This paper has been downloaded from the Building and Environmental Thermal Systems Research Group at Oklahoma State University www hvac okstate edu The correct citation for the paper is Spitler J D D C Hittle D L Johnson and C O Pedersen 1987 A Comparative Study of the Performance of Temperature based and Enthalpy based Economy Cycles ASHRAE Transactions 93 2 13 22 Reprinted by permission from ASHRAE Transactions Vol 93 Part 2 pp 13 22 1987 American Society of Heating Refrigerating and Air Conditioning Engineers Inc No 3055 A COMPARATIVESTUDY OF THE PERFORMANCEOF TEMPERATURE BASED AND ENTHALPY BASED ECONOMYCYCLES J D Spitler ASHRAE Associate Member D C Hittle Ph D ASHRAEMember D L Johnson Ph D ASHRAEMember C O Pederson Ph D ASHRAEMember ABSTRACT Deciding whether to use an economy cycle and then whether to use a temperature based or an enthalpy based economy cycle is a problem that many HVACdesigners face today The controversy about which type of economy cycle should be used was demonstrated at a recent ASHILAE meeting when one author recommended the use of enthalpy based economy cycles Sud 1984 while another discouraged their use in favor of temperature based economy cycles Haines 1984 This study quantitatively examines the potential energy savings for two prototypical buildings due to both types of economy cycles This was done in a systematic manner in order to give designers the guidance that they need Two different buildings one more core dominated than the other were studied at five different locations using the Building Loads Analysis and System Thermodynamics BLAST program Hittle 1979 Two different types of systems terminal reheat and variable air volume VAV were simulated each with and without economy cycles For all locations except Houston the temperature based economy cycle achieves most of the savings that can be achieved by the enthalpy based economy cycle INTRODUCTION The use of economy cycles is a popular means of saving energy in HVACsystems Basically an economy cycle involves bringing in excess outdoor air in order to utilize free cooling when the outdoor temperature is low enough Two means of deciding when to bring in excess outdoor air were studied The first is based on a comparison of the outdoor air and return air drybulb temperatures If the outdoor air temperature is colder than the return air temperature then outdoor air is used to the extent necessary to maintain the mixed air temperature as close as possible to the controller s set point If the outdoor air temperature is higher than the return air temperature minimum outdoor air is used The enthalpy economy cycle is similar but it compares the outdoor air and return air enthalpies The subtle advantage of the enthalpy economy cycle is that it keeps the outdoor air damper at its minimum position under those conditions when the outdoor temperature is cooler than the return air temperature but the outdoor humidity is so high that the enthalpy of the outdoor air is higher than the enthalpy of the return air Therefore we can expect the enthalpy econom y cycle to have advantages in climates where it is often moderate but very humid during the daytime operating hours of the buildings The enthalpy economy cycle also allows the continued use of outdoor air if it is warmer than the return air but so dry as to have lower enthalpy This may or may not be advantageous and will be discussed later in the paper If neither of these conditions occurs frequently temperature alone will be a suitable indication of enthalpy J D Spitler is research engineer and C O Pedersen is associate professor Department of Mechanical and Industrial Engineering University of lllinois Urbana Champaign D C Hittle is associate professor of mechanical engineering Ray Herrick Laboratory Purdue University West Lafayette IN D L Johnson is principal investigator ES Division U S Army Construction Engineering Research Laboratory Champaign IL Temperature basedeconomy cycles use some type of temperaturesensor such as a platinum resistance temperature detector RTD Enthalpy economy cycles require both a temperature sensor and some type of humidity sensor Sensors used for temperatureare in general simpler and much more reliable than those used for measuring humidity Humidity sensors on the other hand tend to drift from calibrationand have extensivemaintenancerequirements Problems with humidity sensors and enthalpy economy cycle controls are discussed by Hittle and Johnson 1985 A study of the effects of sensor errors on building energy performance Kao and Pierce 1983 demonstratedthat moderatehumiditysensor errors 10 F error in dew point temperature can cause significant increases up to 13 in system energy consumption An increase system energy consumptionof this m gnitudewill negate any additionalsavings obtainedby using an enthalpy basedeconomy cycle insteadof a temperature basedeconomy cycle This finding highlightsthe need for an extremelyaccurate humiditysensor if one wishes to use an enthalpyeconomycycle The objective of this study was to determine potential energy savings that could be achieved by temperature based economy cycles and enthalpy based economy cycles The potential energy savings were determined through simulation using the Building Loads Analysis and System Thermodynamics BLAST program Hittle 1979 The first step in this study was to select two prototypicalbuildings Five locationsin the United States were chosen and both buildings were simulated in each of the five locations For each building location six systems were simulated 1 a terminal reheat system with no economy cycle 2 a terminal reheat system with a temperatureeconomy cycle 3 terminalreheat system with an enthalpyeconomy cycle 4 a VAV system with no economycycle 5 a VAV system with a temperatureeconomycycle and 6 a VAV system with an enthalpyeconomy cycle The economy cycles simulated in this study are idealized economy cycles No accounting is made for potential errors in temperaturesensors humidity sensors or controllers PROTOTYPEBUILDINGSAND SYSTEMS Building Sites The first building studied was a dental clinic and was intended to be typical of a smal office2building The dental clinic is a single story building with approximately 9000 ft 836 m of floor space The building occupancy lighting and electrical loads are typical small office buildings The second building was intended to be typical of a large multi story office building and was ap ly desigF ted large office building The
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