An OC Overview of the HVAC Vacuum Pump
We here at Outdoor Chief like to dig deep when we start to research a particular tool or product. Sometimes these means are articles can reach stupid word counts in our attempt to cover a topic comprehensively.
However, we know you are busy people. You’re not going to read through a 10,000 account of a particular subject while flicking through your mobile phone.
That’s why we have broken our study of the HVAC vacuum pump down into 3 manageable pieces. You can read about the best pumps on the market here, and learn what you need to know to purchase one in our buyer’s guide here.
And if you want an overview and some background of the HVAC vacuum pump, you are in the right place as we will go into that below.
You can always trust us to go above and beyond when it comes to bringing you the most in-depth information on anything that we choose to publish.
So if you are ready, let’s get down and dirty with the HVAC vacuum pump.
- A little history on the HVAC machine
- The 3 main functions of the HVAC unit
- Introducing the Vacuum Pump
- Means of operation
- Classification via a type of gases that can be pumped
- Piston-based positive displacement
- Compression ratio
A little history on the HVAC machine
The present modular climate-control system called the heating, ventilation, and air conditioning (HVAC) unit is a complex system made up of diverse parts that operate synchronously to create thermal comfort and ventilate a room, hall, motor cabin, or any other enclosed space.
Its modular design allows for convenient installation and efficient troubleshooting. The air conditioning unit has fundamentally evolved from its primitive prototype built in 1884.
This was followed by the incorporation of refrigerator components in its build design in 1939, and then assembly of all its components into a modular stock in 1954 by the Delphi Harrison Thermal Systems.
The latter two developments mark the quantum leaps that lay the foundation for the development of the modern HVAC unit, which can effortlessly, efficiently, and seamlessly transition from cooling to heating air depending on the ambient temperature.
This unit also serves to remove moisture from circulating air, and this reduces humidity to prevent misting or icing of glass panes.
Even so, this progress has come with the adoption of environmentally-degrading fluorocarbon-based chemicals as refrigerants, or their substitution with ozone-depleting thermal fluids, which must not be allowed to escape into the atmosphere.
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The 3 main functions of the HVAC unit
Evidently, the HVAC unit performs 3 key functions; temperature control, humidity control, and control of air circulation.
The key components of the HVAC unit are the compressor, condenser, pressure regulating unit (usually made of an orifice tube and thermostatic expansion valve [TEV]), drier, evaporator, and accumulator, as well as a system of valves interspersed along with the piped stock.
Because these components must work synchronously, then damage to one component can impair the (functioning of the) entire HVAC unit.
Should this HVAC unit be defective, the key repair tools that a technician needs are a vacuum pump, a manifold gauge set, and leak detectors.
Introducing the Vacuum Pump
The vacuum pump is used when servicing the HVAC unit, or after a leak repair has been done, and dehumidification and air evacuation is needed to avoid compromising the internal pressure of the unit and preserving the efficiency of heat transfer.
Usually, air and moisture in the piped stock would raise its internal pressure, and accelerate the carbonization and acidification of compressor oil and refrigerant.
Pumping the fluid contents out of the (HVAC) system also allows for refrigerant recovery (after filtration and desiccation), and its subsequent recycling.
It can also allow for recovery and recycling of ethylene glycol antifreeze during repair or servicing operations, thus saving one the high cost of purchasing a new antifreeze compound.
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The process of moisture and air removal from the HVAC system is described as evacuation. The thermal fluid is the heat-exchange fluid in the system, with fluid meaning a flowing substance, be it in liquid or gaseous forms, and this appropriately describes the refrigerant fluid that easily undergoes phase transitions.
The thermal fluid within the HVAC unit is contained within a closed pressurized system which has low-pressure and high-pressure zones that allow for fluid streams to be created to support its circulation, and this makes heat exchange efficient.
The compressor creates the high-pressure zone, with fluid pressure steadily falling as it flows towards the evaporator and accumulator.
This thermal fluid serves as a good heat transfer medium as it can easily undergo phase transitions to gas or liquid medium when the heat is added or removed respectively.
The vacuum pump is used to create a low-pressure zone inside the HVAC system which allows for the generation of a pressure gradient that supports fluid flow towards the pump.
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Classification of Vacuum Pumps
Vacuum pumps are classified based on their operational capabilities, and the five main capabilities considered are as follows:
- operating pressure range (which hints at the pressure capacity),
- flow rate,
- means of operation,
- capacity to pump continuous gas flow,
- and capacity to pump different types of gases, as well as cleanliness.
Still, the main merit used for classification is operating pressure range, which allows for classification of pumps into roughing, booster, and hi-vac (word contraction of high-vacuum) pumps.
The pressure is measured based on an absolute scale whose manometric unit is the Torr (a shortened eponym of Torricelli), with 1 Torr being equivalent to 1/760 of atmospheric pressure, or roughly 1mmHg of mercury in barometers.
Roughing pumps are designed to evacuate chambers of fluid held at 1 Torr to 0.001Torr (10-3 Torr), while booster pumps are capable of achieving evacuation pressures of 101 Torr to 10-5 Torr.
As expected, Hi-Vac pumps should be able to achieve very low evacuation pressure, and this is true with their pressure range spanning from 10-1 Torr to 10-11 Torr, with the downside being that they can only function when the pressure in the chamber to be evacuated is 10-1 Torr or less, and this mandates one to use a booster pump (hence its designation) to reduce the chamber pressure to a level that allows for operation of the Hi-Vac pump.
Means of operation
Classification of pumps based on their means of operation yields the mechanical/positive displacement, momentum transfer, and entrapment pumps.
The mechanical pump has rotating parts, and are also called positive displacement pumps because the volume of their pumping chamber can be expanded and compressed, hence pushing out the chamber contents at a higher pressure than the intake pressure (pressure of fluids when entering the chamber).
Positive displacement pumps are usually roughing pumps.
Examples of positive-displacement pumps are rotary vane pump, piston pump, diaphragm pump, scroll pump, Wankel pump, zero-oil contamination pump, Toepler pump, liquid-ring pump, and lobe pump.
A unique mechanical pump is a centrifugal pump that uses a rotating impeller (a wheel with attached blades) to accelerate fluids to the exhaust port.
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The momentum transfer or molecular pump uses an accelerated stream of fluid (vaporized liquid or liquid streams) to generate a vacuum in a chamber maintained at sub-atmospheric pressure.
The fluid is accelerated from the intake towards the outlet/exhaust port, which is maintained at a lower pressure than the intake so as to create a pressure gradient that accelerates the exhausting of molecules from the evacuated chamber.
In high-quality models, high-speed rotating blades, or pressurized jets of high-density fluids, are used to impart momentum to the evacuated fluids.
Even so, the momentum transfer pump is a hi-vac pump that operates best when connected serially to a positive-displacement pump that allows this 2-stage pumping system to clear large volumes of sealed chambers (or space that need to be evacuated).
The difference between molecular pumps
Turbomolecular and diffusion pumps, along with vapor pumps, are the main types of molecular pumps.
The entrapment pump uses a very simple, yet salient principle – absorption of gases into adsorbent materials, or entrapment of gases by forcing it to undergo a phase transition into the liquid matter, either by freezing it or allowing it to react with chemicals that create a liquid product.
Ion pumps ionize the gas stream in strong electrical fields, which then allows this electrical field to accelerate the ionized gas atoms towards the adsorbent material.
Cryopumps freeze the gas into a liquid substrate. Chemical vacuum pumps include titanium sublimation and getter pumps.
Another unique pump is the regenerative pump that uses the stationary hollow grooves of centrifugal pumps and takes advantage of the vortex behavior of gases to exhaust gases directly into the atmosphere, which makes suits it as a booster pump.
They usually have teeth on the rotor, which allows the spinning rotor to generate a vortex that is directed towards the exhaust by the hollow grooves in the wall of the pumping chamber. It is also called a side-channel pump.
Classification of pumps based on cleanliness indicates whether the displacement (or working) chamber is dry, or if it can be coated/layered with lubricating oil, and this allows for the classification of pumps into wet and dry pumps.
- If liquid, usually back-streaming lubricating oil or oil streams of diffusion pumps, enter into the working chamber, then the pump is considered a wet pump (previously known as a dirty pump). Some positive displacement pumps are wet pumps.
- If vapor is not introduced into the working chamber, then the pump is described as a clean pump or dry pump. Most molecular transfer and chemical vacuum pumps are usually clean pumps. Backstreaming in momentum transfer pumps occurs when the pressure gradient in the displacement chamber (also called the pumping chamber) collapses and allows for fluid in the exhaust port to flow back into the displacement chamber. This also results in collision between the backstreaming fluid and accelerated fluid streams, with both fluids dissipating their momentum in the ensuing inelastic collisions, which subsequently reduces the amount of fluid evacuated by the pump, and this phenomenon is called stalling. Stalling in entrapment pumps is usually caused by outgassing, which is the release of previously absorbed or adsorbed gas, as well as vaporization of condensed gas.
Continuous flow pumps allow for the pumped fluid to be exhausted in a continuous stream till the pump stops working. Those pumps that cannot achieve this continuous flow, especially entrapment pumps, are called non-continuous flow pumps.
Classification via a type of gases that can be pumped
As mentioned earlier, pumps can be classified depending on the types of gases that can be pumped, which is acutely important for HVAC systems that use non-standard volatile refrigerants.
Normally, each vacuum pump evacuates different gases at different levels of efficiencies depending on its sensitivity to the evacuated gases, with low-mass gases and inert/noble elemental gases presenting most challenges because of their low mass per unit volume, which demands more pumping effort as compared to pumping dense gases.
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Piston-based positive displacement
There is also need to mention that the piston-based positive displacement pump uses the same operating principles as the four-stroke engine – just without the power stroke.
During the intake phase, the displacement chamber increases in size as the piston moves down, which also gradually creates a low-pressure zone that allows fluid to flow from the HVAC system into the working chamber.
The compression phase is marked by gradual reduction of the volume of working chamber by the ascending piston, till the piston reaches its top position, and this results in compression of the fluid and attendant increase in fluid pressure; which allows the next phase to occur.
During the exhaust phase, the outlet port is opened and the compressed fluid flows out passively as the working chamber is inevitably depressurized in readiness for the next intake phase.
The ratio of the fluid pressure at the exhaust port (outlet pressure) to the fluid pressure in the intake port (inlet pressure) is called the compression ratio; and mechanical pumps with high compression ratio tend to work more efficiently as HVAC pumps as compared to models with lower compression ratio.
HVAC Vacuum pumps differ in their functional capabilities because they are designed to utilize almost all the operational capacities that are used to classify vacuum pumps.
This allows for production of a range of pumps whose vacuum salience allows for evacuation of different HVAC systems, ranging from industrial HVACs, laboratory HVACs (where legal policies are sensitive to refrigerant gas leakage), office HVACs, to home HVACs.
Understanding the classification of vacuum pumps, and their operational principles allows one to know which type of HVAC vacuum pump to acquire, or whether an existing pump can match his/her HVAC evacuation needs.