Refrigerant Line Sizing

Refrigerant line sizing is an overlooked aspect of system application and installation. Incorrectly sized refrigerant lines can rob your system of capacity and reduce its efficiency. In a worst case scenario, incorrectly sized and applied refrigeration lines can cause multiple compressor failures. For most applications, the two lines involved are the liquid line and the suction line. 

Pressure drop in the liquid line can create flash gas in the line before the refrigerant reaches the metering device. This causes a drop in system capacity because it reduces the amount of liquid entering the evaporator. Some pressure drop is inevitable. It is important to have enough liquid subcooling to offset the pressure drop through the liquid line. 

Suction line pressure drop also hurts the system performance by increases the compression ratio and reducing the amount of refrigerant being circulated. The general rule is to try and keep suction line pressure drop below an equivalent saturation temperature drop of 2F. The actual amount of pressure this represents depends on the refrigerant and the evaporator saturation temperature. Suction lines have another very important design criteria: the refrigerant traveling through them must have sufficient velocity to return oil. While larger lines help reduce pressure drop, they also decrease refrigerant velocity. In general, you want your suction line to be as large as possible while still having enough velocity to return oil. 

There are far too many variables to mention in a blog post, but I can point you toward some excellent materials available online. DuPont and Lennox both have excellent refrigerant piping handbooks available on the web in pdf form. The Dupont document is applicable to all forms of refrigeration while the Lennox material is primarily for air conditioning. Virginia Air has a great excel file which does a lot of the heavy lifting for you in calculating pressure drop and velocity. They also have several line sizing tables on another tab of the excel file. I guarantee that you will learn something about line sizing if you download and examine these three wonderful resources. I know I did.

DuPont - http://www.hvaceducationaustralia.com/Resources/PDF/DuPont%20Refrigerant%20Piping%20Handbook.pdf

Lennox

http://www.mechanicalconceptsllc.com/downloads/Refrigerant%20Piping%20Manual.pdf

Virginia Air

http://www.virginiaair.com/index.php/contractors/documents/doc_download/487-refrigerant-pipe-sizing

Cold Weather Heat Pump Charging

Charging an air source heat pump during cold weather has always been a bit of a problem. The problem is that the amount of refrigerant circulated decreases as the outdoor temperature drops. Why is this? Well, as the outdoor temperature drops, the evaporator temperature has to drop in order to be able to absorb heat from the outdoor air. The lower evaporator temperature produces a lower evaporator pressure. The lower evaporator pressure increases the compression ratio because there is now a greater difference between the suction pressure and the discharge pressure. The higher compression ratio means that the compressor does not circulate as much refrigerant.

At a 45°F outdoor temperature,  a typical air source heat pump produces a heating capacity roughly equal to its nominal cooling capacity. At 17°F outdoor ambient, it produces about half as much heat as it does at 45°F. This difference in capacity is directly related to the amount of refrigerant being circulated. The rest of the refrigerant is just sitting somewhere – normally in either the accumulator or the charge compensator. So a system operating at 17°F outside could have perfect pressures even if it only had half of its factory charge. That is why you can be way off checking a heat pump by pressures in the heating mode.

Some manufacturers provide heating performance pressure charts, but refer to them as “check” charts. They are intended to check the system operation at specific conditions, but are NOT intended as guides for adding refrigerant. The problem is that you don’t have a good way to judge how much refrigerant is stored out somewhere in the system. I can hear a bunch of you saying that measuring superheat and/or subcooling solves that problem. While I AM a fan of checking both, they still just measure the refrigerant that is circulating.

There have been some interesting methods used, such as measuring discharge superheat. For discharge superheat, you measure the temperature and pressure of the discharge line right as it leaves the compressor. It should be somewhere around 60°F warmer than the discharge saturation temperature. So if you have a 410A system running at a discharge pressure of 318 psig (saturation temperature 100°F), the discharge line should measure 160°F. A lower temperature reading indicates an overcharge and a higher temperature reading indicates an undercharge. The surest way to charge a heat pump in the winter is to recover the refrigerant, evacuate the system, and weigh in the correct charge. If you have performed a repair on the refrigerant system, then this will save you time and insure a correct charge.

Cranking Out The Rules

 With only weeks left in the Obama administration, federal agencies such as the Energy Efficiency and Renewable Energy Office (EERE) are finishing up work on a number of initiatives and publishing final rulings. Who are the EERE? They are a subset of the Department of Energy (DOE). These rulings have an impact on the HVACR industry. While the rulings do not have a direct effect on technicians in the field, they have a big effect on HVACR manufacturers. Indirectly they will affect us as manufacturers respond to the directives. Most of the rulings have to do with how different equipment is tested by the manufacturer, and in some cases, what equipment is covered.  Here is a list of recent rulings with links for more information.

12/02/2016 DOE Issues a Notice of Final Rule Pertaining to Test Procedures for Walk-in Coolers and Walk-in Freezers

https://www.regulations.gov/docket?D=EERE-2016-BT-TP-0030

12/01/2016 DOE Issues a Notice of Final Rule Pertaining to Test Procedures for Compressors

https://www.regulations.gov/docket?D=EERE-2014-BT-TP-0054

11/30/2016 DOE Issues a Final Rule Pertaining to Test Procedures for Central Air Conditioners and Heat Pumps

https://www.regulations.gov/docket?D=EERE-2016-BT-TP-0029

11/21/2016 DOE Issues a Comment Period Extension Pertaining to Energy Conservation Standards for Residential Furnaces

https://www.regulations.gov/docket?D=EERE-2014-BT-STD-0031

 11/15/2016 DOE Publishes a Final Determination of Compressors as Covered Equipment

https://www.regulations.gov/document?D=EERE-2012-BT-DET-0033-0012

11/10/2016 DOE Publishes a Final Rule Pertaining to Test Procedures for Commercial Packaged Boilers

https://www.regulations.gov/document?D=EERE-2014-BT-TP-0006-0049

10/07/2016 DOE Issues a Final Determination Pertaining to Energy Conservation Standards for Direct Heating Equipment  

https://www.regulations.gov/docket?D=EERE-2016-BT-STD-0007

10/04/2016 DOE Issues a Direct Final Rule Pertaining to Energy Conservation Standards for Miscellaneous Refrigeration Products

https://www.regulations.gov/docket?D=EERE-2011-BT-STD-0043