Your questions answered: HVAC: How upcoming part-load efficiency regulations will influence rooftop system designs
Questions not answered during the April 27, 2017, event are tackled here.
Eric Walthall, marketing manager air conditioning scroll compressors at Danfoss in the Greater Atlanta area, tackled unanswered questions from the April 27, 2017, webcast on the upcoming part-load efficiency regulations.
Question: How can part-load variable compressors reduce or eliminate the need for hot gas reheat for dehumidification?
Eric Walthall: Hot gas bypass is an inefficient way of achieving capacity modulation. Variable speed compressors that are properly designed into a system are excellent at dehumidifying a space because they can precisely meet the capacity, temperature, and humidity requirements.
Q: How will part-load operation of compressors affect electrical demand (kW) and energy usage (kWh)?
Walthall: For energy consumption, compressors that operate efficiency in part-load, such as variable speed or intermediate discharge valve (IDV) are able to better match the required load through capacity modulation. For electrical demand, variable speed compressors differentiate themselves due to the low inrush current (greatly reduces the high peak when equipment is turned on) and in some high-load cases, the utility can directly increase the setpoint to reduce the overall demand.
Q: How much energy can be saved by using a variable speed compressor?
Walthall: It depends on what you’re comparing it to (single fixed speed, mechanically modulating, staged, tandem). The most common would be single fixed speed, and variable speed can save more than 30% in energy consumption.
Q: What are the details for U.S. Department of Energy Step 1 and DOE step 2 (it was hard to read on the chart/slide)?
Walthall: View the website that has the latest DOE ruling for rooftops. If you click on the “Notice of Effective Date and Compliance Dates DFR” link, you can find the table with the 2018 and 2023 IEER minimums.
Q: What is IDV?
Walthall: IDV stands for intermediate discharge valve. IDV technology allows the compressor to avoid over-compression losses and, thus, extra work by the motor during part-load operation, enabling Danfoss Scrolls DSH and DCJ to save energy and improve IEER from 3% to 10%.
Q: Have manufacturers explored MAGLEV technology to reduce friction loses?
Walthall: It is a technology that’s being evaluated for several applications, for the reason you listed as well as to eliminate oil in the compressor.
Q: How does a compressor with an inverter work?
Walthall: Many of the functions are similar to how a fixed-speed compressor operates. The difference is that instead of operating at one frequency (50 or 60 Hz), the variable speed compressor with a drive can now ramp up and down across a speed range. The speed that the compressor is running at a given time is driven by the design of the system, the building load, and the ambient conditions.
Q: What are the piping restrictions for field piping distances and vertical heights?
Walthall: We define this in our compressor applications guidelines for OEMs to consider when they design a system. It varies by applications, compressor platform, design base, and so on. If there’s a specific case you are curious about, we can get you this detail.
Q: With the application of the variable speed technology, are compressor expected usable lifespans improving?
Walthall: Not necessarily. We still have the same lifetime spec. Even though the compressor isn’t cycling on and off as much, its run hours are quite a bit more since it is operating in part-load. Many of the internal components (and their lifetimes) are similar between fixed speed and variable speed compressors.
Q: What types of compressors does your firm make?
Walthall: Danfoss makes reciprocating, scroll, and centrifugal compressors.
Q: What technologies currently use IDV scroll compressors?
Walthall: In terms of applications, rooftops, dedicated outdoor/outside air systems, chillers, ground source heat pumps, and IT cooling.
Q: What’s the typical cost premium (as a percent) for a variable speed scroll compressor with a permanent magnet motor compared with an equivalent fixed speed compressor?
Walthall: Without getting into detail about pricing, there is a premium for variable speed compressors (IPM motor) versus fixed speed, but it’s typically not the roadblock from a cost standpoint. The major issue is typically the cost of the drive for the variable speed compressor.
Q: Are these efficient compressor technologies applicable to rooftop unit (RTU) heat pumps?
Walthall: Absolutely. Variable speed especially is very efficient in heating mode for a heat pump.
Q: What is the payback for investing in the new technology compared to a baseline of, for example, 2012 IEER of 11?
Walthall: As I mentioned during the webcast, it depends on the design of the unit and the profile of the building. Your average payback is 3 to 5 years, but could be as quick as 1 to 2 years.
Q: Can we reuse R22 compressors with R134a?
Walthall: You will need to refer to the compressor application guidelines to see what refrigerants are qualified for the compressor you’re referring to.
Q: When a compressor is discharging through the IDV at part load, is the discharge pressure the same as at full speed?
Walthall: No, IDV is completely driven by pressure (no electronics). At full load, the discharge pressure is high enough over the suction pressure that the IDV stays closed (operates like a normal fixed-speed compressor), but as the discharge pressure starts to reduce (part-load), the IDV starts to open.
Q: How sophisticated are the controls in an RTU with IDV?
Walthall: There are no different controls than today. The IDV opens and closes based on the suction and discharge pressure (driven by the building load and ambient conditions). So, no electronics are used.
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