Pti l .Commercial heat recovery an appraisal I by A.J. Johnson, BEng,MCIBS, Electricity Council H EAT recovery has a lways been practi. ed in commercial buildings. Neverche les . it o ft en ma ke . en e to recover this free heat to get lt is onl y wi1h increasing awarenes. o f ene rgy cosis thal de ·igner it under the contro l of the syste m pending a decision on what to do a nd building o perators have considered how to ma ke more with it. We will re turn to me thod o f achieving this late r. effective use of heat recovery to reduce their bills. A s e ne rgy costs ri e, compared wi1h capita l costs of equipme n1 to recover nnd DELIBERATE HEAT RECOVERY re-use th at ene rgy. mo re sophi. ticated idea · become vi abl e . the If the crite ria for successful heat recovery are ful filled. there arc ra nge of optio ns increases . a nd the choice of the most suitable va ri o us mechani sms by which it can take place. T he si mplest system for any gi ve n applicatio n become mo re difficult . This method is to recirculate a proportion of the extract air from the appraisal consider the o ptio n which could be desn ihed as heat space which has inad verte ntly picked up heat. Other methods recovery and which are open to building services designers and de pend o n me de vi ce which is e ithe r active or passive . the operators. di tinctio n being between tho ·e \ hich use further energy t function a nd tho ·e whi h d not. The active me thods go beyond FUNDAMENTALS FOR HEAT RECOVERY discrete devices for heat recovery. into syste m where several It is co nvenie nt to de fin e heat recove ry as the process of compone nts interact to give mo re tlexible a nd mo re comple te reclaiming and re-u ing ene rgy o ri gi na ll y de ·tined for some other control o f heat e ne rgy tl ows within a building. and the re ult ca n bl' purpose. for heating a space for co m fort or for pr cess heating. In eve n less de pe ndent o n paid fo r e nergy brought into the build ing. the commercial wo rld we a re m re concerned with the fo rmer end At this level the contro l philosophy mu t be care fully thought use fo r the heat e ne rgy . and so in gene ral we can make be Iler U$e of through if the heat recovery sche me is to be effective . fairly row te mperature, o r ' low grade' heat. In this appraisal of heat recovery techniques I will examine each The re-use of heat energy is often regarded as intrinsically 'a good of these methods in turn. thing ,' without apprecia ting that it i not always advantageous. The conditions for a successful heat reco very in ·tallatio n a re that: (a) There must be heat available in the right quantities at the right AIR RECIRCULATION This is the simple t fo rm o f heat reco e ry am.I in many ~· ases thl' temperature and at the right time. cheapest and most cost effective. Air fmm the: cxtrnct duct i ~ (b) There must be a use for that heat once it has been recovered. a ll owed into the supply duct. Hence. lu w grade heat in the extrac1 (c) There must be an economic case for the scheme: that is. the net air i. used to reduce the <1mo unt o f hea t added to the supply air present value of the energy savings should exceed that of the cost of stream . Heat pic ked up from li ghting. pellpl e and hu si n c ~' installing the heat recovery measures. machines which would othe rwise he lost in the extract ai r i' (d) The operating taff must be educated on the rea ons for the the refore recovered a nd re-used . heat recove ry sche me: carelessness or misunderstanding of the However. one must ask what the original reasons for the air opera ti on of plant o r the building itself can undo all lhe e fforts of a handling system were. If one of its purposes was to disperse odours. well meaning design. or infectious airborne material. or high moisture content air. then It is impos ·ible in an appraisal such as this to advise whether in a obviously recirculation is not feasible . Likewise, if the system w;i ~ particular case these four co nditio ns co uld be fulfilled. What it can trying to cool the space. then there is not much point in recirrnlati n ~ do is to point out that certain opiio ns are avail able and that they warm air. These points might seem self-evident. hut it is necessar; should be explored. bea ring in mind the abo c: points. in a system which incorporates air recirculation to remember tt1 allow for such circumstances in the control system. INADVERTENT HEAT RECOVERY In any case. there may he a helter opportunity for energ~ Most build ings recover ene rgy in the fo rm of heat whe ther they reduction by reducing the volume of air supplied and extracted in want it d r no t. Elect.rica l e ne rgy o rigi na ll y b rought into the space the first place . In many circumstances. circulation volumes are se t fo r li ghting o r operating business machines ei ther immediate ly o r to suit the design conditions rather than those commonll ultimately degrades to hea t. Thi heat ene rgy hali therefore been experienced . and it makes sense to control the volume of ;11 1 recovered by the space. If the second conditio n fo r effociive heat throughput to that required to pt:rform its job. Variahle spec1 ~ recove ry . that there ho uld be a use for that hear. is nm fulfilled motor drives and controls are more freely availahle than thcy use• then the advertent heat recovery co uld be an e mba rra · ·me nt ! to be, and so the additional benefits of reduced energy rnnsumptit11: Cooling is required in such circumsta nces if co mfortable by the fan motors can be received. conditio ns are to be maintai ned . This can o ftcn be i.1ccomplished Where the circulation volume is reduced. or varied. the .iii witho ut furthe r ene rgy consumptic>n . but if the inadvc rte nt heat distribution system must also be considercd to ensure that ;iii recovery is associated with s lar heat gains or me tubo lic gains from movement in the room space is still adequate. In the case of the people in the ·pace. mechanical cooling ma we ll be rcquired . permanent reduction in volume then new. smaller diffusas an , and further ene rgy e pe nded . o . ·onH! inad e rtent heat recovery registers may he needed to maintain the same air throws as hefo1 . is not de ·irable . (or perhaps to remedy defects that exi~ted before!). With variabl· 44 Refrigeration, Air Conditioning and Heat Recovery, October. 19 3 .. Cool fresh air and latent heat by absorbing into the medium. The apparent ,, problems of cross contamination between air streams are overcome by close tolerances between the edges of ducts and the wheel itself. Cooled exhaust air and by a 'purge' segment between the extract and supply sides where contaminants are blown through in reverse flow. Air streams -• • Cold end are generally arranged to oppose one another to dislodge any . • • • debris which falls onto the wheel surface . • -· Control over the rate of heat transfer can be maintained by Hot • .• • end varying the wheel's rotation speed relative to the air velocity through the medium. Refrigerant warm supply air / -return along . Altho ugh efforts are made to avoid cross contamination. it is Liquid wick inevitable that some transfer of materi al occurs between ai r refrigerant Wann• extract air streams. This is particul arl y of concern where ·te rility is needed. Fig. 2-Heat pipe. such as in hospital surgical wards. or where moist ure should not be.-.---­ transfcrred, as in swimm'ing pools. In these case recuperati ve DJ wick and gravity. (separated) methods must be used , so !'hat onl y heat is transferred ......--- [t can be seen that the e ffi ciency of operation will depend on the A further limi tati on o f a ll the method o far described is that the position of the heat pipe - where the evaporating end is below the heat ·ource and sink must be adjacent to one another. It can be condensing end. the unpumped liquid refri gerant return is inconve nient to arrange supply and extract air stream to run obviously enhanced by gravity whereas when horizontal or inverted parall el to one ano ther. particula rl y if retrofitting heat recover ~ the return reli es solely on the capillary action of the wick. into an ex i ting building. T his can be overcome by using a flu id Efficiency of heat transfer at eac.h end of any heat exchange r medium to transport the heat between air streams. depends on how close the air stream are to the heat tran fe r The run aroun d coil sy te rn (fig.4) comprise two finned tube medium.