CPD PROGRAMME


Professional development


The CIBSE Journal CPD Programme


Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.


Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).


CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.


Simply study the module and complete the questionnaire on the fi nal page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current.


You can also complete the questionnaire online, and receive your results by return email.


Saving energy through simple HVAC heat recovery


In this module we look at heat recovery in mechanical ventilation systems using plate heat exchangers and methods to compare annual heating performance


The reduction of energy use in buildings is not only an economic issue but also increasingly linked with opportunities to lessen the demand on the primary energy source (and distribution networks), as well as reducing life cycle environmental impact. This CPD article will consider heat recovery in mechanical ventilation systems using plate heat exchangers and show how to compare annual heating performance using binned weather data.


Heat recovery systems Heat recovery in HVAC systems will typically exchange heat between the discharged room air and that being


Counter-fl ow plate heat exchanger (sensible heat recovery)


O


Discharge room air R


R’ B


Moisture content kg/kgda


Room air dew point X O R’ B R


gR’ gB


=gR =gO


Cross-flow Plate Heat Exchanger (sensible heat recovery)


introduced from outdoors. The system may be designed to exchange only sensible heat or both sensible and latent heat. Details of the principal system types may be seen in Section B of the CIBSE Guide. An example of a sensible heat recovery


process is shown in Figure 1. This psychrometric process indicates


an increase in the sensible heat of the incoming air that could take place in a cross-fl ow plate heat exchanger, a (regenerative) thermal wheel or a run around coil. The process is a basic sensible heating or cooling process – depending on the temperature of the opposing airstreams. The heat exchanger sensible heat effectiveness, εS where O


= O(θB and R – θO )/ R(θR the room temperature, and θB – θO ),


fl ow rates of air at outdoor temperature θO θR


are the respective air mass ,


being the


temperature of the outdoor air after is has been through the heat exchanger. Considering Figure 1, if the temperature , is below the dew


of the incoming air, θO point temperature of the extracted air, θRdp


Dry-bulb temperature °C


θRdp θO θR’ θB θR


Fi gu re 1: Psychrometry of cross-fl ow plate heat exchanger


www.cibsejournal.com ,


there is likely to be some condensation in the airstream that is coming from the occupied space, so providing increased heat exchange. But, in the case of a simple impermeable plate heat exchanger, there is no transfer of water vapour. The heat


Figure 2: A section through a combined counter- fl ow and cross-fl ow plate heat exchanger


of condensation will add to the recovered heat. In recent years, the simple cross-fl ow plate heat exchanger has been developed to provide an additional counter-fl ow component, as illustrated in Figure 2. Owing to the extended heat exchange surface, the effectiveness is increased (as is the air side pressure drop). The manufacturer1


reports a seasonal sensible


heat exchange effectiveness of 85%; such a process is illustrated (approximately) in Figure 3, where the condensation from the discharge air further increases the dry bulb temperature of the incoming outdoor air. The additional resistance to air fl ow will


require energy for fan power (W) given by: Power = Q·∆P/ηfan


where Q is the air volume fl ow (m3 is the additional pressure drop through


·s-1 ), ∆P


February 2013 CIBSE Journal


61


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