Wet & dry air receivers in compressor house can save energy

The industry can focus on compressed air treatment to save on energy, says Ashok S.

After compressed air generation in the compressor house, it is time for industry to look at compressed air for smooth, steady, non-pulsating cool, dry air as feed to pneumatic actuated loads. This will save energy.

Considering compressed air generation system in the compressor house as a whole, the inefficiency in each sub-system adds to pull down overall efficiency of compressed air within a few months of installation. So we need to improve compressed air energy loss parameters like pressure drops and temperature rise in the compressed air header before it leaves the compressor house.

Why do we need two receivers after compressor and dryer?

Benefits of the wet control air receiver:

The wet receiver should to be directly connected to the compressor, post air cooler with no water separator/filter in between.

1. Damping pulsations caused by the load and unload pressures in the compressor kilo watt zigzag chart can be avoided.

2. Providing location for free water to settle at the bottom of the receiver, so it can be removed by an auto drain valve.

3. Reducing load/unload or start/stop cycle frequencies, to help compressors run more efficiently.

4. Slowing system pressure changes, to allow better compressor control and more stable system and header pressures.

5. Performing the above function, the receiver smoothens the harsh compressor loading pattern, improving the health of compressor systems and avoiding breakdown inside the hood.

6. This wet receiver acts as a buffer to post air cooler, aiding its slow and steady heat transfer efficiently.

7. Once the temperature is reduced near the ambient of the wet receiver, the downstream dryer will not de-rate more.

Benefits of dry demand air receiver:

1. The dry receiver will act as a demand buffer between the load demands and compressor delivery pressure.

2. The load unload pressure control sensor should to be fixed in the dry receiver as this will take the user demand directly.

3. The dryer functioning will improve as the dry receiver acts as a buffer to flatten the process demand spikes.

4. The system is to provide general pressure stability in systems with undersized or tee connections in distribution piping.

5. The refrigerated dryer thereby improves, allowing the steady drying with minimum pressure drop across the dryer.

6. This ensures the safe working of the dryer, even catering to the highly fluctuating downstream loads.

What is the need for relocation of water separator after wet air receiver?

The above figure explains how water separator plus AO filter works better in the wet receiver and not when placed between the compressor and receiver. The compressor Original Equipment Manufacturer (OEM) gives the same in the package of compressor, post air cooler, WS and the integral refrigerated dryer. This all-in-one package sometimes becomes inefficient gradually due to poor workings of compressor.

So, it is better to physically isolate the post air cooler from the compressor, shift the WS from entry point of wet receiver to the exit of the receiver and place it before the refrigerated dryer for healthy working of the sub systems. The discharged air, due to the compression by compressor, immediately gets cooled, instantly at the post air cooler. Here, the wet receiver begins its work after the post cooler will act as a buffer and allow the discharge to stabilise before entering WS. Presently, the water separator's location is just outside the post air cooler or after the hood, and is prone to choke often inducing artificial pressure drops.

Let us study the compressed air case study given by Energy Efficiency Best Practices by Raymonds, India in 2015. The same is mentioned in Workshop on Knowledge Exchange Platform site.

This company had a battery of six compressors, all of them linked to a common header and the header is feeding one main air receiver. The layout is such that the six micro filters are all positioned in each compressor discharge line before entering the header. Here, they achieved a lot of savings in terms of energy due to the uniform loading of six compressors because they relocated the six microfilters from their existing location to the new location after the air receiver outlet header. From there, compressed air was fed to the user department. Here too, instead of putting multiple small filters in each sub-header say six in number, put one big duplex and over-sized filter as one main and the other as standby sized to 50 per cent more-than-rated cfm (cubic feet per minute) of compressors. One big sized filter instead of multiple small filters gives minimum pressure drop and the operating efficiency is better. The oversized filter or the refrigerated dryer at the time of installation of compressors leads to minimum pressure drops in compressor house.

Is the post air cooler working alright inside the compressor hood?

The post air cooler functioning is important for the health and safety of the compressor. It reduces the load and pressure drop across the refrigerated dryer. In a 160 KW 1000 cfm (cubic feet per minute) rated screw compressor, around 1000 liters of water are sucked inside over 48 hours running weekly. Out of this, the post air cooler removes 68 per cent of water, say 735 liters at 35oC. The percentage values may vary from place to place but the percentage removal of say around 70 per cent removal is done by the air cooler. But inside a compressor hood, the Heat Exchanger (HX) poorly transfers the heat as it sucks only the hotter air from the compressor sub systems inside the hood.

The compressor OEM isused guidelines to discuss this aspect to decide about laterally shifting the heat exchanger position say by 200 to 300 mm away from the compressor hood and connect to the other subsystems by extended piping. By this, we see to it that we provide abundant ambient air supply surrounding the compressor hood to effect better heat transfer and heat exchange at the air cooler.

Water removal takes place each week from 500 l/sec (liter/sec) of 7 bar(g) air. Since ambient air contains 12.5 g of water vapour for each m3 of free saturated air at 15oC, a 500 l/s (1,000 cfm) compressor will inhale 1,080 litres of water vapour per 48-hour week. This quantity is still in the air at terminal pressure and discharge temperature. The vapour will begin to condense as air temperature is cooled to, or below, that of the pipe downstream of the compressor.

What does the compressor OEM need to guide the user in the installation?

Practically we understand that the compressor OEM has to ship the compressor-in-hood as package. But the OEM can instruct the site team or a knowledgeable buyer and user to partially knock down the sub systems and protrude the air intake filter, keep out the post air cooler HX, remotely shift the pressure control sensor to the dry receiver so that the compressor obeys to the process that pressure needs through the pressure commands. The compressor can suck cool dry outer air in the compressor house instead of starving on hot harsh air inside.

1. The post air cooler inhales and exhales better, outside the hood instead of poor heat transfer inside.

2. The remote pressure sensing gives steady process air pressure input to the compressor load and unload, logic and not pulsating pressures as measured now, since it is now inside the compressor hood before and then after cooler.

3. In fact, stand alone post air cooler will be more efficient than an inbuilt post air cooler inside the dryer. This leads to ease the maintenance, monitor routinely the pressure drop and the temperature drop across the air cooler. Efficiency is visualised with higher temperature drop and lower pressure drop across the post air cooler.

The industry thinks that they are losing energy in their compressor KW/CFM. They can concentrate on the compressed air treatment, at the compressor air intake pre-filter to the existing filter and at the compressor delivery air. Let us revisit the basics of compressor and air treatment that suit this condition monitored approach towards energy saving, routinely. Give attention to the OEM guidelines to each of the compressor sub systems and to the compressed air treatment, update to enhance energy efficiency in the existing system that is bought few years back.

Reference:

1. Knowledgeplatform.in Energy Efficiency Best Practices by Raymonds, India in the year 2015.

2. Compressed Air Energy Efficiency Reference Guide by (CEATI) Customer Energy Solutions Interest Group

3. Air filter OEM 'Parker' compressed air treatment

4. Good Practice Guide 216 - Energy saving in Drying of compressed Air