The textile mill segment now understands that by logical running a plant and by feeling the positive cross-ventilation effect in conditioned premises, they can comfort-process parameters to achieve around 10 per cent more productivity, says Ashok Senthuraman.
The textile mill now has increased production and production-related automation; so, heat rejection by machines is more and machines have less space in between to exhaust out hotter air. But they still run their old AC plants with obsolete practices without monitoring.
If the moisture is inserted gradually, in stages to the input raw material as cotton and to final product as yarn, say at the blow room, preparatory at the spinning especially and definitely at the post spinning at auto coners; then a mill can achieve higher productivity, less breaks, higher speeds at sustainable quality.
The industry has understood that the distribution losses to be reduced start from the poor functioning of nozzles as well as the pumping losses and simultaneously can improve the humidification effect immensely. Apart from the time-based preventive maintenance schedules for a humidification plant, its condition-based monitoring helps in total productive maintenance (TPM) in a practical way.
Existing operating conditions and symptoms
Kindly go through the following conditions of an AC plant and please try to match to the existing plant that has similar symptoms of working. If so, many of the symptoms can be corrected at no greater extra cost; one can visualise shortly that the humidification plant improves the productivity. These finer symptoms show how a mill treats its AC plant and the effects seen practically.
1. The plant is partly used, like switch off fans and pump for sake of power saving;
2. Water is hard, higher TDS (total dissolved solids), and sedimentation is more in pipes inside and outside;
3. Fans have fluff scaling at hub, blades, densely loaded with dust and fluff;
4. Conditioned premises RH (relative humidity) and temperature not sling-measured across the conditioned area;
5. RH and temperature are sensed at one stagnant spot, giving deviations within the conditioned premises;
6. Water in the sump is very chilled due to choking mist louvers, compared to ambient wet bulb;
7. Water pump discharge pressure is not measured and when measured, it is deviating from normal;
8. Sharp pipe elbows immediately at pump discharge to header and to vertical raiser pipes;
9. Suction and discharge piping mismatched to fluid velocity in pipes;
10. Rusted GI (galvansed iron) piping at the water circuit and need to change over to rigid PVC water pipings;
11. Half of nozzles water jetting, the tail nozzles are plugged, pissing or oozing;
12. Vertical raiser pipes are not tee-linked to each other and to tail end discharge header;
13. Water piping with high-friction head losses, heavily scaled inside and outside;
14. User conserves electricity by not running water pump, fan often and falsely satisfying;
15. Sharp bends instead of contour at supply plenum at masonry, at air ducts;
16. Positive cross-ventilation is not there in premises, but hot pockets existing;
17. Hotspots near equipments, motors are becoming hotter day by day;
18. The user thinks humidification plant is unwanted, but mandatory liability;
19. Dept temperature and RH is not maintained as per norms and the same to be publicly displayed and alerted;
20. Machines are modernised and run at higher speeds, but AC plant is not fully matched to use effectively.
*Nozzle pisses at 14 Hz* Nozzle jetting at 50 Hz. * Energy efficient nozzle increases Spray Dwell Time by swirl flow.
The closed loop pump discharge pressure control VFD (variable frequency drive) can be retrofitted to pump circuit to maintain the optimum head and delivery water pressure between 1.5 to 3 Kgsc band to suit to the plant operating conditions. Many mills are using VFD for namesake only and manually setting the Hz to set discharge pressure. The pump can only eject at 10Hz at 20 per cent loading and mills need to optimise setting in the mid range of head band in metres of water column as mentioned in pump nameplate.
Water online metering is a must for a humidification plant. This helps to assess the plant performance daily. When water circuit gets choked in the spray segment, this is shown in reduced water intake per day and we can take early steps to rectify it. Moreover, whether right amount of water to be mixed with air per hour as stipulated by manufacturer is daily followed or not, can be studied. Less water per hour consumed by plant today compared to yesterday means humidification efficiency is deteriorating day by day for the given load.
The sump water temperature in the spray chamber is the practical day-to-day indicator of Humidification plant efficiency versus Scaling and choking in the spray chamber. This shows the water circuit resistance in the air washer area and its choking status and the need to clean up the air washers. Daily monitoring of sump temperature will indicate plant correct working. The cooler the sump water temperature today compared to yesterday, the more is the choke in the distribution at the spray chamber.
For a humidification plant, water softening plant is the first priority. This is similar to giving cool dry air intake to the compressor instead of hot dried stale air, as this aggravates air compressor problems and losing energy. Frequent water analysis is a must before putting the water to use inside the equipments so that we know the TDS, pH, etc, parameters of incoming water and used water; this will ascertain the scaling effects in the wetted parts of the equipments and pipe lines, etc. Rainwater harvesting done in industry premises definitely helps to improve the condition of existing water to reduce its hardness, etc. Planting trees around the AC plant area and around the building helps to retain the moisture inside the mill.
The textile mill segment has an excellent landscape area compared to other industry segments. The same can be utilized, and one can go for rainwater harvesting pond / multiple recharge pits and ponds. Some textile mills have gone for 50 lakh litre ponds which is just a collection of rainwater inside the premises and predominantly the roofwater collected and brought to the pond. This pond serves the mill for the whole year with excellent rain water with the lowest TDS to the mill AC plant, and thus helps a mill to sustain throughout the year with no need of RO / external water resource.
Water circuit: Ring main distribution to adopt now
The existing plant water circuit distribution through nozzles is typical of a radial system. If we can think of ring main distribution as switched over by all the compressed air user industry from radial distribution, we can employ similar logic to the water circuit here. For a given load, in the CA ring main system, the air can come from right side or from left side of supply and ultimately the load is starved of air supply. We could ensure the right amount of volume and pressure of air in this ring main than the radial and as well we could reduce the header pressure; this is energy saving too. Similarly, the pressure and flow at the nozzle end must be constant to achieve steady humidification efficiency. So we see to it that the nozzle gets water at correct flow and pressure either from top or bottom of the raiser pipe. This reduces water circuit pressure, energy saving is possible, and steady functioning of nozzle too is possible at constant efficiency.
Let us consider the pump discharge circuit consists of one main header and four sub headers are tee - branched off from the main header. In those horizontal sub headers, the vertical raiser has many nozzles clamped on to it. The problem is that, of the four sub-headers, after few days, half of the nozzles in the given raiser pipe are choked and not working. The plant operator does not open to see daily whether all the sub headers and the corresponding nozzles working.
* Tee-type linking of vertical Raiser pipes * Replace Monobloc pump with Open well SUB. Pump * Plant Trees around AC plant
Alternatively, to circumvent this problem and elongate choking intervals in water circuit, we provide additional piping arrangements from the main header to each of the raiser pipe from the top. So, each nozzle will get the adequate flow with pressure either from the top or bottom of the raiser piping. This modification is proposed to the existing humidification plants keeping in mind existing operating practices. This modification brings in round-the-clock steady pressure to the nozzles and the existing pressure gradient, i.e. the pressure drop comes down automatically. Here the water-air mist cone after the nozzle must spread, stay in the air more, with optimum breadth and length. With this modification, the tail end nozzles neither starve and ooze piss out water; nor first sub header nozzles jet out with higher pressure. We have to bear in mind that unlike electrical distribution wherein the load ends are active devices, which grab the energy from distribution; here the nozzles are passive load end devices in water circuit. The nozzles are to be fed at the rated pressure and flow and this will maximise the mist efficiency.
Thinking macro at the breakup of T&D losses for any system like electricity, water, air, gas, fuel oil distribution in the industry, the losses are more and varying in the distribution, whereas the losses in transmission are fixed and relatively constant. Hence, always keep a frequent-lens vision on distribution circuit compared to transmission circuit, and that is what a clever industry does. Here, in water circuit transmission losses account upto the nozzles and the distribution losses start from the nozzle.
Now, the industry pumps with higher discharge pressure, more water to spray chamber in order to achieve better RH. This leads to short circuiting of water from pump to chamber to and fro only, and the portion of humid air going inside the duct goes less only. From the energy saving angle, if the water pressure drop in the pump discharge circuit is minimised, then this nozzle-rated-pressure warrants only less power from the pump. We see to it that the discharge header pressure at the nozzle end must match the nozzle-rated pressure for its maximum efficiency. This is the focus area in the plant now.
Industries have circumvented this problem and now adopt less number of swirling jet nozzles instead of conventional nozzles. Also, they optimise pump delivery to match nozzle-rated pressure. This critical change ensures uniform and constant distribution of whirl-water. This resuls in a changed size of air-water mist spray to dwell longer in the air throw area in the chamber. Finally, the conditioned area gets quickly the requisite RH and temperature.
Do we measure humidification plant efficiency?
Condition-based monitoring of the plant is done from outside and it is not possible for the operator to measure the operating parameters of the plant by opening the plant door frequently and daily. So, we have to fix remote monitoring instruments and audio-visual alarms outside the plant to monitor the operating parameters of the plant frequently (the operator need not go inside). Either the instruments can be relocated outside or at least the provision and impulse piping are brought out of the plant room with end fittings for intermittent fixing and measuring of parameters.
The plant can be measured at the water and air circuits. The industry thought earlier that we need to monitor essential parameters that are prone to breakdown. But now to improve efficiency of the auxiliary, the industry wonders what else can be measured to get the maximum output from the sub-systems.
The water pressures that can be measured in the plant are:
1. Water pump discharge pressure in Kgsc;
2. Water spray header pressure at the tail end of the last sub-header;
3. Hour meters for pump, fan as required to study seasonal changes in premises.
The pump discharge pressure gives an indication of the pumping status and the pumping range is within the normal limits of operation. The tail end pressure shows what is the choked-up pressure in the sub-headers after few weeks of operation. During commissioning the plant itself, the industry must do the trial of choking quarter, half, three-quarter and full nozzles and study the pump discharge pressures readings. This helps us to compare the system pressure in the normal running of the plant.
Pressure gauges are to be fixed outside the plant wall. The impulse lines to the gauges can be flexible hosing-type or diaphragm-type sensor gauges. The range is selected to match discharge pressure of pump along with factor of safety. This is to measure the water pressure at pump discharge and at the tail end nozzles. Provision can be made in the AC plant to measure the air circuit resistance across HP fans, filters. We can measure and record the actual delta pressure in mmWC (millimeters for water column) / Pascals drop during commissioning or routine cleaning maintenance schedules and we can follow the same after few months' operation. Fix U tube manometer / magnehelic gauge to daily measure at each fan and filter. The user needs to average automatically, the readings from the 4 no temperature and RH. Sensors are to be located in the conditioned area and the response will be fast and accurate and repetitively reliable.
Pumping system: Water and power savings
The AC plant OEMs (original equipment manufacturers) offered only monobloc pumps till date. Now, the trend has changed and the user must go for open-well sub-pump instead of this monoblack, and this gives around 50 per cent power savings straight away due to the NPSH on the suction side and improved sub-pump efficiency to match to the pump delivery pressure of say 2 Kgsc and a band of 1.5-3 Kgsc, which is the BOP (best operating pressure) for both the pump and nozzle flow to ultimately deliver above 95 per cent RH at mist eliminator area.
The above listed are the possibilities mentioned in the existing pumps from old star rating to 5 star ratings in the pumps as found in PCRA (Petroleum Conservation Research Association) bulletins. The Indian farmer has switched over from GI piping to rigid PVC piping. Here too, when we switch over, the friction losses reduce, and consequently the pressure drop is reduced. The line sizes are matched to the pump system curve so that the high discharge pressure as well the heavy pressure drop at the tail end is avoided.
Humidification: Air circuit
The industry has understood now that by running HP (humidification plant) scientifically and practically by feedback sensing and control, they can improve production. To be precise, if we concentrate at the spray dwell time of humidified air in the spray chamber, then we can improve the performance and this definitely gives a boost in product output apart from improved in quality.
Usage of high efficiency pressure die-casted aluminum fan blades, GRP/ FRP fan blades (properly weight-sized to the air throw and cfm specs and installed correctly) consume less power input for a given air output. The thrust point is compared to metal fans, the weight of the blade is concentrated at the centre than at the ends and by flow curves. This gives more air power with less weight, hence with less power.
Here again, care is taken to avoid or remove masonry sharp bends in the chamber and in the duct for smooth flow inside premises. This is what is concentrated in vaastu as free flow of air and no obstacle in the air path, the bends to guide the air smoothly in change of direction but not block and detour the air path. In some plants, we find the supply air fan blasts the air against a sharp and blind wall, then take 90o turn to diffuse turbulently thro supply ducts. The velocity of air is reduced, and is fluctuating then and there itself.
Always bear in mind that the break-up of the T&D losses for any energy medium input, the distribution of air-thro diffuser is the focus area. As the industry has understood now that lighting levels in the given area must first satisfy visual tasking of the workmen than the general lighting. So, industry has reduced the height of the luminaries to focus the lighting on the work area.
Likewise, industry has to drop down the supply diffuser so that the air is diffused to the focus work area and hot pockets are avoided at the work area. Here, we must not see the aesthetics of dropping the supply diffuser. In fact, the ideal alternative is to bring down the false ceiling to the minimum possible convenient height. This helps in minimum buffer space in between, and the plant conditioning is faster now due to frequent air changes.
Ultimately, the premises should be kept at positive cross-ventilation air currents. Care is taken to avoid hot pockets especially at the user and equipment area. Existing conditions indicate either the fans are switched off to save power or the exhaust trenches are not cleared off dust. These result in suffocation of equipment, localised heat buildup, and the product suffers due to the harsh environment. These parameters are essential for machine health.
Humidification plant and its load
The load to the plant is mainly due to the equipment, say three-fourths of the load and the balance quarter load is due to ambient dry weather conditions. We have to reduce the load on the plant, and to make conditioned area more comfortable. It is very much rupee-wise and paisa-unwise move to reduce the above-ceiling temperature and is easy too.
Now we are facing solar heat, entering the premises from the top and here we install false ceilings to suppress the heat further. What is happening to us, for example, is that we feel the comfort differ in three ways as when we stand under the sun, walk under the sun, go by two-wheelers under the sun. The impact is least felt under the sun when we go by two-wheeler because of the breeze.
Similarly, the roof or the attic is ventilated by force like the lateral high volume low pressure exhaust fans or by self-propelled roof extractors. This brings down the under roof temperature by say 5oC. The existing attic hot stale air is forced out and heat is not felt in false ceiling as well in supply ducts. The industry has insulating the ducts in the attic area, alternatively replacing with cylindrical FRP ducts to avoid frictional losses and reduce chokes in the ductings.
Generally, the roof heating by solar radiation causes 50 per cent of the heat load in any premises. Though we have gone for false ceiling to avoid the same, still the plant needs to be insulated from the climate extremes of day and night and that of summer and winter. What we suggest is that two-stage insulation at an affordable cost will improve the ambient. This is an alternative only, a cost-effective method of primary cooling.
There are other methods like green housing, pucca comfortile type weatherproof tiling, roof extractors, lateral ventilators, acrylic based reflective coating, or an eco-friendly idea of growing tall trees around conditioned premises to provide shade before and after noon. This can be done above AC plant chamber, MCC, compressor house and wherever open terrace on RCC roof. This primary insulation is meant to avoid the solar heat load. In the secondary insulation, i.e. under deck insulation, the attic fan definitely helps to remove the stale hot air under ceiling and insulate false ceiling from solar heat load. This method of attic ventilation and cross-ventilation inside premises is low-cost energy saving type compared to high-cost cooling of equipment and premises.
Remember: A few degree temperature drop of air inside duct at the start; and at the outside duct due to attic ventilation is easy to achieve and consumes less power. This is less compared to the power required to temperature reduction inside the premises
Industry must give importance to the humidification auxiliary right from purchase of good quality plant and user-friendly maintenance schedules. Industry needs to rope in the manufacturer even after the supply and warranty of the plant. Industry and the vendor have to work as a team to sustain and improve energy efficiency of the plant. Even in the cold climate outside the mill, the temperature and RH inside the mill would be harsh and stale now, due to heavy heat rejection owing to machines and the processes involved in the spinning area. So, instead of running the AC plant as a passive utility and running only for namesake, please thrust your focus to run the plant actively. A healthy AC plant not only comforts the spinning process and machines, but can be turned into a catalyst to improve yarn production by way of speed, quality and weight of yarn.
We have to frequently ask the manufacturer what best can be done on his equipment often to improve plant efficiency. This is not only for a humid plant but for other equipment too; the industry has to coordinate with their existing vendors to improve the product's energy efficiency.
Now many members of industry understand that humidification is a tool to improve productivity. This utility is under our control, the parameters are defined and they point to the user a road to better productivity. They even run the mild mist fogging type cooling utility in spinning areas to maintain the RH around 55-60 per cent.
In the field of medicine, the old saying by the experts say, "Shake the bottle before use and shake the body after use for better results." The same concept can applied to the industry in that humidification is like medicine for the industry. Instead of running the plant constantly, it is better to run the plant practically based on the feedback parameters from the premises.
Now, industry needs to go in for variable speed drives for fans and the same will function on the basis of MP-based automation of premises' parameters. One humble thinking now is that the first and foremost pre-requisite for the plant is to a healthy running of the plant in air and water circuits. If the water circuit is healthy, then the air circuit can be further reduced and optimised to maintain the RH and temperature. Then, we have to think of fine-tuning of process parameters, and later retrofitting of energy-saving gadgets to the process that will definitely achieve better results in the overall plant productivity.
About the author:
Ashok. S is a BEE Empanelled & Accredited Energy Auditor from Coimbatore and the Chairman of IAEMP Coimbatore chapter, India.