Classroom

Lesson 1

Heat recovery in fresh air systems.
By Roger Palamarczuk dpac Ferroli Sales Manager.

June 2009

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.

The next leap in client welfare and energy conservation is to use a Ferroli rotary enthalpy recovery wheel. Not new technology certainly, but in the past they have been mounted vertically in a supply / extract air handling unit. Ferroli have now positioned a wheel diagonally in a standard recuperater chassis which means it can be mounted in a 600mm ceiling void. Using the enhanced design criteria -5oCDB / 80%RH Outdoor ambient and 20oCDB / 50%RH Indoor we can predict an efficiency level of:

Winter: Between 70% to 85% Sensible and between 63% and 75% Latent

Summer: Between 77% to 95% Sensible and 63% Latent.

Put that in perspective, the smallest system rated at 310m3/hr (.086m3/sec) will develop a yield of 85% Sensible and 75% Latent, recovering 3.5kW of heat that would normally be thrown away. We can further enhance the energy efficiency by controlling the operation with a CO2 sensor. At off peak occupation times the fan is switched off or reduced in speed. Calculating this effect is difficult but a good assumption would be a 30% saving in operating time of the fan. When you do not need the outdoor air to dilute the indoor air you do not run the fan, when you do, you get high efficiency operation which benefits the occupants and the facilities purse.

Price must also be a consideration. Typically a rotary enthalpy wheel system would be 40% more expensive than a conventional sensible square recovery unit. This increase will buy an average 150% increase in heat recovered and recycled. In our example above the increase would be in the region of 250%. The carbon foot print will be reduced, going towards the energy assessment criteria, not to mention the payback period which will be dramatically reduced.
I would suggest if you consider incorporating heat recovery units into your terminal system you should seriously consider upgrading to enthalpy wheels with CO2 control. It could be the difference between passing and failing the system energy assessment.

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Lesson 2

Air & water cooled products for refrigeration & air conditioning
By Roger Palamarczuk. dpac Ferroli - Sales Manager

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.
Chilled water temperature: Inlet 12°C Outlet 7°C Ambient temperature 35°C
Recovery water temperature: Inlet 40°C Outlet 45°C
Evaporator fouling factor: 0.44x10^-4m²°C/W Recovery fouling factor: 0.88x10^-4m²°C/W
Refrigerant: R134A Compressors: Twin Screw
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.

Ferroli RVH Air Cooled water Chillers with Helical Fans
Model	EER	SEER	SWL (dbA)	SPL (dBA)
RVH Basic	2.8	3.73	100	73
RVH AS Low Noise	2.66	3.66	95	67
RVH ASS Extra Low Noise	2.55	3.56	90	62
Energy Efficiency Ratio	EER
Seasonal Energy Efficiency Ratio		SEER
Sound Power Level			SWL
Sound Pressure Level at 5m free field				SPL

Every chiller is attached to a building that will have a need for potable hot water if not space heating or even process hot water. This is achieved by incorporating a refrigerant to water heat exchanger into the discharge of the compressor before the condenser coil. A diverting valve will direct the superheated refrigerant to the appropriate heat exchanger, and allow a degree of recovery. The recovered heat can be stored for use when required. This will amount to approx 30% of the cooling capacity at design conditions, and we can expect the low noise versions to better this by approx 3% for each version.

In perspective, there is 152kW of recoverable heat on a 511kW capacity chiller. There will be diversity for seasonal and occupational variation, but thermal storage systems will make that heat capacity available almost the whole year. There is an additional option to increase the leaving hot water temperature to 50oC, and is some of the smaller range it can exceed 60oC. Now we are in the realms of making the conventional boiler redundant.

Ferroli RVH 510.2 Air Cooled water Chillers with Helical Fans and De-super heater
Model	Cooling Capacity kW	Recovered Heat kW
RVH Basic	511	152
Ferroli RVH 510.2 Air Cooled water Chillers with Helical Fans and Total Recovery
RVH Basic	511	680

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.

Ferroli RVW Water Cooled Liquid Chillers
Model	EER	COP	SWL (dbA)	Recovered Heat
RVH Basic Cooling only	4.73	4.25	98.5
RVH Basic Cooling only with Soundproofing	4.73	4.25	94%
RVH Basic Cooling with De-super heater	4.73	4.25	19%	19%
RVH Basic Cooling with Total Recovery	4.73	4.25	113%	113%

These values do not take into account the type of heat rejection plant that would be adopted. The choice is between a water cooling tower and dry air cooler. From an energy perspective the tower is more efficient and takes up less space. Whichever is chosen the energy cost must be factored into the above efficiency levels.

There is a third option for both air and water cooled plant, and that is low temperature glycol systems. There is a further choice of Ethylene Glycol and Propylene Glycol, the main difference can be seen in the correction factor table below.

Correction Factors for Ethylene Glycol in Condensers
Percentage Glycol in Mass / Volume	0 / 0	10/8.9	20/18.1	30/27.7	40/37.5
Freezing point (°C)	0	-3.2	-8	-14	-22
Refrigerating power	1	0.995	0..985	0.975	0.97
Input Power	1	1.01	1.015	1.02	1.03
Water Flow Rate	1	1.038	1.062	1.091	1.127
Water Pressure Drop	1	1.026	1.051	1.077	1.103
Correction Factors for Propylene Glycol in Condensers
Percentage Glycol in Mass / Volume	0/0	10/9.6	20/19.4	30/29.4	40/39.6
Freezing point (°C)	0	-3.3	-7	-13	-21
Refrigerating power	1	0.99	0.975	0.965	0.955
Input Power	1	1.01	1.02	1.03	1.04
Water Flow Rate	1	1.018	1.032	1.053	1.082
Water Pressure Drop	1	1.026	1.051	1.077	1.103

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.