Classroom

Lesson 1 – Heat Recovery in Fresh Air Systems

An occupied space requires fresh air to maintain an acceptable level of welfare. CIBSE recommend 12 l/sec per person as a standard, and this can be delivered by various means, natural ventilation, mechanical extract with trickle ventilation or mechanical supply and extract. Commercial office applications would use either a, All air, VRF system or Chilled water system to maintain the comfort conditions required.

All air systems would incorporate outdoor intake and exhaust systems to deliver conditioned air into the space. A VRF and Chilled water system utilising in space terminals could use natural ventilation, an extract system to pull outdoor air into the space, or a dedicated outdoor air supply and extract system. Whichever system is adopted there will be an amount of control of the outdoor air introduction. Minimum would be an extract system controlled on temperature and/or humidity. Optimum would be a dedicated supply and extract system, which is what is installed in a lot of older premises. The incorporation of a heat recovery system goes a long way to maintain the internal comfort levels by recycling the heat energy within the space, be it heated or cooled. As a by product it is also energy efficient compare to the basic extract system.

Square profile heat recovery systems fit neatly into the ceiling void and join up the supply and extract ductwork so that sensible heat is transferred and not wasted. The elements will be either paper or aluminium plates forming a cross flow arrangement and we can expect an efficiency level of between 50% and 60%. They are controlled either separately, or switched on with the comfort system, but in both cases are working at maximum ventilation rate. The efficiency measurement is taken at -5oCDB ambient fresh air temperature and a nominal 20oCDB space temperature or exhaust air temperature.

Lesson 2 – Air & water cooled products for refrigeration & air conditioning

The basic concept of commercial water chillers have not changed dramatically over the years. The components to drive the system have benefited from material and engineering technology which has increased the efficiency of the whole package. The environmental concerns have pushed the development of new refrigerants and the overall efficiency levels have increased with each new introduction. The heat reclaim aspect of modern water chillers have made them an exciting addition to the mechanical services industry, and should be incorporated where ever possible. Saying that, it is important to recognise there must be a need for heating for the system to work. Don’t generate hot water or warm air if you don’t have to.

Air cooled systems are the most popular and form the majority of chillers sold into the UK at present. Special versions of air cooled chillers and heat pumps have been developed to meet the particular and specific requirements of the design engineers and installers who wish to build systems able to operate in the most efficient way and achieve the utmost in energy savings. Ferroli have developed a step by step approach to the design of each version of water chiller and heat pump chiller so the designer has the versatility to select the most appropriate product.

Cooling only systems can be just that, and have no reclaim capability at all. Here we look at the efficiency levels published and ratified by an independent laboratory, like Eurovent.

For comparison purposes we will select typical design conditions and a capacity range.

The Ferroli RHV has a capacity range of 330kW to 1400kW and we have taken the average efficiencies with the appropriate noise pollution levels, both of which will have an effect on the environment.

If there is a greater need for hot water, say a process plant that is continually washing down, then we can look at total heat reclaim. This would average out to be staggering 130% of the refrigeration load. We do have the opportunity to inflate the leaving hot water temperature to 50oC, but this gives 125% heat output from the chiller. It would be difficult to obtain the higher temperatures achievable with the de-super heater as we lose some high temperature heat from the compressor discharge within the refrigerant to water condenser. By incorporating two condensers in parallel we must make allowances in controlling the head pressure in the air cooled condenser. This is done automatically in the factory to ensure the machine runs to specification all the time.

The same principles can be used in water cooled liquid chillers, recovering heat from the compressor discharge by diverting it to a secondary heat exchanger. The use of water cooling towers has fallen out of favour due to the health issues associated with them. If proper maintenance is in place that monitors and controls these issues there would not be any problems, but, maintenance is more critical for the water cooling system than the air cooled equivalent. The efficiency levels for water cooled plant are marginally greater than air cooled, as can be seen in the chart below.

Propylene Glycol has a marginally better energy performance characteristic than Ethylene, but again the choice has to be made against the application. There would be another set of correction factors for the evaporator side of the chiller. Using glycol on the condenser side would tend to suggest cooling at low ambient conditions so anti-freeze protection is the requirement. Using glycol on the evaporator side would suggest a process application or an ice storage system which would warrant the low temperatures seen here.

There is a lot to be gained from heat recovery and using the right system for the application. The limitation is the vision of the whole process and being able to integrate all appliances and systems to complement and support each other. A comprehensive range of products, engineered to meet global demands in commercial markets for efficiency and value engineering will help that vision and produce an award winning process.