1.1 How to determine the rated pressure of air compressor?

The running pressure of air compressor is determined by the resistance of equipments, duct between the environment inlet and outlet.

The rated pressure is based on the estimate with some margin.

Power consumption is directly related to the pressure, so calculation of every segment and item should be careful.

1.2 How to determine the configuration of an air compressor?

Large scale fermentation usually uses centrifugal air compressor, while small to mid scale use piston machine and low pressure screw machine.

Although in terms of compression efficiency, multistage is more power saving than single-stage compression as for the same compression ratio, but take account of the overall investment, operation cost of the air pretreatment system, it is more economic to choose single-staged compression.

Single-stage compression has a high outlet temperature, these heat could be recycled in the air pretreatment system. So there is no need to buy post air cooler when purchasing air compressors.

1.3 How much shaft work is taken up due to the temperature raise of the air compressor?

In view of a piston compressor, cycle work calculation of a one-time compression is as follows:
a、isothermal compression cycle work

b、isentropic compression cycle work

among them:p1、p2——inlet/outlet pressure,V1——intake air volume

Assuming that, the gauge pressure after compression is 0.2Mpa, P2/P1≈3 air entropy index k=1.4,
Then L1≈1.1P1V1,L2≈1.29 P1V1,L2≈1.2L1. Isentropic compression has 20% much more work than isothermal compression. Actually compression is a polytropic process, the index k is less than 1.4, L2<1.2L1. That is to say, in the real shaft work consumption, more than 80% is due to the pressure raise, less 20% counts for temperature rise.
As for centrifugal compressor, there are similar formulas, but the polytropic exponent is larger than the isentropic exponent, which is generally between 1.4 and 1.6. As assumed above, in actual compression, up to 36% shaft work is consumed for the temperature rise.

2.1 How to determine the cool air temperature in the fermentation air pretreatment system?

Air cooling temperature is determined by air temperature after heating. Generally the fermentation temperature is about 30℃, and about 40℃~50℃ after heating; due to the saturated moist air the temperature will rise by around 15℃ under the same pressure, and relative humidity is below 50%; considering factors of temperature decrease of main air duct, air moist mix(supersaturated) and so on, the actual heating aspray bymplitude is between 20℃~25℃, therefore the deducted cooling temperature is between 15℃~30℃. Real cooling temperature has to take account of fermentation product, process and cooling liquid and other factors.

Low cooling temperature is not what we pursue, since the lower the temperature, the more cooling capacity required, then the higher the operation cost; Besides, the lower the cooling temperature, the lower the absolute humidity, air will be in saturation when misted with fermentation liquid under the same fermentation temperature, the more moisture it will take away, so the liquid level will drop, this counterproductive fact is obvious in long-cycle fermentation. Therefore, in the premise of satisfying membrane filter requirement and fermentation process standards, air cooling temperature should be controlled in a proper range.

2.2 The more water collected the better for the air cooler? Why does the liquid yield change all the time?

Cooler water yield is associated with air temperature after cooling, and inlet air temperature, relative humidity and air pressure before and after the compression. The latter conditions can determine pressure dew point. When cooling temperature is lower than dew point, the condensate comes into being. Assuming cooling temperature stays still, when the climate is dry or air pressure (after compression, similarly hereinafter) is low, pressure dew point is low, condensate is less; When air humidity is high, or the air pressure is high, pressure dew point is high, condensate is much more.

Therefore, condensed water yield does not directly reflect the cooler working state. Under a certain air condition (pressure dew point keep still), the lower the cooling temperature, the more the water condensed, however the higher the operation cost.

It is important to note that during the winter, moisture condensed before reaching the air cooler, pipelines, storage tanks could have condensed water precipitation. It is normal that condensed water yield drops.

2.3 Why there is nearly no leakage of tubes in the new type heat exchanger?

Manufacturing issues such as manufacturing defects, welding problems (air traps, slag and other), plus heat stress are possible factors leading to leakage. But the new type heat exchanger is assembled by high quality 304 stainless steel, which has a good performance against pressure, temperature and corrosion. The flow speed in the shell is low, and tubes are in a vertical arrangement with small length, so the heat stress is small, the equipment is strong, robust and safe.

In comparison with chemical reactor, the temperature (below 200 ℃), pressure (below 0.35 Mpa) of the fermentation system is mild with little fluctuation, also there is no failure of vibration and fatigue involved. Therefore, in the premise of accurate design, conforming manufacturing and test and correct operations, the heat exchanger can service reliably without leakage in a long term.

2.4 Why sometimes the outlet water of horizontal gas-liquid separator seems not much?

This phenomena is credited by several factors as follows:
2.4.1 The moisture amount upstream the exchanger is small
Air temperature after cooling is above the dew point, or near around, the total condensation is not obvious.
Moreover, most of the condensation are released in the water outlet of the cooler, only a small amount of moisture goes into the separator.。

2.4.2 Drain of the separator
There are two forms of drain in the bottom of the separator, drain constantly and periodically. If the water release is small with a constant drain, due to pressure difference inside and out, when the high pressure in the drain tube drops with a great gradient, gasification happens with little visible water out; if using periodical drain ( could use automation valve), liquid water are visible.

2.4.3 Climate factors
The separator gives away little water or even no water when the environment is dry; In hot weather, there is temperature difference between cooler air outlet and separator air inlet, the air is heated and the droplets are gasified again, so there is little water in the outlet. If the cooler outlet and separator inlet are arranged with a long distance apart, the gasification is very obvious, just like the mog dissipate quickly in the sunshine.

2.5 Why it is better to use hot air out of the air compressor as the heat source than steam vapor, hot liquid and other sources?

The humid air needs to be heated up after dewatering, to reduce its relative humidity. Traditional heat source is steam, or we can use hot air, hot water for the sake of energy saving. The comparison of the several heat sources are tabled below

[table style=”simple”]

Steam vapor Hot hair Hot water
Sources Boiler Air compressor of SAS Residual heat of SAS or other
Convective heat transfer coefficient High Medium low medium
Device footprint Small Big Medium small
Investment (auxiliaries included) Low High Medium high
Heat source operation costs Very high No Medium low
Operation reliability Normal Very good Normal


To analyze the above table, at a first glance, the hot air heating sources has a relatively low efficiency, large footprint, a one-time increase of investment costs, indeed it brings a long term production stability, system safety, reliable operation and extreme low operation costs. And the increase of the one-time investment is far more less than the saved operation costs. The most important, as long as the air compressor is in a normal operation, the system stability is ensured, prevented from the influence of accidents and other disturbance apart of the SAS system. if steam or hot water is used as heat source, the system shuts down when heat supply disrupted. And there is barely no pressure deviation in the tube and shell of the heat exchanger, it eliminates the problems of scaling, tube cracking, vapor or liquid leakage, and avoids accidents.
If use hot water, constant supply of hot water must be ensured. If hot water is heated by steam, then better use steam as source directly.
1. High complexity, low reliability, and high investment of auxiliary equipment. It has to build the whole circulation system (pool, pump, auto-control, etc.) To facilitate debugging, water pool must have enough capacity. Back-up pump with auto-startup function is necessary. Malfunction of pump or the pipeline may lead to severe loss.
2. Low overall heat transfer efficiency. Air carries limited heat due to its small enthalpy. After air to water, and water to air transfer, plus pipeline heat dissipations, the overall heat loss is significant. That’s why there is a compensation heat exchanger to balance heat fluctuations.
3. Scaling. Water with a temperature higher than 60℃ is prone to scaling. As a consequence, scaling problem is more severe in heater than in cooler, and it impacts the heat transfer. Therefore, it is mandatory to deionize water and to keep the heater clean.
4. High risk of water leakage. Remarkable pressure difference exists between shell and tube in the case of gas-liquid heat transfer. Once water leaks out, outlet air humidity soars up and it even becomes saturated, which damages downstream filters, and increases contamination risks. In contrast, there is no pressure difference in SAS air heater, hence leakage problem is non-existent.
5. Lag of heat conversion and instability of air quality. As Fermentation production aeration rate changes, or air temperature changes due to the climate factors, heat of hot water from the cooler will change correspondingly, However, since the whole water circulation system is quite complex and has redundant heat exchanges, hence it has a slow response so the feedback and adjustment lags behind the demands of air heating, leading to heated air temperature fluctuations and intermittent instability of relative humidity. That will exert a fatal impact on the life span of membrane filter as well as its sterilization capacity, even causing severe contamination losses.
As mentioned above, using hot water as heat source has many risks. Especially when pumps or the auto-control go wrong, the whole production line may shut down. Thus it requires careful considerations to adopt this method, which is usually not recommended in view of system safety and robustness.
In a conclusion: Hot air source has an advantage over steam and hot liquid. The real application turns out that, hot air is the best choice of heating source. And we strongly suggest this scheme to project designers and decision-makers.

2.6 What's the temperature demand of hot air source to be used in the heat exchanger?

As a kind of heat source, when other temperature parameters keep constant, the higher the hot air, the bigger the average temperature difference (driving force of heat exchange), the smaller the exchange areas, the lower the investment. From this point, there is no need to attach a cooler in the air compressor, the hot air could be utilized before cooled down together later.

Generally, the outlet temperature of the air compressor is above 100℃, high enough to be used as a heating source. In real situations, air temperature after cooling and specified heating amplitude should be taken account of. If the hot air temperature is below 100℃ while the demanded heating amplitude is high, the manufacturing cost will increase obviously, however, this amplitude of cost is totally acceptable in contrast with the steam vapor saved. Overall, it turns out to be an excellent value of money invested.

2.7 1 When temperature is low in winter, can heating meet operation requirement?

In accord with the heat transfer formula
In which: Q:Thermal load
K:Total heat transfer coefficient
A:Heat transfer areas
Δt:Average temperature difference

From above, Q is directly related to Δt instead of the temperature of inlet or outlet, therefore it’s temperature difference that matters.
In winter, the temperature of air heating source may be low, but the environment is dry with a low pressure dew point; after cooling the temperature would be very low if condensation happens, if same heating amplitude is applied as in summer, Δt and Q will not differ much, so in winter the system could meet the heating requirement.
If no condensation happens, and the relative humidity meets filter’s requirement, then heating is unnecessary. For example, the inlet air has a temperature of 0℃, a relative humidity of 80% and a pressure of 0.2MPa, a pressure dew point of 12.7℃, when cooled to 40℃ the humidity is only 20%. In that case, downstream filter works fine without air heating. So in a conclusion, heating issue in winter is usually a pseudo-proposition.

2.8 How is the resistance of the hot air heating? Why it is under good control?

Resistance is more of a relative term. In contrast with other heating source, the resistance of air heater is slightly bigger. But the resistance is counteractive to heat exchange efficiency and investment cost. As for a certain amount of heating fluid, if the resistance is big, it indicates that it has a high flow speed and high convective heat coefficient and a high K, so the transfer area could be small, and investment could be reduced however the power consumption of air compressor goes up. On the other side, small resistance indicates slow flow speed, lower heat exchange efficiency, increase of the heat exchange areas, large footprints, higher investment cost. But if resistance is set to be the priority, through accurate design can we find a sweet point with a high efficiency, a balance of equipment investment and operation cost. So the resistance of hot air heating can definitely controlled in a reasonable range.

Not only can resistance of hot air heating be well controlled, also the resistance of the whole SAS system could be controlled within 0.01Mpa. Comparing to the traditional system, the resistance is only 10%~50%.

2.9 Is it needed to tune the hot air heater?

Different air compressor has different outlet temperature. But usually we don’t need all the hot air to heat the cooled air, a part of it will go through the bypass and directly to the cooler. And in different seasons, temperature of air compressor outlet differS and while also the bypass volumes differ.

Therefore, one can set the bypass valve on the pipeline, to achieve balanced flow distribution and resistance.Practice has proved that the system do not require frequent valve operation, only a moderate adjustment in the seasonal change.As for adjustment, both manual and automation work well.

2.10 Does hot air heater need cleaning?

Since in the heater, the air in the shell and tube have the same pressure. So expect for out shell of the equipment, all the internal parts works under no pressure difference, so there is hardly possibility of leakage. Besides, air is relatively cleaner than water, and comparing to the cooler, the heating tube is less easy to get scaled, so there is no need to clean often.

Wet and cold air is heated, and the temperature of the hot air drops but usually still above the condensation point. Therefore, the inner part is dry, even manufactured by carbon steel rust will not happen. With an adequate budget, stainless steel is undoubtedly preferable.

In a conclusion, there is no need to clean the inner part of the heater, and visible maintenance is enough. The heater is robust and reliable.

2.11 What are the potential safety risks to use hot water heater?

There are several advantages to use hot water as heat source, such as high heat exchange efficiency and low single investment cost. However, we must take the following problems into considerations:

a.Hot water source, whether in the air pretreatment system or outside?

The former comes from the cooler inside the system, which requires establishing a complete hot water circulation system (tank, pump and automatic control systems, etc.). The storage tank must have enough volume in order to facilitate maintenance. And the water pump must have a back-up, with an automatic start function. Both the invest cost of auxiliary system and reliability of automation would be considered. The malfunction of pump and automation will directly impact the system operation, which will impede the fermentation production and lead to cost damage or even overall production shut-down. Outsourced hot water does not need auxiliary system. But we need consider the reliability of the outsourced system to ensure the continuous supply. If sourced water is directly converted from the vapor, then better to adopt vapor heating instead.

b、Scaling problem
Water with a temperature above 60℃ is easy to get scaled, so the heater is prone to scaling than cooler. Scaling damages the heat transfer performance severely. So we must deal with the water quality and heater cleaning properly.

c. Assessment of the potential danger of water leakage
If hot water is used as the heat source, we have to assess the impact if leakage happens. As the last work step of air pretreatment system, once leakage happens, then the outlet relative humidity rises even with oversaturated water, which will have a serious impact on the membrane filter, Cannot guarantee that the filter sterilization effect, increasing the chance of contamination of the fermentation. This will damage the overall production severely.

Therefore as is mentioned above, the use of hot water air heater kept many security risks. Especially the failure of circulating pump and automation control directly affect system operation. Use of hot water air heater should be particularly careful.

3.1 What is fermentation exhaust?

Aerobic fermentation needs the input of sterile air from the bottom of fermenter, and the air goes out from the top, which is also called fermentation exhaust. Exhaust air consists of air, a variety of metabolites in the fermenter; especially the foam in the exhaust is a mixture of gas, particles and liquid.

3.2 Is it necessary to process the fermentation exhaust?

Fermentation products vary a lot, so the composition of exhaust vary for different products. But undoubtedly it will impact the production, environment, and other economical & technical index, therefore it must be well handled.

3.3 How to dispose the fermentation exhaust air?

According to disposal targets and requirements, exhaust air-handling can be divided into the following methods:
1 Open processing by cyclones
2 semi-closed processing by efficient helix-strike separators
3 spray absorption treatment
4 ozone oxidation treatment or photo catalytic oxidation treatment
5 boiler combustion treatment

As for method one, open processing by cyclones, fermentation exhaust is leaded to cyclones, and liquid goes out from the cyclone bottom to drainage, duct or storage containers, while the air goes out of the top, released to the environment or moving to the next processing step. Method 2 is different from Method1, separator transports the collected liquid back to the fermenter and the air is released or for transported to next processing, therefore, it is categorized as semi-closed processing. Also, the internal structure of efficient helix-strike separator is totally different from traditional cyclones.
Method 1 and 2 are usually applied as first treatment of the air exhaust, and the latter three are applied as the second treatment. Determination of the second treatment depends on the components of fermentation exhaust or other environmental factors. One thorough treatment is to lead air exhaust to the boiler and burn it , if the factory has one.

3.4 Is the efficient helix-strike separator prone to the contamination?

No. Beginning from design, manufacturing to installation and operation, in every single chain we have corresponding standards and regulations to turn down the contamination.

Equipment design is based on fermenter structure and fermentation process; The equipment is manufactured with stainless steel by precision manufacturing, double sided polish; and it is anti-clot, without sterilization dead corner; Installation will guided by the drawings and instructions, and we provide corresponding operation manuals for cleaning and sterilization.

Not only a single separator, we take account of duct value installation and other operation guide, forming an efficient, anti-contamination fermentation air handling & recycling system.

3.5 Is it necessary to install fermentation exhaust handling equipment if there is no broth escaping?

The reason of no feed escape needs cases by case study. Whether the fermentation constant volume is too low? The usage of defoamer is excessive? If the volume packing rate is high and the fermentation itself is not easy to produce foam, then it is not economic to add exhaust handling equipment – efficient helix-strike separator. But in terms of environmental protection, the gas-liquid separation efficiency of helix-strike cyclone is as high as 98%~99.99%, and it removes the droplets and moisture brought by the elevating air, and avoids the run-off of fermentation medium. And the density of the viable bacteria in the exhaust air drops significantly (four orders magnitude comparing to system without separator). The operational environment and stability is improved, phage is effectively curbed. The helix-strike separator is playing a role of first-step exhaust air disposal; dramatically reduce the burden of deodorization and sterilization. So considering the indirect benefits of production stability and environmental benefits, it is strongly suggested to install helix-strike separator. And regulation in developed country demand similar air-handling equipment. This is the right direction.

4.1 Is the vortex gas-liquid mixer prone to contamination?

The vortex gas-liquid mixer is precisely manufactured, surface polished inside and outside; it has a internal streamlined profile with any corners, so it is not easy to get contaminated. After content release of the fermenter, one can simply wash the mixer or soak it or steam purge it. This process can be done simultaneously with tank cleaning, and sterilization can also be done at the same time.
Note that bypass steam duct and water duct (normally both) must be set in the air duct leading to mixer inlet.