During maintenance of a HVAC system, the HVAC vacuum pump and HVAC system become complementary units, at least operationally; and they should be matched with each other.
This implies that a low-capacity vacuum pump should not be used to evacuate a high-capacity HVAC unit.
There is also a need to consider the HVAC design which has evolved substantially, in tandem with new environmental regulations and need for design code compliance, as well as market competition, and need to incorporate new technologies to improve efficiency and reliability of the pump.
This is bound to affect how the vacuum pump is connected to the HVAC unit, as well as the effectiveness of refrigerant evacuation under low-pressure conditions.
Still, there are basic considerations that you – as the buyer of a HVAC vacuum pump – should factor when choosing the right vacuum pump, or when looking for the best HVAC vacuum pump that suits your projected needs.
These considerations are discussed below.
The vacuum pump you choose should be well-matched with your existing and projected HVAC evacuation needs.
Your needs should be based on minimum draw levels, that is how much fluid and existing contaminants need to be evacuated (drawn out) of a HVAC unit, with the minimum draw informed by recommendations of the HVAC system manual.
Another important fact to remember is that deep vacuum allows for complete evacuation of dry particulate contaminants and fluids, including the hard-to-remove non-condensibles and thermal-stable gases, from the HVAC system.
It also allows for identification of leaks in the HVAC piping stock. Therefore, it is recommended that you choose a pump that can generate deep vacuum that slightly exceed your projected evacuation needs.
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The vacuum pump must never allow for gas leakage, as this would eliminate the vacuum generated inside the working chamber – due to atmospheric air back-streaming, and because atmospheric air is usually dense than most refrigerant gases, then it would stall the displacement of refrigerant gases.
Therefore, the pump needs to be sealed completely against leakage, and this can be achieved by using rubber seals and oil caps.
Also, the housing must be strong enough to withstand pressure gradients (between the working chamber pressure and atmospheric pressure) which prevents buckling or implosions, as well as withstand falls and accidental damage that would have otherwise caused cracks or outright breakage.
This means that a durable construction is a must-have feature.
How should the pressure in the working chamber be measured? Linearly or logarithmically (so as to address the large pressure ranges)? And if linearly, what is the degree of sensitivity required?
It is advisable that pressure inside the vacuum pump be measured in a linear manner, as this allows for very high levels of sensitivity to be achieved when compared to a logarithmic scale.
The linear measurements must start from an absolute level – in this case, the absolute pressure which is zero pressure which indicates a perfect vacuum.
The HVAC system needs to be connected to a pressure gauges that indicate the pressure in the unit. Most pressure gauges are calibrated in inches, with high-quality gauges being calibrated in microns.
One inch is equivalent to 25,400 microns, and therefore microns provide a more accurate reading of pressUre than inches.
Therefore, if the vacuum pump, comes with a pressure gauge; it is recommended that you acquire a model whose pressure gauge indicates the depth of vacuum in microns. Usually, digital gauges are preferred over bourdon tube-type gauges.
To begin with, there is need to dispel a common myth that states: the bigger the pump, the faster it completes evacuation.
The reason for this is that the HVAC piping network, which is composed of some small-diameter tubes with returns bends, and metering devices along the piping stock create flow restrictions during evacuation.
This is further compounded by the fact that service valves have quarter-inch wide male flare ports that have orifices whose diameters is only 3/16 inches, which means that flow out of the service valves is dependent on flow speeds rather than the size of orifices; and higher flow speeds can be achieved by increasing the pressure gradient across the flare ports.
Even so, the vacuum pump does not generate substantial vacuum to create this pressure gradient. Additionally, as evacuation proceeds, pressure inside the HVAC pipes reduces and this reduces the pressure gradient, which would reduce flow, even if a very large pump is used (for evacuation).
Therefore, rather than using the size of the pump to match it with your evacuation needs, it is better that you use a more objective measure – the flow rate.
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This is amount of gas that can be moved through the pump per unit time (which is usually per-minute), and it indicates the volume of fluid that can be evacuated from the HVAC system per unit time.
It is usually rated as cubic-feet-per-minute, or CFM; and the ASHRAE industry standards matches the pump CFM to the HVAC system size, for instance, a 1.5 CFM is matched with a 10-tonne (ton) HVAC system, while a 4.0CFM pump is matched with a 30-ton HVAC system.
It is recommended that you use ASHRAE recommendations to match your HVAC system size with your soon-to-be acquired vacuum pump.
ASHRAE stands for the American Society of Heating, Refrigerating and Air-Conditioning Engineers. You can also use the following formula as a rough guide:
CFM X CFM = CFM2 = HVAC system tonnage, for example,
A 6 CFM pump should be suitable for a 36-ton HVAC system, which is within ASHRAE recommendations which lists the tonnage range for a 6 CFM pump as 30-45 tons.
Usually, a 10 CFM pump can handle almost all medium HVAC evacuation needs.
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One of the principal factors that determine the flow rate – and also make pump size irrelevant – is the amount of power generated in the vacuum pump to drive evacuation.
This is important as most vacuum pumps are mechanical pumps whose rotating parts are driven by electrically-powered motors, and the power generated by these motors are measured in horse-powers (HP) or watts (W), with 1 HP being roughly equivalent to 745.7W.
Generally, a compact pump with a high-power rating can deliver high CFM flow rates while being energy-efficient and less-bulky than a large pump with an equivalent power rating.
The electrical motor usually needs alternating current (AC) to work, with the two common voltages required being 110 volts (V) or 240V, with the polarity frequency being 60 Hertz (Hz).
How much vacuum is generated by a pump should give you an idea of whether deep vacuum is achieved, and how suitable is this vacuum for your evacuation needs.
As mentioned earlier, vacuum needs to be measured in microns, with 0 micron indicating perfect vacuum.
ASHRAE provides recommended micron ratings for different CFM ratings, for example, a 5 CFM pump is expected to generate vacuums of up-to 25 microns.
Still, it is advisable that you choose a pump whose micron rating is 75 microns or less. The micron rating of the pump indicates the ultimate vacuum that the pump can generate.
Pumping capacity is also affected by flow resistance. It is therefore important to describe how resistance is generated against flow, and how it affects pump function.
The HVAC unit is a piped system, and fluid flow through the pipes must overcome friction created by the pipe wall, and inside the pipe lumen due to collision among gas molecules, and this impedance against fluid flow creates resistance (which also manifest as fluid drag) that the vacuum pump must deal with when drawing fluids towards its working chamber.
This explains why a low-capacity pump cannot be used to evacuate a high-capacity HVAC unit as the HVAC system with its intricate piping network (containing pipes of different diameters) is liable to generate high flow resistance in the pipes, and if the vacuum generated in the pump cannot overcome this resistance, then evacuation will be greatly hampered.
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Type of Pump
The main types of vacuum pumps are vacuum generators, rotary-vane pumps, and diaphragm pumps; though high-end models can use the principles of chemical vacuum and momentum-transfer to achieve very high vacuum levels.
You should also remember than piston pumps generate less vacuum than rotary vane pumps, due to incomplete evacuation of the working chamber by piston motions.
Most of the HVAC vacuum pumps used for evacuating home and office HVAC units are rotary vane pumps which are a special type of centrifugal pump that use vanes mounted on rotors rather than an impeller to create positive-displacement.
Other pumps use a piston, while other are basically modified centrifugal pumps. Because the operation of rotary vane and centrifugal pumps are based on almost similar principles, the following description of what to look for in a centrifugal pump should give you an idea of what to look for in a rotary vane pump.
In the centrifugal pump, fluid enters the working chamber though an inlet port aligned to the center of the impeller core, hence its designation as the suction eye, and this allows the impeller spinning in an anti-clockwise/counter-clockwise direction to thrust it (the fluid) outwards towards the chamber wall in a radial manner.
This impeller-driven radially-oriented displacement creates centrifugal acceleration, which also creates a vortex that generates vacuum in the suction eye.
However, in the rotary vane pump, there is an inlet and outlet port and fluids are thrusted from the inlet to the outlet ports by the rotating vanes.
On the other hand, the amount of centrifugal acceleration achieved, flow rate, and depth of vacuum generated, are dependent on the impeller speed; with high speeds generating a higher flow rate that can support continuous flow of the evacuated fluid from the HVAC.
The operational design, servicing requirements, suitable applications, and internal construction allows for segmentation of centrifugal pumps into different groups according to industry standards (American versus European standards, Chemical industry standards, and oil and gas standards), and the following features:
Number of Impellers/Vanes
A single-stage pump has only a single impeller (or vane), and can generate low amounts of vacuum, as well as low exhaust pressure.
This makes it suitable for low head service, that is, it is well-suited for evacuating low capacity HVAC units.
A two-stage pump has two impellers connected serially, either in a single working chamber, a pseudo-twin chamber (which has a volute molded in a single chamber), or in two connected chambers.
It can generate medium vacuum and medium pressure, and is best suited for medium head service.
A multi-stage pump has 3 or more impellers connected serially in a single chamber, or in multiple chambers. It is suitable for high head service.
This describes the location of the suction eye, which determines if the vacuum in the pump generates axial thrust of fluids into the working chamber.
There are two types of impeller suction; the single suction model where there is only a single suction impeller aligned axially to the suction eye, and the double suction model where there is a suction eye on either side of the impeller.
Double suction pumps are prone to uneven vacuum generation due to uneven fluid flow through the suction eyes, principally because of the piping system and the resistance the fluids encounter in the pipes delivering the fluid to each suction eye. Impeller suction is generally non-applicable to rotary vane pumps.
The axial shaft holding the impeller can be oriented either horizontally, which creates a horizontal pump, or vertically, hence creating a vertical pump. Horizontal pumps are easier to service and maintain, as compared to vertical pumps.
The way the impeller shaft is connected to the driving shaft, usually a rotor shaft, determines how efficient motion transmission is transferred to the impeller blades. There are two ways the impeller shaft is connected to the driver:
Integral close-coupled design
The shaft is mounted directly to the driver shaft in a compact fashion/design to create a compact and lightweight pump-driver assembly.
Even though it is lightweight because no additional components are used, this inexpensive design is suitable for light- to medium-duty service because motor torque cannot be increased by the driver-shaft assembly.
The shaft and driver shaft are connected together via a flexible coupling assembly that allows for spacer coupling, and this can allow for adjustment of impeller speeds and power. This is because the speed transmission assembly allows for change of impeller speed.
Volute is the space/volume inside the working chamber that receives the fluid thrust outwards by the impeller, and is physically separated from the impeller by a curved lip that defines part of the impeller chamber.
In single volute (single-lipped) models, the curved chamber casing is unpartitioned and has a single lip, which alongside the chamber wall creates a curved funnel space – called the volute – that allows the impelled fluid to flow out through a single discharge path to the exhaust port.
In double volute models, the volute space is partitioned to create two radially-balanced discharge paths by dual lips oriented 180 degrees apart (to balance the radial load), with their respective discharge paths merging at (or near) the exhaust port.
In triple-volute models, there are three lips oriented 120 degrees apart for radial load balancing. Multiple volute models provide better performance than single volute models. Even so, in rotary vane pumps, the vanes are spring loaded so that they can expand to always maintain contact with the chamber wall, and thus no volute is created.
The aforementioned features allow one to estimate how well a vacuum pump would operate when evacuating a HVAC system.
Depending on your evacuation needs, and how much vacuum is required; you can choose between a rotary-vane positive-displacement pump, or its piston-based congener; or go for a two-pump system where a booster pump supports a high-vacuum momentum transfer or entrapment pump.
Usually, in most cases, a positive-displacement pump based on centrifugal thrust principles would suffice.
Ease of Use
The process of starting the pump, and testing for leaks and vacuum generation should be easy and straightforward.
The pump should also be safe to use indoors, which would greatly convenience the user during HVAC evacuation in bad weather conditions.
An exhaust filling should allow for indoor use, and you should look for a pump whose filling allows for external venting using readily available hoses.
Another important consideration is to choose a model that comes with a continuous-use motor.
Moreover, the process of checking the oil level, and refilling the lubricating oil should be easy. Usually an oil-level sight glass would suffice for oil level readings. Likewise, an oil drain plug allows for easy drainage of contaminated oil.
The vacuum pump must also come with a blank valve (either detachable or integrated into the pump head unit), as this allows you to easily perform isolation tests to find out how much vacuum can be generated by the pump.
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As mentioned earlier, some pumps come with their own pre-calibrated pressure gauges, and this allow for easy installation and usage of the pump without the hassle of connecting a poorly-calibrated gauge, or pairing the pump with a non-compatible gauge.
The other accessories that can come with the pump are mounting hardware that allows one to connect the pump to the HVAC system, as well as stabilize the pump on the floor. A detachable blank-off valve is also a useful addition.
Most HVAC vacuum pumps are electrically-powered as they need electricity to run their motors. They should therefore come with surge protection, protective fuses, and oil level indicators which show how much lubrication oil is there to cool and lubricate the moving parts of the pump.
A gas ballast should also be provided so as to allow for evacuation of water vapor from the pump. This prevents moisture condensation, which leads to contamination of oil with water, hence resulting in corrosion of motor parts and all ferric components lubricated with the oil. Usually, the gas ballast is found in two-stage positive-displacement pumps.
An isolation valve should be a good addition, as it prevents oil from flowing into the vacuum chamber/working chamber, hence maintaining the cleanliness of the working chamber.
The price of the vacuum pump must be within your budget, and if it is beyond your budget, then you need to make trade-offs and accept to lose certain additional features in order to get an affordable, yet high-quality pump.
Nonetheless, HVAC vacuum pumps are a lasting investment, and it is advisable that you weigh your budget against your expectations, so as to make the right choice of whether to acquire a relative expensive and long-lasting, high-performance pump rather than a cheaper model with poorer performance and less longevity.
Operating costs should also be considered, along with maintenance costs, because purchasing a cheap model that is an energy hog and breaks up frequently would impose an unjustifiable cost to the user.
Expectedly, it is sensible to go for a pricier, yet reliable and high-quality HVAC vacuum pump that is cheap to operate and breaks down infrequently.