Consulting-Specifying Engineer - November 2008 - (Page 40)

FH(%/100) = Design month (i.e., January) heating run fraction = January bin hours x (January monthly load Btuh/heat pump capacity at TMIN) CAPC ((EER+3.412)/EER) x (RP + RS x FC) (TMAX – TH) LC (ft) = LC (ft) = Length of ground heat exchanger @ TMAX CAPC (Btuh) = Heat pump cooling capacity @ TMAX (from manufacturer’s data) EER (Btuh/W) = Heat pump energy efficiency ratio @ TMAX (from Piping header collage into a Manheim Township High School building in Lancaster, Pa. Source: Consolidated Engineers pumping equipment similar to the design of many HVAC systems. Provide careful duct design and installation in which ducts are kept in conditioned spaces and are well sealed. With radiant systems, design for low heating water temperatures and higher cooling water temperatures that exploit the efficiency of the heat pump equipment. 7. Estimate the ground heat exchanger loads. Consider the monthly loads and the design month’s load. 8. Size the ground loop. This is the most important element of the geothermal design and is typically performed using thirdparty software. Collaborating with experienced professionals who size and install ground loops is essential. The installation should follow the prescriptions of the International Ground Source Heat Pump Assn. (IGSHPA). The loop installer should be IGSHPA-certified as well as a member of the Geothermal Heat Pump Consortium (GHPC). One straightforward method for determining pipe length for a two-pipe vertical ground loop is as follows: CAPH ((COPH–1)/COPH) x (RP + RS x FH) (TL – TMIN) manufacturer’s data) TH (°F) = TM = Mean earth temperature (from tabular geographic data) TMAX (°F) = Maximum earth temperature = typically 10 F below the coldest outdoor air temperature and not above the maximum EWT specified by the manufacturer’s data FC (%/100) = Design month (i.e., July) cooling run fraction = July bin hours x (July monthly load (Btuh)/heat pump capacity (Btuh) at TMAX) Run iterations with different values of TMAX and TMIN to select a loop length that minimizes first cost while not compromising system efficiency. Undersizing the ground heat exchanger can result in poor operating efficiency, reduced comfort, and nuisance heat pump lockouts on safety controls for the life of the system, while an oversized heat exchanger only increases the installation cost of the system. The cost of ground loop installation often will make or break project economics. Because of this, it is natural to assume that there are more instances of undersized heat exchangers. However, the contrary is true, indicating the potential to further close the gap in the cost of the system through right-sized heat exchangers. There also has been past evidence of greatly varying results from different ground loop sizing software with similar input data.2 Since local hydrology and geology play a big part in ground loop selection, many design engineering companies opt to contract with local geo-installers and certified loop designers to size the ground loops, as these companies are generally the most knowledgeable in the subject. If you are designing a ground loop, one helpful source of information is the Natural Resources Conservation Service Soil Survey (http://soils.usda.gov/survey/) available from NRCS offices around the country. It gives a detailed layer-by-layer description of the soil down to a depth of roughly 6 ft, along with any soil types, rock content, density, and available water capacity. Ground thermal properties can be determined from soil and rock manuals including Soil & Rock Classification for the Design of LH (ft) = LH (ft) = Length of ground heat exchanger @ TMIN CAPH (Btuh) = Heat pump heating capacity @ TMIN (from manufacturer’s data) COPH(unitless) = Heat pump coefficient of performance @ TMIN (from manufacturer’s data) RP(h-ft-°F/Btu) = Pipe resistance (from pipe resistance tables) RS(h-ft-°F/Btu) = Soil resistance (from soil/field resistance tables) TL(°F) = TM = Mean earth temperature (from tabular geographic data) TMIN(°F) = Minimum earth temperature = typically 40 F above the coldest outdoor air temperature and not below the minimum EWT specified by the manufacturer’s data 40 Consulting-Specifying Engineer • NOVEMBER 2008 http://soils.usda.gov/survey/

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Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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