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Interaction of the Building Envelope with Hvac Systems Utilizing Heat Reclaim and Economizers

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Interaction of the Building Envelope with Hvac Systems Utilizing Heat Reclaim and Economizers INTERACTION OF THE BUILDING ENVELOPE WITH HVAC SYSTEMS UTILIZING HEAT RECLAIM AND ECONOMIZERS Surya Anantapantula, Ph.D. Harry J. Sauer, Jr., Ph.D., P.E. Ronald H. Howell, Ph.D., P.E. ABSTRACT Tlte effects of bllilding Iteating, ventilating, and air-condi­ relteat, fOllr-pipe fan coilllnits, constant-voillme dllal-dllct tioning (HVAC) system performance on tlte potential en­ (or IIIl1ltizone), and single-zone reheat. For each of these, the ergy and cost savings witlt tlte combilled lise of Iteat reclaim primary system consisted of a heat reclaim chiller or heat and air economizer cycle operation were stlldied. Variolls pump with a dOllble-blllldle cOlldenser, a sllpplemClltary combillatiolls of heat reclaim and economizer cycle opera­ Iteater, a heat rejection device (e.g., a cooling tower), and tion were simlliated and evaillated ill terms of allllllal en­ optionally, thermal storage. The capability to calclliate ergy savillgs and simple life-cycle costing for economizer hOllr-by-ltollr ellergy reqllirements for combined heat re­ and/or heat reclaim installation. An hOllrly bllilding loads claim-economizer operation with priority to the operation of program (ill C langllage) was developed that IIses an ef­ eitlter heat reclaim or the economizer was then added. A fective temperatllre difference (ETD) method to calclliate simple method of life-cycle costing was then added, wlticlt heating alld cooling loads. To investigate HVAC system per­ enabled identification of the most economically viable I/Cat formance, the bllilding loads program was tlten incorpo­ reclaim-economizer ("best") combination. Resllits are pre­ rated into system simlliation programs for the following sented to show the effect of intel'llal load level, ti/ennal types of terminal systems: variable-air-voillme (VAV) with mass, U-factor, and fenestration area. INTRODUCTION transfer of heat from areas of heat surplus in the build­ ing to areas requiring heat. Such systems would be Energy conservation has been and continues to be of broadly classified as applied heat recovery or heat paramount ilnportance to engineers and enviromnental­ reclaim systems and would include applied heat pump ists alike. Concern for energy conservation in the area of systems. building heating, ventilating, and air conditioning Air economizer cycles have been widely used for (HVAC) during the past few decades has led to the devel­ many years in all-air and air-water types of HVAC sys­ opment and use of various types of heat recovery tems as a means to reduce operating time for chilling (reclaim) systems that recover building internal heat equipment. In an air econonlizer cycle, when cool out­ before it is discarded in the exhaust air or in the condenser side air is available to be mixed with building return air water. 11,e performance of heat recovery (reclaim) sys­ to provide cooler air to the cooling and dehumidifying tenlS used in combination with an air economizer cycle coil and thus provide "free cooling," more (than the was evaluated in this study. minimum ventilation requirement of) outside air is sup­ Many commercial, institutional, and industrial plied and the outside air fraction is increased up to a buildings require sin1Ultaneous heating and cooling for maxinuun of 1 (or 100%). This is done with a variable­ prolonged periods during occupancy. This is especially position outside air damper controlled by a mixed-air true with buildings that have one or more core zones temperature sensor ahead of the cooling coil in combina­ that must be cooled year-round. Frequently, therefore, tion with an outdoor air enthalpy sensor. 11us variable­ the mechanical cooling and heating equipment must be position damper typically is used in conjunction with a operated jointly to provide the necessary comfort. 111is fixed minin1Um outside air damper so that the required work studies the potential for savings in energy that can ventilation air can always be made available for the be realized by the use of HVAC systems that permit the building. V. Surya Anantapantula is a senior staff engineer for Alco Controls Co., St. Louis, Mo. Harry J. Sauer, Jr., is a professor of Mechanical and Aerospace Engineering at the University of Missouri-Rolla. Ronald H. Howell is a professor in and chairman of the Department of Mechanical Engineering at the University of South Florida, Tampa. Thermal Envelopes VI/Whole Bui/dings /I-Principles 477 I I : L - - - - - - - - - - - - .. -- .. --- .. CENTRIFUGAL I I 0 l_ ---- ----© __ _ __ ":':':~~~ 0 '~AA'"~ HEAT RECLAIM SYSTEM WITH DOUBLE BUNDLE CONDENSER AND HEATING STORAGE SERVING FAN COIL UNITS AT TWO ZONES Figure 1 Schematic of heat reclaim-economizer system. Some HVAC system designers have suggested that available). 11,e use of properly controlled and operated heat reclaim chillers and heat pumps should not be used thermal storage consequently will make the use of an in conjtmction with an air-handling system that incorpo­ ecol1Oluizer less cost effective. Control of the econonlizer rates variable-position damper economizer control should not be based on the needs of individual building because the "free cooling" concept (of economizer oper­ zones. TIley also concluded that if a heat reclainl system ation) would result in higher alUlllal operating costs is to be used, cost considerations will favor the addi­ than if only minimum outside air ventilation is used. tional use of a properly controlled and operated econo­ Other investigators have found that outside air can be mizer for high electrical energy rates. used economically in a heat recovery project if properly A simple schematic of the combined heat reclain1- controlled to maintain an overall building "heat bal­ ecollOlnizer system simulated in this study is presented ance." Several have developed design guidelines for the in Figure 1. 111is is an internal-source, vapor-com pres­ use of an economizer 'with heat recovery. They con­ sian-type, four-pipe heat pump/chiller system using a cluded that accurately evaluated, time-related heating centrifugal compressor and a double-bundle condenser. and cooling loads are an integral part of the design/ For purposes of illustration, the schematic shows only selection process to select anlong an economizer system, two zones (there could be any number in the actual sys­ a heat reclaim system, or a combination of both based on tem)-one calling for heating (maybe a perin1eter zone) energy or cost considerations. They recommended that and the other calling for cooling (maybe an interior if an economizer is lIsed in conjunction with a heat zone). 11,e evaporator (water chiller) of the heat pump/ reclaim system, its control is critical and must be such as chiller provides the cooling required, and the corre­ to provide for priority to heat reclaim. If such priority is sponding heat available at the double-blU1dle condenser not provided for, there might be heating cost penalties. can be used to heat water in the heating bundle (to pro­ 11,e economizer operation should result in discarding vide heating for the other zone). If excess heat is avail­ only that heat that is in excess of what the building can able at the condenser (over and above the heating need use instantaneously for heating plus that later needed for the particular hour), it could optionally be stored in a for heating and being placed in storage (if storage is heating storage tank, and beyond the capacity of the 478 Thermal Envelopes VI/Whole Buildings II-Principles storage tank, heat can be rejected at the cooling tower. If of course, the penalty in energy of heating and cooling air the heat available is insufficient to meet the heating supplied to the same zone. When this system is served by requirements, additional heat can be provided by the the double-bundle chiller/heat pump, the needs of the supplementary heater. central cooling coil would be met by the evaporator of the TI,e cooling required at the cooling coil depends on chiller /heatpump. Heat available at the condenser can be whether the economizer cycle is operating at that partic­ reclaimed (or recovered) and used to provide hot water ular hoUl; which depends on either the temperature or for the central heating coil (and preheating needs, if allY). the enthalpy of the outside air (based on the type of con­ trol sensor used). The system shown is always able to METHODOLOGY supply minimum outside air and can provide more out­ To determine the requirements of the building in side air when called for by the economizer (Le., when terms of sensible cooling, sensible heating, dehumidifi­ use of more outside air would result in "free cooling"). cation, alld hUlllidification for each hour, all energy bal­ The opposing effects of economizer cycle and heat allce was performed for each zone of the building on all reclaim can be visualized from the schematic in Figure 1. hourly basis. It must be kept in mind that this analysis If the control system employs the economizer cycle in was aimed at developing a sinlplified, reasonably accu­ zone 1, less cooling is required at the evaporator, but rate procedure that would provide a common basis consequently less heat is available at the condenser for upon which different terminal and prinlary systems heating at zone 2 and the supplementaty heater may could be analyzed and the performance of those systems have to be activated. It is evident that the control system with the simultalleous operation of heat reclainl and is faced with the choice of whether to give priority to the economizer cycle could be studied. economizer or to heat reclainl. TI,is work studied the The approach used in this study to calculate building pros and cons of both priorities and examined the cooling loads may be called all "effective temperature atmual energy consumption in each case. It was found difference (ETD) method with time averaging (TA).
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