REFRIGERATION Controlling discharge and liquid pressure at low heat sink temperatures – Part II

CONTROLLING DISCHARGE AND LIQUID PRESSURE AT LOW HEAT SINK TEMPERATURES – PART II

In Part I of this two-part series in the November/December 2021 issue, we discussed the condensate or liquid drop leg regulation method. In this article, we will cover the discharge gas regulation method.

As the name implies, the discharge regulation method involves the use of a regulator in the discharge line between the compressor and the condenser. Like the condensate regulator, the discharge regulator used in the discharge gas regulation method is an inlet regulator. One can think of this pressure regulator as a hold-back regulator since it works to maintain a certain upstream pressure.

One potential advantage of this method over the condensate drain regulation method is that the pressure regulator setting is reached very quickly on compressor start-up. With the condensate drain method, the process of building pressure tends to take longer, particularly when the air-cooled condenser is subject to low ambient temperatures. The discharge gas regulation method can be very useful for starting up cold systems. Also, during operation, the discharge gas regulation method is often preferred in systems using heat reclaim, hot gas defrost/ice harvest, and so on. Since this regulator is installed in the discharge line, and must regulate hot gas instead of liquid, it will need to have a larger pipe connection and port size than if a condensate regulator was used.

On system start-up the regulator modulates to maintain at least the minimum set point pressure and the discharge pressure upstream from the valve increases rapidly. In the meantime, the pressure will begin to climb. The rate of

Discharge Gas Regulation

Discharge Regulation Method

pressure increase within the condenser will typically lag the discharge pressure as the vapour entering the condenser experiences desuperheating, condensing and then subcooling. As the system warms up, liquid refrigerant will accumulate in the condenser, effectively reducing its available heat transfer surface. Since the amount of refrigerant leaving the condenser is somewhat limited, the receiver pressure may be too low to provide the differential pressure required by the expansion valves. It is also important to keep in mind that as the liquid is flooding the condenser, the expansion valves are also trying to feed liquid from the liquid line to the evaporator. This also contributes to the lowering of the liquid level and pressure in the receiver.

In the condensate regulation method, we discussed how a bypass regulator installed between the compressor discharge line and the receiver inlet, works to maintain a desired receiver pressure. The discharge gas regulation employs this bypass or receiver regulator in the same manner. In fact, for a given system design and load, both methods would use the same regulator size. This regulator is an outlet regulator, so it is always sensing its downstream or receiver pressure. The receiver pressure

Phil Boudreau Phil provides sales and application support for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at pboudreau@bitzer.ca.

regulator will generally be set to the minimum pressure required at the inlet to the expansion valve, plus any additional pressure drop that exists between the receiver outlet and the expansion valve inlet. The discharge regulator then must be set to a higher pressure so that hot gas can flow into the receiver when needed. In addition to the minimum pressure required at the expansion valve inlet, it is also very important to ensure that the discharge pressure is not permitted to drop below the minimum required by the compressor manufacturer.

The Condenser Split Method

Condenser splitting is another commonly-used head-pressure control method, particularly in parallel rack applications. Note that this method is typically used with both the fan cycling or VFD methods, along with the flooding methods previously described.

To “split” the condenser means to isolate one or more sections of the condenser and then pump these sections out during colder weather. The refrigerant that is pumped out of the condenser will end up in the receiver. This is because the expansion valves will only take the amount of refrigerant they need and the remaining refrigerant will be held back at the receiver. The split method reduces the total refrigerant charge required in the system.

A typical temperature at which the split mode of operation is initiated is 10°C or 50°F. So once the outdoor temperature drops to 50°F, a valve in the discharge line feeding the portion of the

Condenser splitting is a commonly-used head-pressure control method, particularly in parallel rack applications.

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condenser to be isolated closes. Since we also need to pump this portion of the condenser out, we will need a connection from this section of the condenser to the main suction header on the rack. We need a solenoid to open in this small 3/8 in. O.D. or so pump-down line when the ambient temperature drops to 50°F.

In addition to the solenoid, some form of pressure-reducing device is needed. This can either be an orifice, or even better, a small thermostatic expansion valve. The expansion valve will allow the refrigerant to be superheated prior to reaching the suction header as it absorbs heat from the surrounds. Note that a check valve is also needed in the condensate line of the isolated section of the condenser, so we don’t draw refrigerant up from the receiver when the pump-out solenoid is open.

In addition to pumping out a section of the condenser, the fans that serve that section are taken offline.

During the warmer months when the ambient temperature is above 50°F, the pump-out solenoid is closed and the discharge solenoid feeding the winter-only portion of the condenser is opened to allow full use of the condenser. As this condenser section fills back up with refrigerant, the liquid level in the receiver will decrease.

Note that it is important to equalize the pressure drops through the two sections of the condenser during the summer mode. This is to equalize the pressures, refrigerant quantity and subcooling in each section. Similarly, it is good to install a check valve in the opposite section of the condenser to equalize these pressures and flows.

Discharge Gas Regulation

In some applications, more than one section of the system is isolated. This can be accomplished by using multiple valves. This further reduces the refrigerant charge requirement for the system. Instead of using solenoids, actuator-driven ball valves are also used. The actuator positions are set by the system controller as needed.

Actuator-driven ball valves allow for more than one section of the system to be isolated.

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Riskin’s first book, Mother Nature is Trying to Kill You, was a Canadian best-seller. In 2018, it was made into an award-winning documentary on the Canadian Discovery Channel.

Dan Riskin:

GOING TO BAT FOR SCIENCE

There is a knack to paring down a scientific topic while still informing and engaging an audience and our very own Canadian science guy Dan Riskin has nailed it. Sought after by Canadian media, the scientist and journalist takes his role as science advocate very seriously, particularly with COVID-19 in the mix.

“As I move forward, it’s given me a lot to think about just what my role is, and how I can be useful. I still do as much science communication as I can, but at the root of it I believe building trust in scientists is a way of making everybody safer and making everything work better. If people know that scientists are moms and dads who love their kids, that they’re trying to save their own families and their communities, and that they care deeply about making the world a better place, that really helps build trust in science. People will be encouraged to get vaccinated when they have the opportunity, because they trust the scientists who make those vaccines.”

Riskin has plenty of opportunities to get the message out there. In addition to being CTV News’ science and technology specialist, he has done countless radio interviews. It was an interview with Newstalk 1010 that launched him as a pandemic commentator.

“I remember my first interview with John Moore, the morning anchor on Newstalk 1010. I had a conversation with him in December of 2019, about this strange disease, this pneumonia that was showing up in China. I remember saying at the time, either this is not a big deal, or it’s a very, very, very big deal. And it turned out to be a big deal. But because of that, I sort of jumped into this role of talking mostly about the pandemic and helping people stay informed and know what advice to follow and things like that. That was really not something that I’ve anticipated as part of my career trajectory by any stretch.”

Opportunistic viruses

important piece of the solution to preventing, not necessarily COVID-19, but the next pandemic that might come from that.” Riskin’s expertise in bats, interest in ecosystems He credits people working behind the and role as pandemic spokesperson have led scenes with advancing conservation him to conclude that protecting habitats is key and battling climate change; to preventing outbreaks. from policy makers to regulators, “In a way, you could say bats are responsible to those on the front lines who for this. But that’s not really fair. Bats have their later implement those changes. own diseases. In fact, people who study bats “I think that the people who are and study diseases that jump from animals to implementing those changes of people, have been saying for a long time that policy are the unsung heroes for when you have environmental degradation, In W5’s The Host, Dan Riskin climate change.” Riskin notes that when you have loss of habitat, it forces the speaks to two scientists who are this effort includes the person who animals that were living in a rainforest somewhere studying how bats can live with comes to replace a furnace with a more to suddenly come and live in close contact with coronaviruses in their bodies energy-efficient technology. people. with no negative side effects. Riskin adds that as attention turned to “What you end up with is more opportunities for a virus to indoor environments during the pandemic, the importance make the jump. People have been saying, you’ve got to of ventilation and indoor air quality became apparent. “We protect bats in their habitat. Because if you don’t, there are have these indoor environments and we don’t realize that diseases these bats have that are going to jump to people. we’re biological organisms. Oh, I’m kind of cold I’ll just turn And lo and behold, the disease jumps from bats to people. up the heat − you don’t think about the fact that you’re an That is a reflection of the fact that we have just really animal living in an environment that is kind of tough. All of a eroded their natural habitats. It’s weird to say, and seems sudden, the people who make those internal environments counterintuitive, but the solution to preventing outbreaks possible are getting some recognition.” like this, and new diseases, is to protect the animals and their habitats, to save wild places. Conservation is a really

Gross, hilarious stuff

Riskin’s wheelhouse is how bats crawl on the ground, and how bats fly in the air.

Riskin’s career has included co-hosting Discovery Canada’s Daily Planet, and Animal Planet’s Monsters Inside Me, writing a best-selling book, leading wildlife tours, and researching biomechanics, primarily using bats as a system for his research. His interest in bats, and science in general, was spurred by “gross, hilarious stuff.” “I was in a class in high school where the teacher made us read a book for 20 minutes every day. I did not want to read a book for 20 minutes every day. I was complaining about this to my mom and we were at the museum together in Edmonton. We went into the bookstore, and she said ‘You pick any book that you think will be interesting and then you’ll have something to read in school.’ I looked around on the shelves and this book had a picture of a bat on the front − I never thought about bats in my life. I thought: I’ll get this one, maybe it’ll be gross. “I read the book when I was in high school and the facts in there, the way it was written, it was just all vulgar and disgusting. You know, animals having sex, animal genitals, poop stories, stories about people getting covered in poop, just gross, hilarious stuff. I just remember reading it and thinking whoa, this is like a whole hidden world that I kind of dig. It was like I’d discovered something that nobody else knew about. I felt like I was pulling one over on my teacher because if I read this, it’s stuff which he would never want me to talk about, but here it is in a science book so it’s legitimate.”

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A P P L I C A T I O N S | T E C H N O L O G I E S

FEBRUARY 2022

AARON GALLANT:

ALWAYS AN ISLANDER

ALL ABOUT ACTUATORS DIAGNOSTIC CHECKLIST FOR THE FIELD REAP THE BENEFITS OF CLOUD CONNECTIVITY INTEGRATING SNOWMELT WITH BAS HYDRONIC PRODUCT SHOWCASE JOBSITE JARGON WITH A HYDRONIC TWIST EIGHT MAJOR REVISIONS TO LOOK FOR IN CSA B214

A P P L I C A T I O N S | T E C H N O L O G I E S supplement 02.22

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CONTROLS: ALL ABOUT ACTUATORS We are surrounded by actuators in our everyday life. In this article we will look at actuators used in hydronic and HVAC applications. Bob “Hot Rod” Rohr

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TROUBLESHOOTING: DIAGNOSTIC CHECKLIST FOR THE FIELD When you are working in the field it is handy to have some basic “go-to” diagnostic strategies, particularly for recurring problems. Ray Wohlfarth

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ROAD WARRIOR: ALWAYS AN ISLANDER Aaron Gallant of Water Tight Plumbing & Heating Inc. says his favourite projects involve working in a boiler room, which he attributes to his first employer. Carolyn Cooper

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DEPARTMENTS

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A showcase of hydronic technology.

56JOBSITE JARGON

Match the correct definition or meaning to the terminology you hear in the field and enter for your chance to win a Visa gift card.