개방형(open type) 압축기는 motor가 압축기의 housing 외부에 위치하므로(motor crankshaft에 의해 압축기와 연결되어 있는 구조) 냉매 누설 또는 외부 공기의 흡입이 발생할 수 있는 구조의 압축기이다.
반밀폐(semi-hermetic type) 압축기는
대형 압축기에 일반적으로 valve와 piston을 보수할 수 있도록 실린더 헤드가 조립가능하게 되어 있는데 이러한 형태의 압축기이다.
밀폐형(hermetic type) 압축기는
개방형 압축기에서 발생할 수 있는 냉매 누설을 방지하기 위해 모터와 압축기가 동일한 하우징(housing)에 위치하도록 설계된 구조의 압축기이다. 과열된 상태로 압축기의 실린더에 유입되도록 설계하여 액압축 현상이 발생하는 것을 방지하는 구조로 되어 있다.
hermetic compressors will have direct connection to the motor, and sealed in a welded casing. This type of construction allows no maintenance except in factories with proper repair tools and skill
semi-hermetic compressors will have direct connection to the motor. However, the sealing is not a complete one. There will be a provision to dismantle the casing for parts replacement, and maintenance
open type, has the compressor and driver separated from each other. The compressor and the driver will be separate entities, connected by a coupling. Sealing of refrigerant gas from atmosphere is achieved by means of labyrinth and mechanical seals. Yes, this compressor construction is suitable for high capacity cooling
3. 압축기 특성
3.1. 압축기 성능맵
압축기의 성능은 압축기 맵(compressor map)으로 살펴볼 수 있다.
압축기 제조사에서 실험을 통해 압축기의 회전속도(compressor's rotational speed)에 따른 압축비, 효율, 유량, 서지/최대압력비(surge/maximum pressure ratio)를 도식화 하여 제공한다.
A : 설계점(Design Point)은 임펠러 설계의 기준이 되며, 압축기의 IGV(Inlet Guide Vane)가 최대로 열렸을 때, 기준이되는 압력대비 유량이다. 설계점의 압력과 유량은 압축기의 성능을나타내는데 이용된다.
B : 시스템 압력저항(System Resistance) 증가로 인한 압축기의 압축비가 설계점보다 높아지고, 유량이 감소한 상태이다.
E : 시스템 압력저항 감소로 인한 압축기의 압축비가 설계점보다 낮아지고, 유량이 증가한 상태이다.
C : 시스템 압력저항이 계속 증가하면, 성능곡선과 서지(Surge) 곡선의 교차점에 도달하게 되고, 성능곡선에 정점에 있다. 'C' 점을 서지점(Surge Point) 라고 하고, 이때부터 압축기의 역류(逆流)가 생기는 서지현상이 일어난다.
3.2. 서지(Surge) 현상
서지(sugre)는 맥동현상이라고도 한다. 서지의 진행과정은
C → D → E → C → D → E → C → .... 으로 진행되며, 연속적으로 반복할 때마다 변동의 폭이 커진다.
구간별로 살펴보면,
C → D : 시스템 압력저항이 큰 관계로 역류가 생겨 유량이 감소하는 과정이고, 유량이 0(Zero)인 D 점에 도달한다.
D → E : 압축기의 생성압력이 시스템 압력저항보다 커, 유량이 정방향으로 바뀌면서 (역류→정류), 점 E 로 이동한다.
이때, 압력변화는 일정하고, 생성유량만 증가한다.
E → C : 압축기의 압력이 과다한 관계로 성능곡선을 따라 점 C 로 이동한다.
A or C →D 과정을 1 사이클로 보며, 약 5 초 사이에 일어나면 서지(Surge) 1 회로 규정하고,
5 사이클(서지 5 회) 이상 연속 진행되면, 진동(vibration)의 폭이 커지기 때문에 장비의 베어링과 회전날개의 손상을 줄 가능성이 높아진다.
3.2.1. 서지 발생 원인
흡입부의 유량이 적을 경우, 흡입부 압력이 올라가는 경우,
토출 유량이 밸브 등에 의해 억제되어 유량이 줄어들면서 압력이 올라가는 경우
컨트롤의 오작동으로 회전속도가 갑자기 올라가는 경우
3.2.2. 서지 해결방법
압축기가 써지를 일으키는데에는 두가지 조건이 있다.
유량이 감소하거나, 압축비가 상승해, 압축기맵의 서지 곡선보다 좌측에 놓이는 경우다.
터빈 압축기가 운전중 서지가 안 생기도록 하기 위해서, IGV(Inlet Guide Vane)의 속도를 알맞게 조절하여야 하고,
터빈 압축기 구동시 서지를 피하기 위해서, IGV 최소값 (IGV Minimum Position)을 정하는 이유도 여기에 있다.
서지발생시 바로 벗어나도록, 무부하(Unloading) 상태로 만들어야하며, 이때 BOV(Blow off Valve)나 By pass 벨브를 열어주고, 흡입유량을 증가시킨다.
안티서지 컨트롤(Anti-surge Control)이란, 서지가 발생하지 않도록 밸브를 통해 흡입 측으로 유량을 유입하거나 Flare gas로 방출시키도록 하는 컨트롤 시스템이다.
3.3. Choking & Turndown ratio
터보 압축기는
서지(Surge)현상으로 최소유량(Minimum Flow)이 정해지고,
초킹(Choking)현상으로 최대유량(Maximum Flow)이 결정된다.
초킹현상은 압축기의 유량이 너무 크면(점 E을 초과하면), 고압을 만들어내지 못하고 오히려 압력비가 급격히 떨어지는 현상을 말한다.
이와같이 서징(Surging)과 초킹(Choking)으로 인하여, 압축기의 최소유량과 최대유량이 정해지며, 설계점(Design Point)에서 서징이 발생하는 점까지의 유량변화폭을 'Turndown'이라고 한다.
Turndown 이 크면 클수록 압축기의 효율이 좋으며, 이것은 곧 회전체(Impeller)의 성능과 직접적인 관계가 있다.
Turndown 을 비율(%)로 나타낸것이 바로 ‘Turndown Ratio’이다. Turndown Ratio 의 공식은 다음과 같다.
Turndown Ratio = (Turndown/최대유량) * 100 = (최대유량-최소유량)/최대유량 * 100
Now it’s time to dig deeper into understanding air conditioner compressors:
I would like to give you an overview of how air conditioner compressors are categorised. These are categorised by,
the seal type (e.g. completely sealed, semi-sealed and open type), and
the operation type (e.g. piston, scroll, rotary and centrifugal)
Let us also consider the basic characteristics that ALL compressors share.
That set of basic characteristics is broken into,
the pressure rating p (in bar or pounds per square inch - psi)
the flow rating ṁ (in m3s-1 or cubic feet per minute - cfpm), and
the speed rating N (in rpm)
These characteristics will be plotted to form a compressor map for a specific compressor.
Performance of manufactured compressors will be tested, to obtain results of the pressure ratio, efficiency, mass flow rate, and surge/maximum pressure ratio limit, against the compressors’ rotational speed. Results will be plotted in a compressor map.
A compressor map will consist of a set of speed curves, and a surge/maximum pressure ratio. The y-axis will be the pressure ratio range of the compressor, and the x-axis will be the mass flow rate of the compressed medium.
A contour of compressor efficiencyranges will be plotted within the speed lines. The maximum efficiency of a compressor will be in the centre contour area.
The centre contour will be,
away from the surge/maximum pressure ratio line, and
away from the maximum achievable flow rate of the compressor, at a given speed
Surge/maximum pressure, maximum flow rate and efficiency:
Any compressor rotating at a specific speed N1 will only be capable of,
sustaining a maximum flow rate of ṁ1, and
a maximum pressure ratio of p1/p0, where p0 is the pressure of the gas at inlet
Maximum flow rate
The maximum flow rate could not be exceeded simply because the speed is already fixed.
The surge/maximum pressure ratio
Now, imagine if the air conditioning compressor keeps compressing the refrigerant, but the overload protector is malfunctioning and the condensate line is blocked. Pressure in the system will rise.
There will come a point, when the compressor could no longer supportthe rising pressure and backflow of compressed refrigerant will start to happen.
This is known as the surging/maximum pressure point. Backflow will occur quite violently from this point onwards, and damage to compressors will be significant, due to high vibration. This happens as the force from compression, is lower than force from the downstream high pressure gas.
The only way to get pass this point is by,
lowering the downstream gas pressure, or
increasing the compression force by increasing the compression speed
However, the latter choice will utilise more shaft power from driver, per compression work. In other words, the efficiency will drop.
Lowering the downstream pressure would be more practical to avoid surge/maximum pressure ratio. The efficiency will increase, as an effect to that. However, lowering the downstream pressure too low will make the compressor inefficient, as the shaft power conversion into compressing the gas will be very low.
Surge occurs only for centrifugal compressors, axial compressors and fans, as backflow may occur between the tip of the blade, and the casing.
Surge can be easily detected by the “low hammering” or pulsating sound. One word. Compressor surging is bad. Ok, that’s four words.
Othertightly sealed compressors will only face the maximum pressure ratio, as the back pressure from the compressed gas, will overcome the driver’s (e.g. an electrical motor) force.
Compressors’ efficiency
Defined as the actual work done to compress gas, per energy input from the driver’s shaft. The driver is usually an electric motor.
Compressors’ efficiency (and that includes air conditioner compressors as well), is proportional to the product of mass flow rate, and pressure ratio.
We have to bear in mind that, the pressure ratio is low when the mass flow rate is high. The compressor will act to transport the gas, rather than compressing it.
On the other hand, the mass flow rate is low when the pressure ratio is high. This makes sense, as the back pressure from the gas will slow down the gas transfer.
Yes, absolutely. The maximum compressor efficiency will be attained at moderate mass flow rates and pressure ratio.
The maximum compressor efficiency is normally around 80%.
The compression power of air conditioner compressors are normally rated in horsepowerhp, or kilowattkW. One horsepower is equivalent to 0.7457 kW.
We have talked about the general features of compressors. It is now time to go a little deeper. You shouldn’t miss this fun.
Sealing of compressors:
There are three types of air conditioner compressors construction. The completely sealed, semi-sealed, and the open type.
Completely sealed compressors are also known as the hermetic type, and the semi-sealed are known as the semi-hermetic type.
What do these suppose to mean? The,
hermetic compressors will have direct connection to the motor, and sealed in a welded casing. This type of construction allows no maintenance except in factories with proper repair tools and skill
semi-hermetic compressors will have direct connection to the motor. However, the sealing is not a complete one. There will be a provision to dismantle the casing for parts replacement, and maintenance
open type, has the compressor and driver separated from each other. The compressor and the driver will be separate entities, connected by a coupling. Sealing of refrigerant gas from atmosphere is achieved by means of labyrinth and mechanical seals. Yes, this compressor construction is suitable for high capacity cooling
Air conditioner compressors for residential, and for average-large sized office buildings, will normally be of hermetic, and semi-hermetic construction.
Moving on, to the advantages and disadvantages of these two sealing types.
Hermetic
Semi-hermetic
Advantages
Higher efficiency, as the possibility of compression leakage is very small
Possible to do maintenance and parts replacement, without having to change the complete unit
Disadvantages
Consider buying a new air conditioning compressor, if you note signs of trouble
Lower compression efficiency. Leakage from compressor body is highly possible
Sealing topic is done, and you’re already prepared to go a little more into air conditioner compressors. Great!
Operation types of air conditioner compressors:
The list is almost non-exhaustive. We have centrifugal, rotary, screw type, roots blower type, sliding vane type, plunger type, ejector type, liquid-ring type, axial type, swash plate type and gear lobe type.
Talking about all of those will be quite interesting indeed, but let’s keep our topic to the most commonly used compressors in air conditioning cycle. The piston and the scroll compressor.
It begins now...
The piston or reciprocating compressor
It’s the oldest type of compressor, going back as far as 1600’s. Very robust and simple in construction, able to deliver very high compression pressures. Just to illustrate, a reciprocating compressor with 40 mm cylinder length, is able to deliver about 12 barG (gauge) of pressure (or 150 psiG or 120 metres of water height).
This type of compressor is constructed using,
piston head – functions as the gas compressing “agent” by continuously reducing the cylinder volume
piston rings – functions as the sealant between the piston head, and the cylinder, to prevent gas leakage from the compression chamber
crank shaft – a shaft that enables the reciprocating motion of the piston
piston rod – the connecting piece between the piston head, and the crankshaft
spring loaded suction and discharge valves – separates low pressure side and high pressure side from the compression chamber. Enables positive displacement of gases, by correct opening and closing of the valves. Suction valve will open as the piston moves away from the valves, and discharge valve will open as the piston moves towards the valves. The valves will otherwise, be in closed position
and, compressor’s cylinder block – functions as the housing for the compressor parts
Those are the basics. However, modifications are carried out in air conditioner compressors of this type, to have two pistons, and oriented horizontally, to improve the compression efficiency.
Identifying a reciprocating compressor is quite easy, even without opening its casing. The casing is almost square in shape, with similar height of its width.
Most approved refrigerants are compatible with this type of compressor. Those are, R-22, R-134a, R-404A, R-407C and R-507.
Robust and powerful as it may be, volumetric efficiency became a concern, as the valve opening do not happen as soon as the piston is on suction stroke. In addition to that, failure rate of reciprocating compressors is quite high due to mechanical stresses from the alternating motion.
Fast forward to three hundred years later, we have the first
Scroll compressor
It was invented in the beginning of 1900’s. As usual, initial inventions need improvements, but all credits to Léon Creux who created it, so we can work on improvements. It was optimised and improved around late 60’s.
Scroll type compressors, able to achieve the flow rate, and outlet pressure, similar to reciprocating compressors, at a smaller size, and better efficiency.
However, cooling for scrolls are quite difficult compared to piston air conditioner compressors. This is the reason for its performance drop against piston type, at higher compression ratios.
Nevertheless, most air conditioning applications require scroll compressors to be used against piston type, due to the advantages.
This type of compressor is constructed using,
a stationary scroll and an orbiting scroll – the orbiting spiral scroll will orbit around the stationary scroll, thus continuously and progressively trapping gas and directing towards the centre of the scrolls, to be discharged pass the discharge check valve, and towards discharge line
crank shaft – used for creating the orbiting motion. This shaft is equipped with counter weights to equalise the centripetal force due to eccentric shaft rotation
casing – to ensure that the discharge part is separated from the suction part, and motor winding is separated from the refrigerant
Courtesy: Sanyo
Sealing between high pressure and low pressure side of the scrolls are helped by the downward force to the stationary scroll, by the discharge gas pressure.
This type of air conditioner compressor, could handle similarrefrigerants to piston compressors.
Courtesy: Sanyo & testequity.com
Identifying this type of compressor is not that easy though. Yes it is normally a vertical cylinder, but rotary compressors share the same shape.
But if you look closely, scroll compressors have a "cap" like top cover, and the discharge line is on top of the casing. Whereas discharge line for rotary compressors is on the side, slightly lower than the top part.
Supporting parts for air conditioner compressors:
Air conditioner compressors are supported by important parts such as,
bearings
spiral grooved shaft for lubrication oil distribution, and
strainers to prevent solid ingress into the compression chamber
What manufacturers normally practice?
Designing air conditioner compressors is a tedious process, as there is the need to balance with the heat extraction requirement at the evaporator.
Manufacturers will normally have a standard list of common compressor sizes and ratings, to be matched with a suitable evaporator and condenser.
The compressor type and size selection, will be made based upon the refrigerant used, and the desired heat removal at the evaporator.
We sure have covered thebasicsof air conditioner compressors.
1 Housing with connectors and baseplates
2 Top Cover
3 Block with stator bracket
4 Stator (with screws)
5 Rotor
6 Valve unit (screws, cylinder cover, gaskets, valve plate)
