This tech tip about tandem compressors was inspired by the recent HVAC School podcast episode with the Copeland team. You can listen to that podcast episode HERE.

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One compressor is great, but how about two in the same circuit? Or three? We see multiple compressors in rack refrigeration circuits, but the concept isn’t as common in commercial HVAC… except that it IS a thing and has some interesting operational benefits. These configurations of multiple compressors in the same circuit are called tandems (two compressors) or trios (three). This tech tip will mostly focus on tandems, especially Copeland scrolls designed for tandem configurations.

These compressors are specifically designed to work with at least one other, and they come in many different varieties. In general, using one well-designed system that uses multiple compressors is more cost-effective than installing and maintaining multiple circuits. 

Why Tandem Compressors?

It’s not news to anyone that the industry and regulations have focused very much on efficiency over the last several years. One such target for commercial DX (direct expansion) circuits is the minimum modulation requirement, which is how low a system can modulate its load (expressed as a percentage).

Let’s use 25% as an example of a minimum modulation requirement; this means that the compressor(s) in the circuit must be able to turn down to at least 25% of their full capacity to ensure better efficiency in part-load conditions. Using a tandem is one method we can use to distribute the load across two compressors and possibly modulate to 25% of the circuit’s full capacity.

In applications where you might need a compressor capable of providing 40 tons of cooling, you could use one circuit with one very heavy compressor, or you could use two smaller compressors that are designed to work together. The goal is to spread out the load and enable the compressors to reach that minimum modulation requirement.

Due to the energy savings, efficiency standard compliance, and simplicity of design offered by tandem compressors, you shouldn’t be surprised to find them in RTUs, chillers, DOAS systems, and data center cooling systems.

Which Compressors Can You Use In a Tandem?

Tandem compressors are specifically engineered to work as a pair, but the individual compressors within those configurations come in all flavors. The compressors in a tandem may all be the same horsepower (even) or different horsepower (uneven).

Copeland’s tandem compressors are often used with traditional (fixed), two-stage, digital, and variable-speed scrolls. Most of the time, a tandem will consist of a fixed compressor with either another fixed compressor or a two-stage, digital, or variable-speed compressor; there’s usually at least one fixed compressor. However, some tandems may be configured to consist of two two-stage compressors. 

Tandems are quite common in data centers due to the turn-down capabilities of each compressor. Additional staging or modulation is critical in these applications, which is why we often see uneven tandems that have three or five discrete steps, and digital and variable-speed compressors come into play when more steps are needed. However, noise is sometimes a concern for clients when digital compressors enter the picture. 

How Are Tandem Compressors Piped Together?

The way tandem compressors are piped together isn’t quite as simple as having a suction header and discharge header in the same way as parallel racks. Instead, the piping usually depends on the compressors in the tandem configuration.

OEL, TPTL, and GEL

The oil equalization line (OEL) below the compressors does exactly what its name suggests; it connects the two compressor sumps to allow oil to transfer freely between them. This pathway allows the oil to balance out. OELs need to be level, as sloped piping can cause the oil to run to one compressor and leave the other one without oil, which leads to increased mechanical wear and premature failure. Copeland tandem-ready compressors come with spacers to ensure that the OEL remains level.

A two-phase transfer line (TPTL) is another common type of line that runs below the compressors, but it’s slightly larger; this line allows oil and refrigerant to flow through and equalize. These usually run through the compressor sight glasses near the sumps, and they may be equipped with a sight glass of their own to allow you to check the oil level. Most Copeland trios use TPTLs.

In the cases of the OEL and TPTL, there is just one additional line, which is why we call it the three-pipe configuration. However, some tandems have a gas equalization line (GEL), which helps equalize pressures between multiple compressors. If a tandem has a GEL, it will be located near the top of the compressor shell, as that is where the refrigerant is. A tandem with a GEL has a four-pipe design.

Which Tandems Require a GEL and Four-Pipe Design?

The design type is dictated by the compressors within the tandem. If we have an even tandem and know the flow rates are similar between the two, then a three-pipe design is feasible and makes the most sense. It’s also worth noting that four-pipe designs are often seen on some of the smallest pairings offered by Copeland due to fitting availability. We can’t add a sight glass to those smaller compressors, so we need a four-pipe design instead of a three-pipe design with a TPTL.

In any case, the compressors will create stress on the tubing, so manufacturers like Copeland conduct extensive stress testing on those tandem compressors. The compressors are tested at different modulation conditions, including leaving one compressor off entirely and the lowest and highest modulation settings for variable-speed compressors. This testing ensures a reliable oil balance. 

Replacement When Just One Compressor Fails

Since piping is a subject that requires a lot of forethought and engineering, replacing the whole tandem may be required, particularly when working with smaller compressors. 

Generally speaking, compressors with diameters under 7” or under 10 HP (or ones that just look like they could be residential compressors) will require full tandem replacement. That’s merely because most distributors don’t carry tandem-ready compressors in smaller sizes. Another telltale sign that you will need to replace the whole tandem is if the failed compressor’s replacement doesn’t have an additional port. Non-tandem compressors only have a suction port and a discharge port; tandem-ready compressors have an additional port, and you CANNOT replace a tandem without an OEL.

However, in many cases, the failed compressor can be replaced by itself if the diameter is 9” or greater (or 10+ HP). Look for stickers on the compressor that indicate it was designed as a tandem; it may be sold specifically as a replacement for a tandem configuration. Tandem-ready compressors usually contain an oil equalization port and a sight glass. However, as we said earlier, variable-speed compressors usually require a fourth line, the GEL, so you may need to be extra diligent about finding an exact match.

Of course, there are also reasons why you might replace the entire tandem, even if just one compressor failed. One such situation is contamination. Tandem compressors share refrigerant and oil, so a catastrophic failure of one compressor might mean that the other compressor may have been compromised in the process. 

How Tandem Compressors Reach Energy Targets

In residential HVAC, we expect air conditioners and heat pumps to meet EER2 and SEER2 targets. Commercial A/C units and heat pumps also have their own efficiency targets to meet.)

Commercial heat pumps and chillers must be evaluated against and meet three different standards:

• IEER — integrated energy efficiency ratio
• IPLV — integrated part load value
• IVEC (to take effect in 2029) — integrated ventilation, economizing, and cooling

It’s also worth noting that IVEC has a heating counterpart: IVHE (integrated ventilation and heating efficiency). You can learn more about IVEC and IVHE in this document from the U.S. Department of Energy.

When you have a single-speed traditional compressor, the compressor is either on or off; meeting part-load conditions, which is part of how IPLV is calculated, is simply not possible. Two-stage compressors also don’t have that flexibility; variable-speed ones do, but those can get expensive. In cases where facility owners can’t shell out the money for variable-speed options, tandem compressors are a more economical means of expanding a compressor’s range of operation. 

Tandem compressors achieve two things: they increase the circuit’s total load (potentially by double or more) and allow one compressor to handle part-load conditions entirely while the other is at rest. Instead of having one single-speed compressor that either delivers 100,000 nominal BTUs per hour (BTU/h), you could have one single-speed compressor that delivers about 33,000 BTU/h and another that delivers about 67,000 BTU/h. During part-load conditions on a cloudy day, there could be just one compressor that runs. If it’s the smaller compressor, you can reach those lower capacities and more effectively meet IPLV standards.

As such, tandem compressors—especially uneven tandems—are sneaky good at meeting stringent IPLV standards in particular. 

Enhanced Vapor Injection (EVI)

If you’re not familiar with EVI, it’s a compressor cooling technology that typically boosts efficiency during heating mode, particularly in cold climates. EVI injects some cool gas directly into the scroll; this adds more vapor for compression (more capacity) and cools the compressor at the same time. This gas often comes from a separate heat exchanger called an economizer. To make injection possible, compressors that use EVI will have yet another port right by the scroll within the compressor body.

EVI is a layer of technology within standalone compressors that can expand their operating envelope; if you have tandem compressors with EVI, you further expand that operating range to meet a demanding set of conditions. Essentially, if we think about efficiencies in terms of load increments, like 25%, 50%, and 100%, EVI merely raises the ceiling for 100%. Instead of having 100% be 100,000 BTU/h, it could really be more like 111,000 BTU/h. 

Additionally, strategies like EVI enable heat pump technology to be sustainable and effective in cold climates where we wouldn’t have imagined installing heat pumps 30 years ago, like Maine and even Norway (which has implemented heat pumps on a massive scale).

Looking to the Future and How Tandems Are Part of It

Regardless of whether you’re a residential or commercial HVAC tech, we can all agree that the industry we know and love is rapidly changing. GWP targets are coming down, energy efficiency ratings are getting stricter (or being newly developed)… there just doesn’t seem to be an end in sight.

Manufacturers are always looking to be prepared, not just for federal regulations but for individual state regulations that may be even stricter (*cough* CALIFORNIA *cough*). Tandem compressors are just one way we can work to meet stricter demands without having our commercial clients break the bank. 

If you’re interested in learning more about the changes that are down the road and how manufacturers like Copeland are preparing, I recommend reading our tech tip about scroll compressor technology and its place in the changing regulatory landscape. In the meantime, you will see that Copeland has developed A2L tandem compressors, and you can always rely on the Copeland Mobile app for information on all things compressor-related.