CUADRO DE CONSTRUCCIÓN Y VALIDACIÓN DEL CUESTIONARIO DIRIGIDO AL PERSONAL DEL DEPARTAMENTO DE CONTABILIDAD
P: pertinente N no pertinente A: adecuado I: inadecuadoS: Siempre CS: casi siempre N: nunca CN: casi nunca
(BoD)
According to ASHRAE Guideline 0, the BoD is “a document that records the concepts, calculations, decisions, and product selections used to meet the Owner’s Project Requirements and to satisfy applicable regulatory require- ments, standards, and guidelines. The document includes both narrative descriptions and lists of individual items that support the design process” (ASHRAE 2005, p. 4). Since the design process is a somewhat linear process, the BoD will be incrementally developed as the design is developed. Once the design is complete, the BoD will need updating only when design changes are a result of the OPR being revised. The expectation then is for the CxA to use the most up-to-date BoD to effectively commission the CDs and ensure that the OPR is being satisfied by the design. This is one of the most important tasks for the CxA during the Planning and Design Phase since it allows issues and concerns to surface as the design is being developed. The technical proce- dures being performed as part of commissioning should also be reviewed, and
DOE REFRIGERATED CASE STANDARDS
The recent work of the U.S. Department of Energy (DOE) on 10 CFR 431, Part III, assigned classification to commercial refrigeration display cases. This work established standardization for the calculation of case energy by class of case, which supports the designer with a higher defini- tion of system load/balance.
Tables summarizing these standards are available on the DOE Energy Efficiency & Renew- able Energy (EERE) Building Technologies Office website under Appliance & Equipment Stan- dards for Commercial Refrigeration Equipment: www1.eere.energy.gov/buildings/appliance _standards/product.aspx/productid/52.
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any information that must be included in the BoD to successfully perform each procedure should be added to the BoD.
In summary, the BoD should address all of the major requirements included in the OPR. To best illustrate how the BoD accomplishes this task, some basic examples are given in Table 2-4, in the order that they would likely
COMPRESSOR PERFORMANCE: RATED VS. APPLIED CONDITIONS AND IMPLICATIONS FOR DESIGN AND COMMISSIONING
The current design practice and difference between nominal rated compressor capacity (CC) and applied conditions results in a large difference between apparent and actual capacity, particu- larly for commercial refrigeration applications. The capacity on paper is not the real capacity, which has implications for the commissioning process and procedures, since commissioning fun- damentally is addressing expected vs. actual performance. This situation provides an example of the necessary interaction and relationship between design practice and commissioning.
In terms of achieving the desired cooling capacity and temperatures, compressors are the most important component in a refrigeration system. Since temperatures in most applications must be met 100% of the time, systems are naturally designed from a conservative point of view—the consequences of not meeting temperatures are expensive to resolve and very unattract- ive for all involved. This is in contrast to commercial HVAC systems wherein, for cost-effective- ness, good design practice expects temperatures will exceed design conditions for a few hours out of the year. A consequence of this difference is that HVAC load calculations and system capacity are honed by “being on the edge,” in contrast to refrigeration applications where a system with “unmet cooling hours” is completely unacceptable.
Low-temperature compressors will most often be much more heavily loaded than medium- temperature compressors. In fact, it isn’t unusual for all low-temperature compressors to be run- ning when only half of the medium-temperature compressors are on. And, on paper, medium-tem- perature systems often have less safety factor. While this may be due to greater load diversity on medium-temperature systems or differences in how walk-in loads are calculated, the main issue is usually a striking difference in how the compressors are rated and applied.
Commercial refrigeration compressors are rated based on ANSI/AHRI Standard 540 (2004), which include a fixed 65°F (18°C) return gas temperature (RGT) entering the compressor, regard- less of the saturated suction temperature (SST). This means that the rated capacity for a compres- sor operating at –20°F (–29°C) assumes all heat required to superheat the suction gas to 65°F (18°C) is productive refrigeration, whereas the actual productive suction gas temperature is likely no greater than 0°F (–18°C), the temperature leaving the display case or freezer box. The discrep- ancy between rated capacity and the actual cooling capacity is often 20% or more. This appears to be addressed by compressor manufacturers’ software, which provides for variable suction gas temperature. However, the software adjustments are approximations (generally a simple ratio of suction gas density), not based on additional compressor testing at more common conditions.
This historical discrepancy in compressor ratings is inherent to the process of design practice, load calculations, and general experience and expertise in the industry. Safety factors and capacity adjustments have evolved from experience and will continue to improve. In this example, the need is for improved compressor rating conditions and testing along with a better understanding of actual loads.
The figures help illustrate these concepts by describing productive and nonproductive superheat by defining points on a pressure-enthalpy diagram and a physical refrigeration system schematic.
Refrigeration Commissioning Guide for Commercial and Industrial Systems
be developed. Please note that these examples illustrate how the BoD should respond and react to some specific requirements that might be found in an OPR. The requirements listed may or may not represent best practices or rec- ommended specifications that owners should adopt. For example, the first requirement in Table 2-4 is to use the 0.4% ASHRAE weather data (2013a) to determine the design-day ambient temperature. It is not the intent to imply How does this relate to commissioning? One point is that commissioning efforts will be suc- cessful only to the extent the current state of the art and design practice are understood and taken into account by all of the right parties. If a CxA sets about to compare expected vs. actual capacity without a sufficient understanding of the technical issues, the results may be disappointing. On the other hand, the commissioning effort helps identify and emphasize discrepancies and contra- dictions, which can accelerate better design.
Summary: Moving beyond the “this is how we’ve always done it” mentality is inherent in the
overall commissioning process and will drive more sustainable design and operations. This requires close cooperation and knowledge sharing between the CxA and design experts. Some improvements will take time and multiple project cycles but will be very valuable.
Graphics courtesy of VaCom Technologies
Refrigeration Cycle with Productive and Nonproductive Superheat (SH): Schematic (top left), Graph (top right), and Pressure-Enthalpy Diagram (bottom)
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© 2013 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.
Table 2-4 Example OPR and BoD Language
Example Requirements in the OPR Corresponding Example Information in the BoD
Ambient temperatures used for design-day calculations are to be the “0.4% dry-bulb” temperatures as published by ASHRAE (ASHRAE Handbook—Fundamentals, 2013). Round 0.4% dry-bulb (db) temperature up to nearest 5 degree increment. Use evaporative- cooled technology when ambient temperature exceeds 105°F.
The 0.4% db temperature for Anywhere, USA = 92°F. The design-day ambient temperature used in design is therefore 95°F. Air-cooled condensers were selected because the calculated design ambient temperature does not exceed 105°F.Air-cooled condensers shall be sized to meet, on the design day, an actual operating temperature difference (TD) between 8 and 10 degrees Fahrenheit to help meet the energy-
efficiency goal for the system.
Given the ambient design temperature on the design day of 95°F, the condensing temperature has been selected at 105°F. The condensers have been selected to meet a minimum of 10°F of temperature difference (TD) without safety factor per the calculation below:
THR
10TD
--- = THR 1TD Condenser
Since the actual condensers have been selected with safety factor, the actual TDs have been calculated per the equation below and fall between 8 and 10 degrees Fahrenheit. T DActual THR THR 1TD --- =
Condensers are to be protected from the outdoor elements when threatened.
Condensers are also to be visually screened when required. If screening is required,
condenser operation shall not be hindered.
Due to the site location, there is no threat of hail requiring hail guards for condensing units; however, since the project is five miles from the coast, condenser fins will be specified with corrosion protection from the saltwater breeze. Furthermore, since there is a residential neighborhood on the hillside behind the premises, condenser screening will be required. Screening will be offset from the condensers a distance of 8 ft minimum on all sides to allow for proper airflow.
This project is to achieve Gold GreenChill certification. Only owner-approved, applicable methods for achieving a reduced refrigerant
charge are to be employed.
Design decisions made that will help the owner reduce refrigerant charge as much as possible are as follows:
• Use microchannel condensers • Use “loop piping”
• Use subcooling on low and medium temp racks.*
* Note: In the commissioning process, the CxA may review this portion of the BoD and realize that the designer missed an important opportunity to fur- ther reduce the refrigerant charge by not using a more effective solution that had been approved by the owner on a previous project, such as using dis- tributed rack systems with water-cooled condensers, for example. The goal would be for the CxA to continually commission the BoD as it is developed to catch opportunities for change early in the design development so that the design can be adjusted while that can be done with little or no cost impact.
Compressor selections per suction group are to be made to meet the number of stages as outlined in the compressor rack specification section based on the number of compressors per suction group. If the number of stages cannot be met, use variable-frequency technology.
The number of Rack ‘A’ capacity steps in the design is 18, which meets the minimum of 14 for a 4-compressor rack as outlined in the compressor rack specification.Refrigeration Commissioning Guide for Commercial and Industrial Systems
that there are days when refrigeration systems are not required to adequately refrigerate. In this specific example, there would be other safety factors built into the design elsewhere per the OPR. Being mindful of the fact that temper- atures may rise higher than the design temperature requires a careful balance of safety factor without grossly oversizing the system—which may create other system issues.