Facilities management paper
Cooling Systems
Chapter 10
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Cooling concepts
Adding to the concepts covered in Chapter 9 Heating Systems:
Cold – there is no such thing! There is only heat energy or the absence of heat energy!
1902 Willis H. Carrier created a cooling system for indoor spaces, prior to 1902, fans and the evaporation process were the means of cooling.
Heating and cooling are closely related, both move air over or across a source that changes the airs temperature.
Heating via electrical resistance, hot water or steam; cooling with cold water or refrigerant (gas).
Both use the same or similar systems.
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Cooling Terms
“Air conditioning” is incorrect, it is “conditioned air”.
Heat always flows from warm to cool, attempts to equalize temp
British thermal unit (BTU) – the amount of energy necessary to raise or lower the temperature of 1 pound of water by 1 degree Fahrenheit
Cooling ton – 12,000 Btu’s
Evaporation – transforming a liquid into a vapor or gas, increases with air flow. Stage in the refrigeration cycle, cold refrigerant chemical cooling air or water.
Evaporative cooling – sensation of heat transfer, air across your skin, no actual temperature change; however, it feels 5°-10°F cooler.
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Cooling loads
A cooling load is any event or activity that leads to the need to remove heat from the space (or, cool the space)
Internal loads – warm things that add heat to the building; we then need to remove the heat
People – temperature (and humidity)
Cooking
Lighting and other appliances (motors)
External loads – warm air (and humidity) that causes heat to transfer through the roof, windows, and walls. Also includes heat that infiltrates through poorly-sealed windows, doors, and cracks in the building envelope.
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How do we make “cooling”?
Refrigeration Cycle
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Vapor-compression refrigeration cycle
Boyle’s Law – at a constant temperature, the volume of a gas is inversely proportional to the pressure upon it
Therefore,
As volume decreases, the pressure increases
As volume increases, the pressure decreases
(volume = the amount of space the gas takes up)
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Components in the vapor-compression refrigeration cycle
Refrigerant – a chemical whose properties allow us to change its temperature by manipulating volume and pressure. The refrigerant will exist as a gas and as a liquid in the vapor-compression refrigeration cycle
Freon is a trademark brand name for the Chemours Company’s
The refrigerant’s specific formula will determine its operating temperatures
Identified by the letter R, followed by a number: R-22, R-341a, etc.
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Refrigeration equipment – the refrigerant chemical will cycle through:
Compressor
Condenser
Expansion valve (or metering device)
Evaporator
Heat exchangers: the condenser and the evaporator are the system’s two heat exchangers. They permit heat to transfer from one substance to another.
Evaporator permits the refrigerant to absorb the heat from the building’s interior
Condenser expels the heat in the refrigerant to the outdoors
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Refrigeration equipment characteristics
Compressor
3 types: reciprocating, scroll or screw, centrifugal
“compresses” gas into smaller volume, gas gets hot (molecules rub each) and motor is hot, refrigerant (gas) absorbs this heat
Condenser
cools refrigerant, causes it to condense into liquid, transfer heat to outdoor environment
Expansion valve (or metering device)
opens & closes to allow a controlled rate of refrigerant to enter a larger space evaporator (higher volume, lower pressure) causing it to expand
Evaporator
Larger space & lower pressure allows gas to return to gaseous state. Refrigerants boil at very low temps with right pressures, absorbs heat from air or water surrounding the coil.
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Refrigeration cycle
Refrigerant enters the compressor as a cold, low pressure gas. The compressor reduces the volume (compressing the refrigerant). The refrigerant leaves as a hot, high pressure gas.
Refrigerant travels to the condenser. The condenser removes heat from the refrigerant, allowing it to condense to a warm, high pressure liquid.
The refrigerant travels to the expansion valve. When the system receives a signal requesting cooling, the expansion valve opens and permits a small amount of the warm, high pressure refrigerant liquid to pass into the evaporator, which offers a larger volume space.
The refrigerant enters the higher volume space and the pressure drops rapidly. The refrigerant is formulated to boil off into a very cold, low pressure gas here. The cold, low pressure gas can now absorb heat from the building.
The refrigerant, having absorbed the building’s heat, now travels back to the compressor as a cold, low pressure gas to repeat the cycle.
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Refrigeration cycle diagram
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Decentralized Cooling Systems
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Decentralized cooling systems
Decentralized: not part of a centralized HVAC system. Often called DX systems, blow air directly across the evaporator pipes/coils.
Window units
PTAC units, for smaller spaces (i.e., guestrooms)
Package units for larger spaces, also known as:
Split systems
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Refrigeration cycle: all-air system
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PTAC Units
Essentially large window units.
Through walls vs. windows “through-the-wall unit”
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Split systems
A.K.A.- package, DX or central air conditioning units
In theory a large PTAC unit.
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Split system (a.k.a., DX system or “package” unit)
This is the same equipment we saw for heating systems
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Heat pumps
In the vapor-compression cycle, we can reverse the direction of the refrigerant flow
The condenser (which is located outside) becomes the evaporator and picks up heat energy from the outdoor air (even when it’s “cold” outside, there’s still come heat energy in the air)
The evaporator (located inside) becomes the condenser, which rejects heat energy to the indoor air
Simply stated: absorbs and transfers heat energy from outdoors to indoors
This heat pump is much more energy efficient for providing heat than an electric-resistance strip heater!
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Heat Pump
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Centralized Cooling Systems
Usually “all-water” systems
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Centralized cooling systems
These systems are all-water systems, meaning the heat in the building is picked up by water and then rejected to the outdoors via water as well. The heat transfer process:
Chilled water (CHW) picks up heat in building in the FCU or air-handler; returns to the chiller to get rid of the heat
At the chiller, heat in the CHW and from the compressor are transferred to the condenser
Condenser water (CW) picks up heat from the refrigerant in the condenser, then travels outdoors to the “cooling tower” to reject the heat to the outdoor air
Building Chiller Condenser/CW Cooling Tower
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Centralized all-water refrigeration cycle system
Chiller
Piping
Fan Coil/AHU
Cooling Tower
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Chillers (refrigeration machine)
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Provides the chilled water (CHW).
Is the compressor and evaporator
Water piping
Moves water from chillers to and from the facility
Highway for heat transfer from building
Label the pipes!
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Fan coil / air handler units
Heat exchangers – removes heat from rooms to the chilled water (CHW)
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Cooling towers
Always located outdoors; transfers heat from the building to the outdoors. Receives the heat-laden condenser water (CW)
CW trickles down through the cooling tower; air blowing over the water picks up heat
The heat, and some water vapor, leave the cooling tower
Water loss due to evaporation must be replenished “makeup water”
Float valve allows for the introduction of makeup water
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Scheduled maintenance: cooling tower shut-down and start-up
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CHW, condenser and cooling tower water
Cleanliness is critical for here, since stuff grows in the cooling tower and can be sent into air. Cooling towers and condenser water require significant chemical treatment and testing:
Ideal breading areas for dangerous microbes, like Legionella
Evaporation leads to higher mineral concentrations
Open system; birds, insects and other animals cause contamination
Control/balance pH, mineral content
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Coil maintenance
Cleanliness – keep all equipment clean to improve energy efficiency, dirty coils will:
Create unwanted insulation, prevent heat transfer
Increase energy consumption and cost
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Coil condensation
Reducing the airs temperature results in dehumidification. Moisture (condensation) must go somewhere. Drip pan.
Clear drain line
Pan can grow fungus, bacteria, viruses, etc.
Overflow can damage: ceilings, floors, walls, equipment
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Refrigeration leaks
Refrigerant gas is in a closed-loop system: continuously reused and recirculated. Leaks should be repaired as soon as possible:
System will not be able to maintain cool temperatures
System will run continuously trying to maintain desired temperatures
Increase energy cost
Cause equipment to fail, compressor overworked
Environmental impacts:
Older refrigerants are harmful to the environment
All refrigerants harmful to humans
OSHA and EPA require continuous monitoring and proper storage and handling
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Environmental Regulations
Until 1990’s refrigerant made of chemicals:
Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs)
Both harmful to the ozone
New chemicals hydrofluorocarbons (HFCs) not harmful to ozone; believe to contribute to greenhouse gas emissions.
Environmental regulations for refrigerants expected to significantly increase
Rule changes for refrigerants more than likely affect properties equipment, refrigerants are not interchangeable (compressors cannot different types)
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Other energy efficiency opportunities
Ceiling fans can provide the sensation of lower temperatures at a fraction of the cost of running a compressor
Lighting – if your property has not converted its lighting to LED lamps, you may want to do so. These lamps emit little heat and will reduce your property’s cooling loads.
Correct sizing: Old practice – install largest even if it was more than needed. Emerging trend: install multiple smaller chillers, use only what is required, additional cooling requires less energy, improves conservation practices.
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One final note…
Almost all of the…:
Refrigerators/coolers
Freezers
Beverage coolers
Ice machines
…equipment that you will use in the hospitality industry at this time uses the vapor-compression refrigeration cycle
For more information, see the chapter on foodservice equipment
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