raise One Pound of Water, One
Degree Fahrenheit.
STN Presentation AC101
STN Presentation AC101
AC System Basics
- BTU’s Listed by Rating Methodology
- IMACA (International Mobile Air Conditioning Association)
- ARI (American Refrigeration Institute) – Refrigerated Transport and Transit Buses
- KW (Kilowatts – European / Metric Measure)
- ASHRAE – (American Society of Heating, Refrigeration and Air Conditioning Engineers)
Used in Residential and Commercial Building Applications
- 1 ARI BTU = 2 IMACA BTU’s (approximately)
-1 KW = 3.415 ARI BTU’s or 6.83 IMACA BTU’s
- Know which standard is being referenced and why
- IMACA used mostly in small bus / school bus split systems
Performance DataCooling System capacity @ARI Rating 43,031 Btu/hr (12.6 kW)
Cooling System capacity @ IMACA Rating 81,964 Btu/hr (24 kW)
AC 101
Each Component has a Rated
Capacity. The system cannot
produce cooling greater than the
least rated component in the system.
Major Components of Split Systems:
1. Compressor
2. Condenser
3. Expansion Valve (TXV) Not
Adjustable
4. Evaporator
STN Presentation AC101
Small Bus (Split System) - Air Conditioning Refrigerant Cycle
Road Compressor
DischargeAccess Valve
Filter Dryer
Evaporator Coil
Condenser Coil
ThermostaticExpansion Valve(Block Type)
SightGlass
Low PressureSwitch
High PressureSwitch
SuctionAccess Valve
Major Components of an Air Conditioning System:
–Compressor•Belt Driven by the Bus Engine•Compresses Low Pressure, Low temp Vapor into High
Pressure High Temp Vapor•Electromagnetic Clutch controls On/Off
–Condenser•Mounted On the Roof or Skirt of Bus•Refrigerant Enters as High Temp/High Pressure Vapor•Condenser Rejects the Vapor’s Heat•Refrigerant Vapor Condenses into High Pressure Liquid
STN Presentation AC101
Major Components of an Air Conditioning System:
–Expansion Valve•Meters Flow of Refrigerant into Evaporator
•High Pressure Liquid turns into Low Pressure, Low
Temperature Sat Mixture in Evaporator
–Evaporator•Evaporator Absorbs Heat from the Passenger Compartment
•The Refrigerant Turns from a Saturated Mixture to a Low Pressure, Low
Temperature vapor in the Evaporator
•Vapor Returns to the Compressor Through the Suction Line
STN Presentation AC101
Major Components of an Air Conditioning System:
–Refrigerant•A Fluid that Picks up Heat by Evaporating at a Low Temperature
and Pressure.
•It Rejects Heat by Condensing at Higher Pressure and
Temperature
•134a is the Refrigerant Used Today In Split Systems
STN Presentation AC101
STN Presentation AC101
Simple, Single AC Circuit
BasicRefrigeration
System
4 Component Flow DiagramSTN Presentation AC101
Condenser
Compressor
DischargeServiceValve
Thermal Expansion Valve
HT HP SHV
HT HP S.M.
HT HP SCL
Legend
HT HP SHV
HT HP S.M.
HT HP SCL
The High Side
STN Presentation AC101
Thermal Expansion Valve
Evaporator
TXV Bulb
Equalization Line
SuctionService Valve
Compressor
Flash Gas
Legend
LT LP S.M.
LT LP SHV
The Low Side
LT LP S.M.
LT LP SHV
STN Presentation AC101
High Side
AmbientAir
Low Side
ReturnAir
Supply Air
Conditioned Space
STN Presentation AC101
Types of AC Systems found in School Buses:
–Single Circuit
–Type A Buses or a Cooler Environment–One Automotive Type Compressor
–Original Equipment Compressor OR
–Add on Compressor
–One or More Evaporators
–One Condenser
STN Presentation AC101
Sample Single Circuit System
Rear Mounted - Free Blow Evaporator
Skirt Mounted Condenser
Supplied Compressor
Tie-In Single System
Rear Mounted - Free Blow Evaporator
Skirt Mounted Condenser
Tied in With OEM Dash Unit
Types of AC Systems found in School Buses:
–Dual Circuit or Loop–Type C & D Buses or Warmer Environment
–Two Automotive Type Compressors OR–Two Add On Compressors OR
–OEM Compressor and One or Two Add On Compressors
–One Transit Type Compressor
–Two or More Evaporators
–One High Capacity Condenser OR
–Two Smaller Capacity Condenser
–Split Systems are Modular–Components can be Mixed and Matched
–You have to know what you are receiving in the system
STN Presentation AC101
Types of AC Systems found in School Buses:–Dual Circuit or Loop
–Type C & D Buses or Warmer Environment–Two Automotive Type Compressors OR
–One Transit Type Compressor
STN Presentation AC101
Two Automotive Compressors
One Transit Compressor
Types of AC Systems Air Delivery in School Buses:–Corner Ducted
–Free Blow
STN Presentation AC101
Corner DuctedRooftop System
Corner DuctedEvaporator on Both Sides
Types of AC Systems Air Delivery in School Buses:–Corner Ducted
–Free Blow
STN Presentation AC101
Free Blow EvaporatorIn Wall Design
Free Blow Evaporator Side Mounted
What Size AC System is Right?:
–It All Depends–Location
–Environmental Conditions–Heat–Humidity–Non AC Season Conditions
–Your Use–Special Needs–Route–Trips
–Do Your Homework–Talk to Your Peers–Talk to Your Bus Dealer(s)–Be an Informed Buyer
STN Presentation AC101
What Size AC System is Right?:–Performance Specification–Actual Performance of the Installed Air Conditioning System in a
Controlled Environment–No Industry Standards for School Bus Air Conditioning Systems–Pull Down Test
–Start at a Designated Temperature in the Test Chamber–Thermocouples in the Bus Measure the Temperature at Specific Locations–The Bus is Heat Soaked until the Starting Temperature is Stabilized–The Bus is Started, Engine Speed is usually High Idle–Data is Captured for the Time Specified in the Test (usually 30 minutes)–The System passes the Test if all Thermocouples reach the required Temperature in the Time Required.
–Houston Pull Down–Las Vegas Pull Down–American Public Transportation Association White Book (Transit Bus
Specifications)
STN Presentation AC101
Las Vegas Specifications: (Partial)
No simulated solar load shall be used.Turn on the air conditioner and note that all doors are closed as they would be under normal operation.The engine speed for this test is the manufacturers recommended high idle speed.Hold the 110 degrees Fahrenheit +/- 1 degree Fahrenheit temperature ambient while recording data for a minimum of 45 minutes. No humidity need be added but it must be monitored and recorded.
The requirement for passing this test is that the system shallpull the interior (four thermocouples previously noted) from 110 degrees Fahrenheit down to 70 degrees Fahrenheit +/- 3 degrees Fahrenheit within 30 minutes from the start of the A/C system. That is all four thermocouples must be below 73 degrees Fahrenheit in 30 minutes (not an average).
STN Presentation AC101
What Size AC System is Right?:–Performance Specification, Continued
–A Requirement in most Transit Bus Specifications
–Increasing use in the School Bus Market
–Removes Doubt the System will Perform in that Bus
–Evidence for the District before the Bus is Delivered
–There May be a Cost for this Test
STN Presentation AC101
Basic AC Maintenance:
1. Return Air Filter on Evaporator
–Keep It Clean!
–Any Restriction Decreases the System Capacity
–Wash out Dirty Evaporator with Cleaner–Many Products on the Market
–Fin Comb to Remove Heavy Buildup / Straighten
Fins
STN Presentation AC101
Basic AC Maintenance:
1. Return Air Filter on Evaporator, Continued
Dirty Return Air Filters
STN Presentation AC101
Basic AC Maintenance:
1. Return Air Filter on Evaporator, Continued
Dirty Evaporator CoilsSTN Presentation AC101
Basic AC Maintenance:
2. Condenser Coil
–Keep It Clean
–Mud, Leaves, Paper Can Restrict Airflow!
–Keep the Screen Clear
–Keep the Fins in Good Condition
–Two Types of Coil Construction
–Tube and Fin
–Micro Channel (Automotive Style)
STN Presentation AC101
Basic AC Maintenance:
2. Condenser Coil, Continued
–Tube and Fin Condenser
Fin Comb
STN Presentation AC101
Basic AC Maintenance:
2. Condenser Coil, Continued
–Tube and Fin Condenser
–Do Not Clean with a Pressure Washer
–The Fins can be Damaged
–Use a Standard Garden Hose and Nozzle
STN Presentation AC101
Basic AC Maintenance:
2. Condenser Coil, Continued
–Micro Channel Condenser
–Newer Technology
–All Aluminum Construction
–This Design can be Cleaned with a Pressure
Washer
Close up View of Construction
STN Presentation AC101
Basic AC Maintenance:3. Compressor Belts
–Keep it Aligned
–Keep it Tight
–Check for Belt Cracks
–Check for Belt Wear
–Check for Idler Wear
STN Presentation AC101
Basic AC Maintenance:4. Hoses
–Look for Rubbing
–Oil Leaks/Refrigerant
–Check for Hose Cracks
STN Presentation AC101
Basic AC Maintenance:
5. AC Switches
–Turn Them Off when the Bus not Running
–Easier to Start the Bus with the AC Compressor
Clutch(es) Disengaged
–Turn the AC on when the Engine is Warmed Up
–Simple Switches to Sophisticated Electronic
Control
–Know How to Use the Controls
STN Presentation AC101
Basic AC Maintenance:
6. Water Inside the Bus
–Evaporator Condenses Water from the Air
–Evaporator Catch Pan Drain can Plug
–Water Spills/Drips out when Turning a Corner
–Let the Maintenance Staff Know
STN Presentation AC101
Basic AC Maintenance:
7. Use Quality Refrigerant!
–Cheap Import Brands are Available
–Get References on Import Brands
–Check the References!
–Some are Contaminated, Not Pure 134a
–They Won’t Cool and will Contaminate Your
Systems
STN Presentation AC101
Resources:1. Pull Down Tests:
• Florida DOT
• http://www.tripsflorida.org/pdfs/AC%20Pull%20Down%20Version%208.pdf
• APTA White Book and Houston Pull Down Standards
• http://apta.com/resources/reportsandpublications/Pages/BusParatransit.aspx
• The Houston Pull Down is the Hotter Climate portion of the specification.
• Choose the “Standard Bus Procurement Guidelines” on the Web page
• APTA has Committees working on Para Transit and Coach Bus Standards
STN Presentation AC101
Questions?
STN Presentation AC101
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