Textile mills need to give priority to compressed air savings

Textile mills are aware that of the total electricity units consumed per day towards compressed air, they are losing more than 30 per cent to compressed air leakage.

Energy savings are a major issue in the functioning cycle of a textile mill. In the total electricity units consumed per day (UPD), the mill can reduce only around five to 10 per cent of UPD after energy audit and implementation. Compressed air leakage is a regular hidden cost in a mill. If not identified right away, it aggravates losses.

When a mill goes for modernisation, automated production demands more compressed air. So, instead of arresting existing air leakage, mills often buy more compressors to satisfy production demands. Ironically, leakage increases.

Why are there more losses in the compressed air system now?

The root cause is accumulation of water and dirt in compressed air pipe lines. This makes pneumatic fittings weak and leaky. A pneumatic fitting and component has a life. Beyond its life, leaks start. Soon, a drizzle turns into a shower.

The compressor post air cooler, refrigerated dryer and the zero air loss drain valves fixed in the air receivers at the compressor house, tail end and feed end receiver (Fig 1) are three sub-systems that have to work perfectly so that no water is trapped in the distribution and the load ends. More importantly, water ingress in the air actuation elements and solenoid valves in the machine will spoil their health.

Where is the loss happening in compressed air generation?

Compressed air generation is gets compromised if a compressor takes 5 to 10oC above the surrounding ambient temperature through choked air intake filters. This is two per cent KWH loss. Cool, dry compressors deliver more air output. So, try to give cool, dry air to the compressor air inlet as shown in Fig 5. Keep the load/unload settings to the minimum bar pressure just 0.7 bar, or 10 per cent above minimum required pressure demand from equipments.

Fix one pressure gauge just after the compressor and another gauge at the compressor house air delivery line/receiver. Fix a temperature gauge on the air receiver and keep its temperature 5oC above the ambient, which is a sign of unhealthy compressor package.

Fig 1. Invest in air receiver load feed/tail end model

Many mills have more pressure drop around 1 to 2 Bar in the compressor house. First, bring down the pressure drop to less than 0.7 Bar. We provide an air receiver in the compressor house to arrest the compressed air pulsation and fluctuation due to frequent loading and unloading of compressor.

Fig 2. Pressure sensor located inside existing compressor hood

Fig 3. Fluctuations in KW/KVA in every minute trend graph

The above images show that the location of pressure sensor (Fig 2) matters to us since this aggravates the cyclic fluctuation of load and unload (Fig 3). This can be solved by shifting the compressed air pressure sensor from the hood to the receiver. Many industries achieved savings and safety after shifting this pressure detection and control sensor to air receiver. Other encouraging option is to expand the compressor hot air exhaust so that the air and oil sub systems within the hood work comfortably.

How do you account for the compressed air losses in distribution?

Analogous to the other Transmission & Distribution (T&D) losses, compressed air lost in transmission is less up to the sub-header, compared to the air distribution loss that is more in the machine. The Flanges and O-ring gaskets, fixed in the main header pipe lines, are prone to leak more. To study this and correct the same, provide intermediate or feed air receiver to each of the air consuming department. This will help the mill measure separately, the air losses in the transmission (compressor and header) and the losses in the distribution (sub-header and equipment in the department.)

The air receiver ought to have pressure gauge and isolation ball valve, preferably solenoid operated, and receiver water drain valve system. The Mill Project team always reduces its budget to invest in these sub-systems, but the mill's compressed air running cost goes up because of the absence of these field sub-systems useful to condition and monitor the mill, air daily demand and supply air flow at rated pressure with minimum allowable pressure drops. 

Are we seeing compressed air losses in machine usage?

Many mills operate at 7 to 8 Bar band in load and unload settings. However, only 6 to 6.5 Bar is enough for the load and unload pressure settings. Invest in air bottles to each of the compressed air consuming machines. Many mills fix a digital air pressure gauge (it is the machine OEM initiative) and give only minimum pressure required for the machine.

Say, the autoconers can be retrofitted with three inch PPR pipe under the total machine length and this will give 200 litres buffer tank. All the control and working pressure tapping can be taken from this retrofit pipe. Mills think of pressure boosters to satisfy higher pressure demands of machine OEMs. Discuss with OEM how to reduce the machine operating pressure with spare air bottle as buffer.

How to prioritise compressed air losses in leakage

Losses are happening in the compressed air generation, distribution in the usage. In the usage part, the leakage in the machine is more than the machine usage now. Hence the question of arresting the leakage is becoming a priority now. What is the leakage in the machine during OFF time? Measure this first to arrest next.

This is similar to our domestic 1000 litre overhead tank where, 100 litre /hr or (%) water enters the from the top of the tank. 50 litre /hr or (%) is drained silently through the bottom drain plug. And the balance 50 litre /hr (%) only is used for the house. Like that, some mills record more than 50 per cent compressed air leakage.

By arresting leakage losses to the minimum, mills stand to gain now. Returns on investment are around six months only. But keep in mind, no mill can achieve zero leakage. So, the aim should be to contain air leakage to the minimum possible per cent.

How the air consuming machine oem & compressor oems can help now?

The machine OEMS in the other segments have realised the preciousness of compressed air. Please try to keep OUT, the air sub header, FRL, etc input components visible to the user daily. Do not keep the inlet air header under closed door but it must stand out of the machine for easy and timely attendance of air input problems.

Provide a buffer bottle or pipe line so that air supply to the machine individual pneumatic actuation mechanism is not restricted or starved of air. Always give open access to the worker so that he can daily remove the water collected at the machine end water drain valve at the bottom most point. If possible, provide digital air pressure gauge to the machine inlet. Some mills are already planning to put Rotameter type or digital type air flow instant metering.

Provide air intake Delta Pressure gauge outside the compressor hood so that each mill can around 2 per cent KWH on condition monitoring the same. Kindly educate the air consumer on how to keep compressed air useful to the pneumatic machine health.

When installing the machine, give the consumer the best installation possible. The compressor and compressed air inefficiency starts from compressor house. Allow no Tee but only Y pipe connections in the house.

Why is the installation of the compressed air system not professional?

Fig 4. Do's & Don'ts of compressor hot air exhaust ducting

During my energy audit of more than a few hundred industries, I noticed that eight of 10 mills have fixed same size duct to match mouth size, but of longer lengths (Fig 4). The damage is done to the machine right from the installation! They ought to provide the duct double the mouth size so that hot air from the compressor hood may go out. That one-time expense enables the compressor breathe better forever. 

Mills buy the latest and efficient compressors or machines. They do not plan to mark a tiny fraction of budget cost towards installation and never give time to commission the same professionally.

I request users to revisit their compressor installation manuals and commissioning reports and check for non-conformance. The mill must have a commissioning report of what kw/cfm has ben achieved as reported by the OEM. Then, the mill must do an FAD test twice a year using the pump-up capacity method.

Many mills are forced to change their compressor air end elements - which cost half of what the compressor costs -- within their life period prematurely because of poor installation and because the post air cooler is getting choked. The latter is bound to happen due to textile micro fluff floating. This situation can only improve if the machine HX suction is oriented towards the outer ambient. If the HX sucks only the machine's hot air, then it functions poorly.

Fig 5 Pre-filter to existing air intake filter and its extension pipe to fresh air entry

Does compressed air metering help?

As energy professionals, we are aware that efficiency of compressed air generation is poor. So, first install a multi-function meter to know daily kWh of compressor during loading, unloading and total units. The relative condition monitoring of the machine input KWH will show the relative compressor consumption deviations.

It will be more accurate to provide compressed air flow meter in the compressed air main header. This will prompt the user to know what each compressor gives per kWh input power, say 3 to 6 cfm. When the OEM says it is 6 cfm/kW, the mill user finds it only around 3 to 5 cfm/kW.

This input kW/output cfm ratio will encourage the mill user to find out healthy parameters in each compressor. The user can also close and open the intermediate air receiver valves in each department to know consumption in each individual department. 

So, header flow metering will give a macro view to the management about the efficacy and per cent compressor costing in the mill power break up in units/day. The portable digital flow meter can be used in each machine to routinely measure machine air consumption at the time of switching on and off.

What are zero- and low-cost ways to reduce compressed air consumption?

Give priority to arresting leakage. Outsourced today, leakage detection and correction exercise will give results in three months. It would be a wise decision.

Many mills have stopped one of three running compressors after arresting leakage. When you can hear the compressed air leak in any compressed air pipe line or machine, it means one third, or about 33 per cent of air flow in that airline is leaking. First, arrest audible leaks. Then use an Ultrasonic Leakage Detector to arrest minute leaks.

BEE says, any machine above 10 HP, operated for more than 6,000 hours a year i.e. two shifts a day, needs to fix a kWh meter to assess the machine's productive health. Here we can measure both compressor's air power and tare power now.

Investing in an air receiver can stall other retrofits. These include comforting the compressor and zero air loss compressed air drain valves with pre-fixed Y-type strainer cleanable daily on-line to save the main drain valve. They also include adding the daily cleanable pre-filter to the existing air intake filter, positive cross ventilation inside the compressor hood, load and unload pressure sensor shifting to air receiver and many finer points of daily monitoring that bring down compressed air energy consumption by one third. 

Courtesy : 
S. Ashok, BEE Accredited Energy Auditor