Packaged Rooftop Units: Chapter 1
In this module, we will look at packaged rooftop units. By the end of the lesson, you will understand the purpose of a rooftop unit. Skip to quiz!
Introduction to Rooftop Units
A rooftop unit is a type of HVAC system mostly found in commercial buildings. These systems provide heating, cooling, and ventilation to a building. Rooftop units are also sometimes found in residential homes. These systems are called rooftop units as they are placed on the roof of a building. We generally call a rooftop unit an RTU.
A rooftop unit contains all heating and cooling equipment in one compact box. These systems do not contain indoor and outdoor units. As all components are placed in a compact box, we call it a packaged system.
Some of the advantages of rooftop units are:
An RTU’s noise does not disturb people inside a building, as it’s installed on the roof.
As all components are fit into a compact box, it comes in small sizes and saves space.
It does not need any security because it’s placed on the rooftop.
We do not commonly find RTUs in residential homes. Some reasons for this are:
Installing an RTU on the roof is costly.
Trained technicians are needed to install a rooftop unit.
Large cranes are needed to install them on a roof of a building.
Applications of RTUs
RTUs have different capacities. The capacity of a rooftop unit is measured in tonnage of refrigeration, or TR for short. RTUs are available in different capacities between 5 TR to 100TR.
As per cooling and heating requirements, rooftop units are used in different places like:
Supermarkets,
Commercial buildings,
Office buildings,
Banks, and
Movie theaters.
A rooftop unit is a packaged HVAC unit installed on the roof of a building. We commonly find RTUs in
Supermarkets,
Commercial buildings,
Office buildings,
Banks, and
Movie theaters.
Components of Rooftop Units
In this module, we will learn about the different components of a packaged rooftop unit. Skip to quiz!
Basic Refrigeration Cycle
Recall that in the refrigeration cycle, refrigerant flows through a system. The four main components of the refrigeration cycle are:
Evaporator,
Compressor,
Condenser, and
Metering Device.
Compressors
Recall that the job of a compressor is to increase the pressure of refrigerant. When gas is compressed, its volume decreases, whereas its pressure and temperature increase.
The common types of compressors used in rooftop units are:
Scroll compressor, and
Reciprocating compressor.
Let’s refresh on how these compressors work.
Recall that scroll compressors use a circular type motion to compress refrigerant. Let’s watch a video to understand how scroll compressors work. In a scroll compressor, there are two matching scrolls, as shown in the videos. The scrolls are like spirals that are placed together. One scroll is stationary and does not move. The second scroll moves and is rotated by a motor.
Reciprocating compressors have been used in refrigeration systems for a long time. They have a piston that compresses the gas inside a hollow cylinder. Let’s watch a video to understand how reciprocating compressors work.
Condensers
Recall that the role of condenser coils is to reject the heat from refrigerant to the outside air. In condenser coils, refrigerant rejects heat and changes its state from gas to liquid.
Some of the types of condensers commonly used in rooftop units are:
Fin tube condenser, and
Microchannel condenser.
Recall that a fin tube condenser has small metal plates attached to the tubes carrying refrigerant. Remember that fins are basically metal plates. In fin tube condensers, fins are attached to the refrigerant tubing.
Recall that microchannel condensers are new in the industry. These condensers are the most efficient types of condensers.
Microchannel condensers are made from thin zig-zag aluminum fins. These wavy fins are the most distinctive feature of microchannel condensers. Refrigerant flows through thin channels and the zig-zag fins help to reject heat quickly from refrigerant.
Metering Devices
Recall that the function of a metering device is to reduce the pressure of refrigerant. A metering device permits only a small amount of refrigerant to enter the evaporator coils. The pressure and temperature of refrigerant decreases as it comes out of a metering device.
In a rooftop unit, we usually find these metering devices:
Thermostatic expansion valves, and
Electronic expansion valves.
Let’s look at how both these metering devices work.
Recall that thermostatic expansion valves are also called TEVs or TXVs. These are the most commonly used metering devices. Let’s get started by watching a video about how they work.
TXVs regulate the flow of liquid refrigerant going into the evaporator coils based on its temperature. For example, during summers, if an evaporator’s temperature is high due to hot air, more refrigerant is sent by a TXV. This cools down evaporator coils and the air flowing over them.
Recall that electronic expansion valves are also called EEVs or EXVs. Electronic Expansion Valves (EEV) control the flow of refrigerants electronically. Let’s watch a video explaining how they work.
Electronic expansion valves control the amount of refrigerant going into an evaporator. EEVs receive information from temperature and pressure sensors in evaporators. They change the amount of refrigerant they let through based on these sensor readings.
Evaporators
Recall that evaporator coils provide more surface area for hot indoor air to pass over them. This increased surface area allows more heat to be transferred to refrigerant inside the coils.
A finned tube and drum-and-tube evaporator are commonly used in rooftop units. Recall that finned tube evaporators have small metal plates attached to the tubes carrying refrigerant. Drum-and-tube exchangers are typically used in larger commercial and industrial human comfort applications.
Recall that finned tube evaporators have small metal plates attached to the tubes carrying refrigerant. These small metal plates are called fins. At each fin, there is a transfer of heat to refrigerant in the evaporator coils from the room air.
In an RTU, the placement of evaporator coils is very important. Different configurations of evaporator coils help to:
Decrease power consumption,
Provide better cooling, and
Save space to make an RTU more compact.
Evaporator coils inside a rooftop unit are usually placed in a:
Vertical position, and
Slant position.
In these positions, maximum air makes contact with evaporator coils so that we can get more cold air.
In a packaged rooftop unit, common types of compressors that can be found are scroll compressors and reciprocating compressors. Thermal expansion valves and electronic expansion valves are the most common types of metering devices.
Secondary Components
In this module, we will take a look at the secondary components of a rooftop unit. Skip to quiz!
Blower Fans
Recall that a blower fan is a component within an HVAC system that is responsible for circulating air. Blower fans push cold air or hot air through ducts to the desired locations in a building. Ducts are closed pathways for air to flow from a system to each room and office.
In an RTU, we generally use a belt-driven blower fan. Recall that in a belt-driven system, the motor and the fan are connected by a rubber belt.
Condenser Fans
Recall that, a condenser fan is a part of a condensing unit. It allows circulation of air through condenser coils. It cools down refrigerant by removing hot air from the condenser chamber.
In a rooftop unit, the size of the condenser fan depends on the size and capacity of the RTU. Larger fans are required to remove the heat rejected by an RTU’s condenser coils. For example, a 50TR RTU will have a larger condenser fan than a 5TR RTU.
A larger capacity RTU can also have multiple condenser fans. For example, a 100TR RTU can have more than one condenser fan.
Crankcase Heaters
Crankcase heaters are used to heat compressor oil. When a compressor is not in use, refrigerant mixes with compressor oil and settles to the bottom of that compressor. This does not allow the compressor in a system to start easily.
When a crankcase heater starts heating a compressor, refrigerant forms vapor and separates away from the oil. This allows the compressor to start easily and work efficiently.
Recall that an RTU is generally exposed to outside weather like harsh winds and extreme cold. At times, a system and its compressor become very cold, and refrigerant mixes with the oil in large quantities. As an RTU’s compressor will not start in such cases, a crankcase heater is necessary.
Filter Driers
Recall that the primary purpose of filter driers is to remove moisture from refrigerant and filter it. A filter drier consists of two parts, the shell and the core. The shell is an outer metallic covering. The core is the inner part, which absorbs moisture.
Rooftop units are installed in open spaces, so there are dust particles all around these systems. There is a chance that dust particles or air could enter the refrigerant due to wind, rain, or other maintenance issues. Because of this, a filter drier must be installed to remove all the impurities from the refrigerant.
Liquid Line Receiver Tanks
Recall that a liquid line receiver tank is a storage tank that stores excess liquid refrigerant. A rooftop unit is a large system, so it also requires a large amount of refrigerant. A liquid line receiver tank acts as a storage vessel to store refrigerant.
It’s installed in the fluid line, as close as possible to the outlet of a condenser. A system can have liquid refrigerant back up in the receiver tank whenever needed.
Accumulator Tanks
Recall that an accumulator tank is a component used to protect a compressor from liquid refrigerant. Recall that a compressor is designed to handle vapor refrigerant only. An accumulator tank is placed inside a rooftop unit before the compressor’s suction line.
Economizers
An economizer is a device used to save energy consumption by using cool air from outside a building to cool the internal space. Economizers mostly operate at night when outside air is cooler than inside air. This process is known as free cooling.
Economizers mix enough outdoor air with indoor air to meet the cooling temperature set on a thermostat. This is done without starting a compressor or the refrigeration cycle. Economizers make use of the fact that air at night is cooler than in the mornings.
The fresh outside air that economizers bring in save the energy spent cooling a building at night. This is beneficial to cool spaces in offices that work all night or supermarkets that are open all night.
Heat Recovery Wheels
A heat recovery wheel recovers heat from exhaust air. Recall that exhaust air is the air that is to be removed from a building. Exhaust air from a building is stale air, but it’s comparatively colder than outside air.
A heat recovery wheel is a big wheel that has two compartments. A metal plate in between separates the sections. Cold exhaust air from a building comes into one section and the hot outside air into another section.
The hot outside air gives its heat to the exhaust air and becomes cooler. Without a heat recovery system, cold exhaust air would always be removed from a building without any secondary usage. This system is also commonly known as a Energy Recovery System (ERS), or a Heat Recovery Unit (HRU), or a Heat Recovery System (HRS).
Heat recovery systems will reduce the amount of energy consumed by conventional heating/cooling units. Using a heat recovery wheel in a rooftop unit has many benefits like:
Reduction in energy cost,
Lower operation cost, and
Reduction in environmental impacts.
Crankcase heaters are used to heat compressor oil. A liquid line receiver tank is a storage tank that stores excess liquid refrigerant. An economizer is a device used to save energy consumption by using cool air from outside a building to cool the internal space.
A heat recovery wheel recovers heat from exhaust air. Exhaust air from a building is stale air, but it’s comparatively colder than outside air.
Ventilation Systems in Rooftop Units
In this module, we will learn about ventilation systems in rooftop units. We’ll also learn about different types of rooftop units based on their airflow. Skip to quiz!
Air Flow
RTUs circulate cold air in a building. Also, RTUs bring in fresh air from outside so that occupants don’t feel suffocated. This helps maintain indoor air quality. Recall that indoor air quality is the quality of air inside any building.
Rooftop units are connected to ductwork which provides a defined route for conditioned air to travel. RTUs are categorized based on the quantity of air that they can supply. These are:
Constant air volume (CAV), and
Variable air volume (VAV).
A CAV system always delivers a fixed quantity of air to a building or zone. Recall that any building is divided into zones. A zone can be a couple of rooms or entire floors of a building. A CAV system will cool an entire building at the same temperature, so we cannot set different temperatures for different rooms.
In a CAV system, we can't control the volume of air being supplied. We can control the temperature of air being delivered, but the quantity of air will always remain the same. So, a CAV system is preferred when a building has no zones or only one zone.
CAV systems are primarily used in buildings with no zones, or sometimes in single-zone and multi-zoned buildings. We find some applications of CAV systems in:
Supermarkets,
Auditoriums,
Theaters. and
Museums.
VAV systems can adjust airflow depending on the cooling or heating load. VAV systems are preferred for a building with multiple zones, but can sometimes be used in single-zone buildings. These systems can send different quantities of air to different zones.
VAV systems can maintain different zones at different temperatures. For example, we may not need to keep a supermarket’s storeroom at the same temperature as other rooms. So a VAV system can control and restrict the quantity of air going there.
Some applications of VAV systems include:
Large supermarkets,
Multi-storey buildings, and
Commercial offices.
Recall that the purpose of ductwork is to provide a closed pathway and direction to circulate cool or hot air to different rooms in a building. Ductwork also eliminates the need to install separate HVAC units in each room of a building.
Recall that the main purposes of ducts are:
To not allow air inside a duct to escape, and
To not allow the transfer of heat between air inside and outside a duct.
Ductwork acts as a postal service that transfers cold or hot air from an RTU on the roof to a specific room, and back.
There are generally two types of ductwork that you will see in RTUs:
Rigid ductwork - Maintains an exact shape and cannot bend easily.
Flexible ductwork - Can bend as needed.
Fans
Recall that a fan provides air circulation. The different fans that we find in an RTU are:
Blower Fans,
Condenser Fans,
Return Air Fans,
Exhaust Air Fans, and
Outside Air Fans.
Recall that a blower fan is a component within an HVAC system responsible for circulating air. Blower fans push cold air or hot air through ducts to the desired locations in a building. It pulls warm air from a building and pushes it through evaporator coils to cool it.
Recall that a condenser fan is responsible for circulating air through condenser coils. It pulls air from outside and pushes it through high-temperature condenser coils to cool down refrigerant.
A return fan pulls air from inside a building and returns it to an RTU system. Recall that a blower fan does the work of pulling air from a building. However, in many RTU applications, a building or space is so huge that a single blower fan cannot do this job.
A return air fan helps a blower fan pull air from a building and supply it to the RTU. A return air fan is installed inside a return air duct. Recall that a return air duct provides a closed path for air from a building to reach the RTU.
An exhaust air fan is also installed on the return air side of an RTU. But it’s installed inside the RTU, not in the duct. It pulls return air and throws it out of the RTU. The air thrown outside is stale air from a building that usually has large amounts of carbon dioxide in it.
When we remove stale air from a building, we need to bring in fresh air from outside to avoid suffocation. An outside air fan performs this function. The primary function of an outside air fan is to pull fresh air from outside into a building. Outside air fans are installed near the intake damper of an RTU.
Air Flow Control
In an RTU, we need to control the flow of air going in and out of various rooms in a building. Dampers are used to control the air flow through a duct. Like a building has gates that open and close to allow vehicles in and out of the building, RTUs have dampers.
Dampers can open or close as needed and allow or block the flow of air. Dampers are situated closer to the mouth or end of a duct. Though a fan will be pushing air, dampers can close the end of a duct to not allow any to pass through.
We usually find these damper types:
Manual dampers, and
Motorized dampers.
Manual dampers are turned and set by a handle, and cannot be controlled without human interference. Motorized dampers have an electric motor and can be controlled remotely or automatically by sending electric signals.
The different dampers used in rooftop units are:
Intake air damper,
Exhaust air damper, and
Return air damper.
Intake air dampers control the amount of fresh air going inside an RTU and a building. Recall that a fresh air fan pulls in outside fresh air into a building. An intake air damper is a motorized damper.
Exhaust air dampers control the removal of stale air from a building by an exhaust air fan. Recall that an exhaust air fan throws stale air from a building outside. Exhaust air dampers are a type of backdraft damper.
Backdraft dampers are manual dampers. They allow air to flow only in one direction. Backdraft dampers are commonly found in places where the backflow of air needs to be avoided.
At an exhaust we only want to expel the air out, and not allow outside air to enter an RTU via the exhaust. There are heavy winds at the roof of any building that can force their way in through the exhaust of an RTU. So, a backdraft damper is the type of damper installed for an exhaust air damper to prevent this backflow of air.
Return air dampers are used to control return air coming from a building into the RTUs. A return air damper is placed on a return duct. These are required to stop a building’s air from coming into an RTU when it’s shut down.
Air flow is very important to achieve human comfort. A constant air volume RTU allows a fixed quantity of air to flow into a whole building from the RTU. A variable air volume RTU allows different quantities of air to flow into different rooms in a building.
The different fans we find in RTUs are:
Blower Fans,
Condenser Fans,
Return Air Fans,
Exhaust Air Fans, and
Outside Air Fans.
We usually find these damper types:
Manual dampers, and
Motorized dampers.
Question #1: Rooftop units are also called packaged systems because;
All components are placed in a compact box.
They have an indoor and outdoor unit.
They are installed on a rooftop.
They contain a chilled water system inside them.
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Answer: All components are placed in a compact box.
Rooftop units are also called packaged systems because all the components are placed in a compact box. All the heating and cooling equipment is placed in one confined unit.
Question #2: Where is a rooftop unit installed?
Ground
Wall
Inside house
Rooftop
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Answer: Rooftop
A rooftop unit is a box-shaped structure that contains all heating and cooling equipment. They are generally installed on the roof of any building.
Question #3: A reason why rooftop units are uncommon in residential homes is that:
They’re small and compact systems.
They make less noise.
Their installation is difficult and requires using cranes.
They are too small in size to be installed in residential houses.
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Answer: Their installation is difficult and requires using cranes.
Installing rooftop units is difficult and requires trained technicians. Cranes are often required to lift these units and place them on the roof of a building.
Question #4:RTUs are commonly seen in the following places (Select all that apply):
Supermarkets
Inside individual rooms
Office buildings
Garden
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Answer: Supermarkets
Office buildings
RTUs are used in commercial buildings like supermarkets, movie theaters, and commercial offices.
Question #5: Which of the following types of compressors are commonly used in an RTU (Select all that apply)?
Scroll compressors
Screw compressors
Centrifugal compressors
Reciprocating compressors
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Answer: Scroll compressors
Reciprocating compressors
A scroll compressor and reciprocating compressors are commonly used in RTUs.
Question #6: Which of the following metering devices are most likely to be found in a packaged rooftop unit (Select all that apply)?
Capillary Tube
Piston Valve Metering Device
Thermal Expansion Valve
Pressure Expansion Valve
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Answer: Thermal Expansion Valve
A thermal expansion valve, or an electronic expansion valve, is most likely to be found in a packaged rooftop unit. A capillary tube and piston valve metering device are used in small systems like split AC or window AC.
Question #7: An IT office computer room requires precise temperature control. Which of the following metering devices will you likely find in the rooftop unit installed for this purpose?
Automatic Metering Valve
Piston Valve Metering Device
Electronic Expansion Valve
Pressure Expansion Valve
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Answer: Electronic Expansion Valve
A thermostatic expansion valve and electronic expansion valve are used in an RTU.
Question #8: Which of the following evaporator types are commonly used in RTUs?
Finned tube
Shell and tube
Tube and shell
Bare and tube
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Answer: Finned tube
A finned tube evaporator is used in an RTU.
Question #9: The most common type of blower fan used in RTUs is:
Belt-driven blower fan
Direct drive blower fan
Hermetically driven blower fan
Semi-hermetically driven blower fan
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Answer: Belt-driven blower fan
The most common type of blower fan is a belt-driven blower fan. This type of fan is commonly used in rooftop units.
Question #10: The job of the condenser fan in an RTU is:
To cool the evaporator coils
To cool the economizer coils
To cool the condenser coils
To provide cool air to the building occupants.
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Answer: To cool the condenser coils
A condenser fan is used for cooling condenser coils.
Question #11: The size and number of the condenser fans in an RTU mainly depend on:
The capacity of the condenser
The location of the condenser
The orientation of the condenser
The time of the day
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Answer: The capacity of the condenser
The size and number of condenser fans mainly depends on the capacity of the condenser coils in an RTU.
Question #12: What is the primary purpose of the crankcase heater in an RTU?
To heat the filter drier
To heat the condenser coils
To heat the compressor
To heat the evaporator coils
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Answer: To heat the compressor
The primary purpose of crankcase heaters is to heat compressors.
Question #13: When a crankcase heater heats an RTU’s compressor, what happens to the refrigerant mixed with oil in the compressor?
The refrigerant burns off
The refrigerant converts into liquid
The refrigerant leaks away from the gaps in the compressor
The refrigerant evaporates back into the system
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Answer: The refrigerant evaporates back into the system
When a crankcase heater heats an RTU’s compressor, the refrigerant mixed with oil starts to boil. This refrigerant evaporates and goes back into the system.
Question #14: A filter drier must be installed in an RTU because:
They are installed in the open on rooftops
They are installed in the basement
They are installed in mechanical rooms
The system to protect refrigerant is not so good in rooftop units
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Answer: They are installed in the open on rooftops
A filter drier must be installed in an RTU because they are installed in the open on rooftops. There is a high chance of dust particles entering the refrigerant and contaminating it.
Question #15: A liquid line receiver tank is placed outside of a rooftop unit.
True
False
Scroll down for the answer...
Answer: False
This is false.
A liquid receiver is placed inside a rooftop unit.
Question #16: A packaged rooftop unit does not need a liquid line receiver tank, as it’s a large system and the refrigerant can adjust itself in any part of the system.
True
False
Scroll down for the answer...
Answer: False
This is false.
A rooftop unit is a large system, so it also requires a large amount of refrigerant. The amount of refrigerant is huge, so it cannot adjust anywhere and requires a separate container, the liquid line receiver tank.
Question #17: An accumulator tank is used to store refrigerant oil.
True
False
Scroll down for the answer...
Answer: False
This is false.
An accumulator tank is used to store liquid refrigerant going into a compressor.
Question #18: Which of the following is a function of an economizer?
Heating a space by mixing cold outside air with indoor air
Cooling a space without mixing outside air with indoor air
It works to purify outside air and use its heat to cool air inside a building
Cooling a space by mixing cold outside air with indoor air
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Answer: Cooling a space by mixing cold outside air with indoor air
An economizer is a device used to save energy consumption by using cool air from outside a building to cool the internal space. It cools a space by mixing cold outside air with indoor air.
Question #19: A heat recovery wheel saves energy.
True
False
Scroll down for the answer...
Answer: True
This is True.
A heat recovery wheel mixes fresh air with return air. When both types of air get in contact with one another, the heat transfer process happens.
Question #20: An energy recovery system works on the principle of:
Transfering heat from hot outside air to cold exhaust air
Transfering heat from hot outside air to cold room air
Throwing cold air from a room out and taking in exhaust air
Mixing hot outside air and cold air in a room to provide a stable temperature
Scroll down for the answer...
Answer: Transfering heat from hot outside air to cold exhaust air
An energy recovery system works on the principle of transferring heat from hot outside air to cold exhaust air.
Question #21: Constant air volume is preferable for?
Buildings with multiple zones
Buildings with a single zone
Buildings with at least five zones
Buildings with at least nine zones
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Answer: Buildings with a single zone
Constant air volume is preferable for buildings with a single zone, as we can't control air volume.
Question #22: A variable air volume system can supply different amounts of air flow to different rooms in a building.
True
False
Scroll down for the answer...
Answer: True
Answer is true.
A variable air volume system can supply different amounts of air flow to different rooms in a building.
Question #23: A packaged rooftop unit does not need ductwork, as it is a compact system with everything built inside it.
True
False
Scroll down for the answer...
Answer: False
A packaged rooftop unit needs ductwork even though it is a compact system with everything built inside it. Ductwork acts as a postal service that transfers cold or hot air from an RTU on the roof to a specific room, and back.
Question #24: A return air fan pulls air from ___ and supplies it to ___.
Inside a building; the RTU
Outside a building; the RTU
The RTU; a building
The outside; a building
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Answer: Inside a building; the RTU
A return air fan pulls air from inside a building and supplies it to the RTU.
Question #25: The role of an outdoor air fan is to:
Push the air from an RTU outside.
Pull air from inside a building to the RTU
Throw out air from inside a building
Bring air from outside into a building
Scroll down for the answer...
Answer: Bring air from outside into a building
A rooftop unit is a box-shaped structure that contains all heating and cooling equipment. They are generally installed on the roof of any building.
Question #26: Which of the following dampers is used to control the flow of exhaust air?
Return air damper
Exhaust air damper
Fresh air damper
Open air damper
Scroll down for the answer...
Answer: Exhaust air damper
Exhaust dampers are used to control the flow of exhaust air.
Question #27: Which of the following dampers can allow air to flow in a single direction only?
Return air damper
Manual Damper
Backdraft damper
Motorized damper
Scroll down for the answer...
Answer: Backdraft damper
Backdraft dampers allow air to only flow in a single direction.
Question #28: Which of the following dampers is used to control the intake of fresh outside air into an RTU?
Return air damper
Exhaust air damper
Backdraft damper
Intake air damper
Scroll down for the answer...
Answer: Intake air damper
An intake air damper controls the flow of fresh outside air into a rooftop unit.