Build Your Own HVAC - The Compressor

This is a follow up to the BYO HVAC - The Basics article. For the basics behind HVAC operation and what each component does review that first!

Size Matters a Ton

The primary goal of choosing a compressor is in matching the amount of cooling (or heating) you must provide. If you under-size a compressor, you will not be able to provide enough cooling to keep your home comfortable. If you size a compressor too large, the compressor will need to turn on and off, so that it won’t over cool your home. The compressor size also matches to the unit cost, the larger the compressor, the larger all the other equipment must be, the more materials it takes to make them all, and the more power they all draw.

It is the compressor size that is key in balancing these priorities first.

Cooling or Heating energy as it is referred to often comes in a few common units of heat transfer:

• Tons (most common in the U.S.)
• Kilowatts
• BTUs (More common in heating applications)
• Joules

These units when applied over a period of time, refer to the amount of heat energy that can be transferred by an HVAC:

• Tons - This comes from how much energy it takes to melt one Ton of ice in a 24 hour period (…yeah, logical, gotta love Imperial Units…)
• Kilowatts - How many Kilowatts of energy can be transferred per hour (kw)
• BTUs - How many British Thermal Units of energy can be transferred per hour (BTU/hr)
• Joules - How many Joules of energy can be transferred ($Watt=J/s$, this is more commonly used for fundamental equations)

Since, in the great ole’ U.S. of A. we use Tons, I will stick with that for now as I am used to hearing it. One common shorthand is to assume you’ll need about 1 Ton of cooling for 400-600 sq/ft of living space.

$1Ton=12,000BTU/hr$ $1Ton=3.517kW/hr$ $1Ton=12,660kJ/hr$

But it might not matter as much as how you use it…

The impact of having an HVAC that is too small is pretty obvious, it won’t keep up when you need it.

But what if you over-size a unit? Or what if it’s not THAT hot outside, but you still need some cooling, let’s say you’re having a party and baking a cake, but it’s only 70$°$F outside.

There are 3 primary types of compressors, and they react in different ways:

A Fixed Speed Compressor will at full power until your home is cooled and then stop, in these Low Load conditions, this is a wear on the compressor, since starting it is the most stressful and can also cause issues with Oil.

A Staged Compressor, typically two staged in residential applications, handles this condition by running only one “stage”, giving you partial cooling so the compressor doesn’t start and stop over and over, and helping with wear. This might help reduce electricity use, often, the whole compressor is still operating, but a mechanism allows the refrigerant to escape the compressive mechanism early. This reduces the work the compressor needs to do. You can always include both “stages” if you need to provide for a full cooling or heating load.

Finally, the most flexible options are a Modulating, Inverter, or Variable Speed Compressor. These all refer to a compressor that will more dynamically reduce the flow of refrigerant. This allows for a tuned operation that can be adjusted more dynamically to the load needed.

Fundamentally, there isn’t much different between a Fixed Speed, Single Stage Compressor and an Inverter Compressor mechanically. There is however additional electronics needed to control the compressor and not run it directly from static line voltages.

In addition, inverter compressor systems are usually more complicated because they require more information to control the compressor properly. They also tend to have Electronic Expansion Valves to support preemptive control of the superheat as opposed to a reactive one you would get with a Thermodynamic Expansion Valve.

These additions are what really add to the cost of more “advanced” units.

Expansion Valves!

Stay tuned for a future article on Expansion Valves!

So, now you know your tonnage, you know what overarching kind of compressor you might want (which determines the further required system design)…Now what’s next?

Cars don’t drive themselves! Well…maybe that’s not so true anymore, but assuming it is for the sake of this analogy…neither can compressors!

The easy stuff - Fixed & Staged

Basic fixed speed compressors are typically run directly off of line voltage. That’s 1 or 3 phase power at 120VAC 60Hz in the US. Controlling these compressors takes activation of a relay to allow the AC power to flow through the motor coils and start rotating the motor.

As noted earlier, a staged compressor isn’t much different, but it will have a solenoid valve actuated with lower voltage to dump off the refrigerant partway through compression.

How they work

Both of these control systems are relatively simple to operate, assuming you provide the correct voltage and supply enough current to run your motor, you’re pretty much set. You need to know when to turn it on and off yeah, but the electrical challenge is nearly non-existent. If you need cooling, start your compressor, if you need less cooling (and you have a two-stage compressor), open up your bypass solenoid.

A basic AC motor works by driving electromagnetic coils at exactly line frequency, with the winding voltage intensity pushing or pulling the rotor to its next position. You always want the one winding pushing away the stator and another pulling it closer. A 3-phase wound motor will have poles in factors of three, and 1-phase in factors of two. The more poles, the shorter distance the motor needs to travel between each sinusoidal peak, and the more torque it can apply in that shorter region. For example, 1-phase two-pole has an RPM of 3,600. If that motor had 6-poles it would have an RPM of 1,200 assuming nothing else changed and torque output will increase.

$RPM=60\ast f\ast \frac{2}{n_{poles}}$, 60 converts Hz to minutes, and the $\frac{2}{poles}$ handles the fact that each pole has a north and south side.

This get’s a lot more complicated when you start looking into inverter compressors…

The hard stuff - Inverter

Inverter compressors, instead of relying on the 60Hz line frequency, rely on a changing that frequency synchronous to the motor rotation to keep the push and pull on the stator present. The same equation for RPM holds, but now that driven frequency will change, usually through the use of a Variable Frequency Drive (VFD). Where it becomes challenging is this: in an open motor, you will typically have separate hall effect sensors present that let you detect the motor position and you can easily adjust your frequency based on detected motor speed & position. Refrigerant unfortunately requires hermetically sealed motors, and cannot have hall effect sensors present in that circumstance.

Motor Type	Drive Lines	Sense Lines
Sensed Motor	U/V/W	H+/HA/HB/HC/H-
Open Loop Motor	U/V/W	…None

What this means in practice is that you must sense the motor position through your your drive windings. And knowing this, how do you sense magnetic fields through windings you are pushing voltage over? It’s a great question, I’m glad you asked!

How they work

The key principles stands, you want your most recently passed winding to push your stator and the upcoming winding to pull the stator. But now you have one additional winding not “active” when the other two are driving, you use this to sense the motor electromagnetic characteristics and determine speed and position. To do this your drive needs to understand the electromagnetic properties of the motor to interpret the signal it is measuring. This is what your VFD handles, either you have a pre-tuned drive, and you select your supported motor variant, or you input your motor characteristics manually and it will tune it’s performance as it runs by measuring the non-driven winding.

Motor Parameters

As noted in Open Source & HVAC, manufacturers don’t want to let go of their secret sauce, so what makes this hard is that they rarely share this motor information. You can usually buy a drive tuned to run your compressor suggested by the compressor manufacturer. Or, even if you do have the motor characteristics, you still need to pay a premium for a configurable VFD.

None of these things are deal breakers, but as you read this, you probably hear the distant cha-ching of a cash register opening to collect your sweet sweet money.

And there’s more!

Refrigerant Selection & Oil

Before you select your compressor you will need to select the refrigerant you want to use. This is usually more a factor of regulation and being environmentally conscious, while understanding the implications on product design. Units with A3 flammable refrigerants like R-290 (GWP 3) must handle leaks with care. Higher pressure refrigerants like R-32 (GWP 675) require more expensive components that handle that pressure. Mixed medium refrigerants like R-454b (GWP 465, 68.9% R-32, 31.1% 1234yf) have more chaotic dynamics, and are harder to recover or re-charge.

GWP Reference Table

Your compressor must be compatible with the selected refrigerant and this will also tell you what oil you need to use. This is one of the most important considerations before selecting your compressor.

Impeller Design

There are different compression mechanisms that offer trade-offs on pressures vs. flow-rates vs size. Impeller designs can be Linear, Rotary, Scroll, Screw, or Centrifugal, in order of size and refrigeration capacity. This usually falls out from your other compressor decisions rather than being an active decision you will make.

Vapor Injection

Another feature that is becoming common in low temperature heat pump applications is Vapor Injection. This technology takes liquid from the condenser, flashes it through an EXV and passes it back over that cool refrigerant before dumping it into the compressor. This technology allows compressors that would only be capable of operation to -10$°$C to operate down to -25$°$C and enhance capacity when heating at low ambient.

Ready to choose a compressor?

Let’s go through a checklist:

• What refrigerant are you using?
  • R-32, A2 Flammability, GWP 675, 48 bar rating @ 70$°$C
  • R-454b, A2 Fammability, GWP 465, Mixed, 40 bar rating @ 70$°$C
  • R290, A3 Flammability, GWP 3, 26 bar rating @ 70$°$C
  • Others? Consider regulation and design parameters!
• What tonnage are you aiming to supply?
  • 30-35 BTU/sqft in cool regions
  • 50-60 BTU/sqft in warm regions
  • Consider extra heating or cooling load based on home efficiency
  • Consider the impact of humidity on heating and cooling
• Do you want a fixed, staged, or inverter system?
  • Fixed Speed, lower cost, but needs more precise sizing
  • Staged compressor, mild cost increase, mild complexity increase, fewer short cycles
  • Inverter, higher cost, more complex system, but you can more easily oversize without mechanical risk
• If you picked an inverter you will also need a matching VFD
  • Pick a motor that has public motor specs
  • Pick a motor you can get specs from the manufacturer
  • Pick a VFD that is a paired match that supports your motor
  • Ensure that VFD uses the correct phase and voltage and can provide the required current
• If you want to use your compressor for heat pump application in very cold climates, you will likely need Enhanced Vapor Injection (EVI)

My System

Refrigerant

I was originally looking at R-32, however when I was looking at parts I struggled to find solutions that met the pressure needs. I want to do thorough refrigerant analysis, so having a mixed refrigerant (R-454B) was less Ideal. This led me to consider getting ahead of things and make an R-290 system (not currently allowed per US regulations).

Manufacturers

Regardless, I struggled finding cost effective compressors through US suppliers. Copeland, Danfoss, Carlyle, Tecumseh are at least US based, some source residential sized (rotary) compressors from 3rd parties, others don’t participate in the space at all, but if they do, they still cater to large OEMs and it was harder to find information for any of their products. They also have many paired parts, Copeland compressor with Emerson drive…Danfoss compressor with Danfoss drive. It wasn’t much better for GMCC compressors either, but I was able to find some information there.

My Choice

So, knowing I want an inverter, ~2.5-3.5 Ton, R-290, with motor parameters available and has a market for one-off purchases, I found the GMCC EDTF420D62EMT on Area Cooling and felt it fit my needs! There are a few VFD options available, but I feel confident I can use the GA500 at the very least. It has high enough current output for this motor, and has single phase (what I’ll want for residential) variants available and supports back-emf open look motor control.

1-phase vs 3-phase

One note on line voltage, if you were to keep pushing compressor size, the trade-off will present itself eventually, if you require a high motor current, it will eventually NEED a three phase power to supply that.

Most residential installs will have 1-phase power available.

Next up

So now you’ve chosen a compressor, it has the features you need, can provide the tonnage you want, and you’re on to the next steps! Now that you’ve made all these decisions you need to get datasheets, pricing, 3D models and source the thing. Good luck!

Next, we will discuss the “easy” decisions that fall out from picking this compressor option.