Click this Mollier diagram showing the proces of drying before
return air 4 and 3 ,
supply air 2-1-2-3
The Hemmes Ventilation-unit
Drying by cooling and heating is also known practice and gets a warm interest when the chiller is called a heatpump. However, the use of heat recovery is hardly known in this field.
Why drying the air?
For comfort ventilation this process offers you in summer the possibility to obtain room conditions according to the Fanger comfort area with the additional advantage of a better working adiabatic cooling when humidifying the return-air.
In the case of ventilating swimming pools fresh air is used to extract the moisture, however in some systems the return air is recirculated and dehumidified with the use of a heatpump and sometimes a heat- pump and heat-exchanger
ing. B.J.M. Hemmes has submitted a request for a patent on a ventilation-system in which the moisture level of the fresh air is reduced by a dryer consisting of a heat-exchanger and cooler and in which the temperature is controlled by a second heat-exchanger.
Secondly the patent request covers the process of dehumidication after heat-recovery. This configuration offers many advantages for the realisation of a true air-conditioning system.
In the configuration with drying after heat-recovery the use of a rotor (regenerative heat-recovery) is promising because of its advantages in summer and winter (pre-drying and moisture-recovery).
A higher efficiency of the used heat-exchangers causes a better working system and bypassing the heat-exchangers gives than the flexibility for meeting all the different demands.
The description hereafter is based on high efficiency (>90%) heat-exchangers.
For manufacturers who are used to work with crossflow heat-exchangers there is good news: you will also find remakable results with crossflow. For instance because one of the exchangers is working under condensing conditions.
· The high efficiency heat-exchangers may result in a long airhandling unit however with clever cross-sections we might fold it once or twice. Currently HRV units are made with these heatexchangers in the range of 200 to 8500 m3/h
· moisture in airhandling units is a undesirable situation because of the growth of human-threatning items.
The basis of the ventilation-unit described here consists of a first high efficiency heat-exchanger (plates counterflow) with a cooler taking care of dehumidifying the fresh air. Hereafter a second high-efficiency heat-exchanger interacts with the return airflow. When using heat-exchangers with an efficiency of >90% the supply air will have about the same temperature as the return air and a humidity as low as achieved in the cooler. In drier periods the fresh air can be bypassed directly into the second heat-exchanger.
A patent is pending for the system of two heat-exchangers (not necessarily high efficiency) and a cooler achieving a dehumidification before heat-recovery or a dehumidification after heat-recovery).
In the following the operation will be explained of the system of dehumidification before heat-recovery for both the state of drying and cooling (airco) and for the state of drying (ventilation of swimming-pools).
Fresh air is led through the heat-exchanger and giving sensibel and latent heat to the other flow, the condensing water vapour is giving the heat-exchanger a further increase in effeciency.
The cooler brings the saturated air to a lower temperature thus drying the air to the setpoint of the humidity. This cold and saturated but dried airflow is then led through the same heat-exchanger and will warm up while cooling the fresh air intake. A temperature efficiency of >90% gives the dried air almost the same temperature as the outside air but the airflow is as dry as the cooler has made it.
The result of this way of drying can be represented in the Mollier diagram by a line almost horizontally to the left (almost isotherm).
The air thus treated interacts with the return air from the room or building and the heat-exchanger can bring the supply air to almost the return-air temperature. The fan used can already be called a heater.
By partly bypassing this heat-exchanger the supply-air temperature can be brought to the desired level (freecooling with minimum supply temperature).
By humidifying the relatively dry return air an adiabatic cooling is obtained and when a indoor condition of 24 oC / 50% rel.hum. is considered the return air can be cooled to below 18 oC. The second heat-exchanger cools the dry but warm supply air in summer down to almost 18 oC.
By keeping the return airflow wet when it enters this heat-exchanger the temperature-efficiency can be improved.
It is possible to lower the temperature of the supply air by further cooling or humidifying, but a humidifier can also be placed in each room. Thus keeping the roomtemperature on the same level by evaporating water.
In this way the complete Fanger comfort-area can be crossed from bottom-left to upper right. Which gives the air an enormous cooling capacity.
When a room is not occupied, or has no significant heatload the return air will stay dry and this will enhance the central adiabatic cooling.
However when cooling is obtained only with a lower supply temperature the same room would be cold and the return air as well which results in a larger external heatload due to transmission.
Most indoor swimming pools will not need the cooling facility. In that case the ventilationunit is only used for demudification of the outside fresh air.
Suppose the indoor air is at design conditions of 31 OC and 14.3 gr/kg . Because of the German VDI using 9 grams as the maximum to calculate the amount of air needed, a drying capacity of 5.3 gr moist per kilo dry air is the result.
If the outside air is dried to 4.5 grams of moist per kg dry air with this ventilation unit, then a drying capacity of 9.8 grams is obtained and almost half the airflow is needed while the extracted enthalpy can be given to the waterbasin. When the outside conditions show a higher level of water vapour than 9 grams it is possible to maintain the inside climate.
The introduction of a cooler in the airhandling-unit of a swimming pool is advantageous when introducing heatpumps at the same time. And if the outside air is dry enough there is still the exhaust air after the second heat-exchanger to extract heat from. At lower outside temperatures the return air has already become saturated in this heat-exchanger and the further extraction of enthalpy will be efficient because of the higher heat-transfer-coefficient.
The heatpump can deliver this enthalpy to the swimming-water that has lost it by evaporating the water into the inside air. By throttling the cooler in the exhaust air a surplus of heat will not occur. This is after the use of fresh air, the second advantage of this ventilation principle compared to the ventilation systems that dehumidify the recirculated air with a heatpump (with or without a heat-exchanger around the cooler).
When the outside air is drier than the setpoint for the supply air the dehumidification section of the unit is bypassed and the pure heat-recovery situation with the second heat-exchanger is obtained. Temperature-efficiency can rise far above 90% due to the condensation in the return airflow.
Disadvantage of the plate-type heat-exchangers in winter is that there will be no moist-transfer to the supply air. However, in the case of storage of heat/cold in the soil (aquifer) it is possible to pre-heat the fresh air (charging cold) and humidify with water in this stage or just hereafter up to the freezing point. The second heat exchanger will take care of the (almost) complete afterheating.
Another possibility is to humidify the supply-air because the temperature might be high enough when using the high efficiency heat-exchangers and humidifying is in that case a better option than bypassing.
The regenerative heat-exchanger already mentioned (rotary or intermittent) can transfer moist from return to supply air and this heat-exchanger is very interesting to use in the situation of dehumidification after heat-recovery (summer pre-drying and drying and in winter moist-recovery and bypass of dryer section) and therefore less applicable for swimming pools.
· when condensing in the heat-exchanger and in the cooler the fresh air is “washed” (large wet surface) resulting in a reduction of dust, pollen, smell, Carbondioxide etc.
· thanks to the true air-conditioning in summer there is a room condition with a low temperature and humidity allowing for an efficient adiabatic cooling of the return air.
· when using the high efficiency heat-exchangers the low temperature obtained is transferred optimally to the supply air.
· the condense out of the first heat-exchanger and the cooler can be used for the adiabatic cooling process and only needs filtering and no softening. Only in dry hot periods some suppletion of water is needed (who doesn’t)
· The cooler can be fed with water from a cold aquifer or with an direct expansion refrigerant, and the cooler doesn’t need an expensive control because this cooling has no effect on the obtained supply temperature but only on the humidity.
· When supplying extra dry air a high cooling potential is obtained. This potential is released by evaporating water in the rooms with the large heat load. The rooms with a lower heat load can have the same temperature but will be drier. Thus there is no extra external heat load due to too low temperatures in rooms that are out of use.
· A relative humidity that is 30% lower allowes for about 1oC temperature rise for obtaining the same comfortlevel.
· A relative humidity that is 30% lower gives a greater cooling capacity than the 1oC temperature rise.
· The calculations of hours of exceeding the maximum temperature allowed should be followed by calculations of the comfort level (Fanger). The largest number of hours with an exceeding temperature disappears when the relative humidity is taken into consideration.
· The capacity of the cooler is “small” because a part of the dehumidification takes place in the first heat-exchanger and the cooler merely cools away the latent heat. The sensible heat is given back by the first heat-exchanger but cooled out in the second heat-exchanger.
· Because the air can be dried thoroughly without a drop in the supply temperature an enormous cooling capacity is possible with a small airflow. Thus giving the possibility to cool the present buildings in which the fresh air flow is kept at a minimum, and not having to invest in local cooling devices (ceilings, fancoil units, induction units etc).
· Because less cooling capacity is needed less condensorcapacity is needed as well. The condensor might be placed in the relatively cool exhaust air and with the surplus of condense a wet condensor will show much better COP’s . This condensor might work as an aftercooler in winter (heatpump).
· The full condensing cooler in the drying section has a high heat transfer-coefficient and will have thick fins or no fins at all. The evaporating temperatures of the refrigerant are normal to better compared to current systems.
· Expensive duct-isolation is not needed any more (small temperature difference and low dewpoints)
· This ventilationsystem is very suitable when using heat/coldstorage.
· where heat pumps(+boilers) are cooling down the exhaust air or outside air they can now be used to dry the supply air in summer and be used as an aftercooler for the exhaust air in winter (smaller though because of the high efficiency heat-exchangers).
· Supplying drier air reduces the air quality problems due to low ventilation and no infiltration.
· Only half of the normal (VDI) airflow is necessary with a smaller heat pump capacity with its consequences on investments and exploitation costs.
· The ventilation air flow will still be larger than what is needed for the swimmers.
· The dehumidification of outside air takes less capacity than the dehumidification of the inside air and the second heat-exchanger will bring the supply air to an acceptable temperature.
· the heat generated by the heatpump can be given to the water in the swimming basin where it is needed to compensate the evaporation.
· in addition the exhaust air can be cooled to generate extra heat and this will have no effect on the supply air temperature.
· at night a 100% fresh air intake is suggested instead of recirculation because the chemical processes in te water continue at night and fresh air will improove the indoor air quality.
· recirculation or reducing airflow: perhaps still needed in winter to prevent humiditiesfrom dropping too low .
· By short-circuiting the exhaust and the fresh air intake the unit works as a recirculation-drying-unit, and is already in use in this form (with crossflow-heat-exchangers).
Close control air-conditioning in computer rooms
Archives (beautiful separation of humidity control and temperature control)
Hotelairco's (alternative for the split-unit and the windowtype unit)
Wash dryers (a rewarded design of AEG has a heat pump but no heat-exchanger!)
Dryers for compressed air etcetera
There are manufacturers of the >90% counterflow heat-exchangers.
For example in Holland:
– Recair (HR Control ,material PS) picture
– Brink Luchtverwarming at Staphorst (material PE).
– Heatex Sweden
– Klingenburg Germany
These heat-exchangers are mostly used for the balanced ventilation in houses. But HRV units up to 8500 m3/h are also made by
– JE Storkair
The unit as described can be devoloped in the small edition using the DC-fans.
Larger air-handling units will demand extra effort in developing because of the modular (maximum 50cm high) heat-exchanger-size.
An air handling unit according to the Hemmes-system might resemble the present airhandlingunits but than with the airflow passing not the smallest side of it but the longest side. The length of the unit is linear to the number of heat exchangers. This “sideflow” will result in a low velocity in the unit.
It is inevitable that a manufacturer might choose the crossflowheat-exchangers to test the proposed system.
For example Menerga would only need to change the place of the fresh air intake and exhaust in the Thermocond-unit and they would obtain a unit conform the Hemmes system using 60% heat-exchangers.
But also the combination of a crossflow+cooler for drying and a rotor heatexchanger is a promising concept for the situation of drying after heatrecovery. An engagement of an Nautica dehumidifier and a Klingenburg Seco-wheel could lead to a happy marriage.
In this case the sorption wheel will have the great advantage of pre-drying the fresh air in summer and humidity regain in winter.