Insulation

Types Of Insulation For Refrigeration Applications

Refrigeration systems cover a broad spectrum of application temperatures and environments, many of which are discussed in the 2006 ASHRAE Handbook–Refrigeration. But they all face the same issues relating to both condensation control and moisture. Since moisture is a good thermal conductor, its presence in an insulation system is highly detrimental. Unlike hot systems, where marginal insulation may result in increased energy use (and added cost), refrigeration systems face condensation, which often leads to complete system failure. Even with today’s high energy costs, the design thickness in most refrigeration applications is dictated by what is needed to prevent condensation, rather than by economic payback.

Refrigeration systems typically operate in the range of 20°F (for Freon systems) to as low as -50°F (for ammonia systems). They can use a variety of refrigerants and fluids in addition to Freon and ammonia, including glycol, brine, and other specialty fluids. Copper, iron, stainless steel, or other piping materials may be used to carry the cooling medium. Typical applications include those in supermarkets; beverage-dispensing lines; chillers; and food processing, freezing, and storage facilities (for example, meat processing and dairy, vegetable, and frozen-dinner cases). Other applications include those at ice rinks and morgues, as well as various unique applications. All of these applications share common concerns regarding condensation control and long-term reliability, but they also have particular issues with installation, required thickness, and the environmental conditions in which they operate. Guidelines for insulation selection, thickness, installation, and maintenance are found in the2006 ASHRAE Handbook–Refrigeration in Chapter 33, “Insulation Systems for Refrigeration Piping.”

Reliability should be the primary concern when considering the design and installation for any application. Design must consider factors including the application temperature, environmental considerations, consequences if a failure occurs, and expectations of the job by the owners (longevity of the system, aesthetics, etc.). Installation considerations include environmental conditions during installation, time frame allotted to complete the job, and worker training.

Below-ambient refrigerant lines are installed primarily to accomplish the following: 

Minimize heat gain to the internal fluids 

Control surface condensation 

Prevent ice accumulation 

Operation is generally continuous, so the vapor drive is unidirectional. Water vapor that condenses on the pipe surface or in the insulation remains there. The vapor retarder must be continuous and effective 100 percent of the time to limit the amount of vapor entering the system. The following are some important features of the insulation in various refrigeration applications: 

Thermal conductivity, or k-value 

Water vapor transmission (WVT) properties 

Water absorption properties 

Coefficient of thermal expansion 

Moisture wicking 

Fire and smoke performance to meet building codes 

The ASHRAE handbook recommends the following insulation materials for refrigeration applications: cellular glass, closed-cell phenolic, flexible elastomeric, polyisocyurante, and polystyrene. All of these materials have one property in common: all are closed-cell foam materials, which means they will have good WVT and low water absorption characteristics.

In all cases, the entire system (seams, butt joints, and termination points) must be completely sealed with adhesives to protect against air intrusion into the system, which would carry moisture and result in condensation between the cold pipe and the insulation. Relying on a single, concentrated vapor retarder is not recommended. Generally, closed-cell foam insulations are used for these applications. Seams should be minimized. On multilayer systems, the seams should be staggered. Taped seams are only allowed as a complimentary closure system.

Supermarkets are one of the biggest and most noticeable applications for refrigeration systems. They face several issues, including changes in refrigerant, colder line temperatures, higher temperature hot gas defrost cycles, changing store designs, and pressures to reduce installation time to decrease store build time. Chapter 16 in the ASHRAE handbook, titled “Retail Food Store Refrigeration and Equipment,” details many of the issues related to this market segment. Mandated changes in refrigerants are resulting in colder line temperatures, which require increased insulation thickness to prevent condensation.

In typical conditions, a 1-inch thickness is the standard; however, in some cases where humidity is high (over 80 percent), a 1½-inch thickness may be used. (See “Insulation Thickness Versus Design Relative Humidity.”) Availability of 1½-inch wall elastomeric insulation that has a 25/50 rating when tested according to ASTM E 84 is a new development on the market. This eliminates the need for sleeving materials to obtain the 1½-inch thickness.

Hot gas defrost cycles require insulation products to withstand spike temperatures up to 250°F. Open-ceiling store designs are mandating a change from the standard black insulation to a white product for better paint coverage and appearance. The installation procedures used on supermarket applications require that copper pipe (20-foot sections) be insulated on the floor and then taken to the ceiling and hung. To expedite this process and eliminate seams, many installers have gone to 18-foot coils of insulation that can be easily slid onto the 20-foot copper lengths. The reduction in seams saves time and improves the reliability of the job.

Using preinsulated pipe hangers is a concept that is gaining acceptance in supermarkets because it saves time and improves reliability by reducing condensation at hanger locations. The majority of piping on a supermarket is indoors, but for the outdoor and rooftop sections, the use of flexible jacketing-polyvinyl chloride (PVC), AL laminates, etc., is being evaluated, either installed at the job site or factory applied to improve the longevity and appearance of the job. Use of protective coatings that need periodic maintenance is becoming less specified.

Refrigeration piping on most supermarkets is found inside, but some stores are designed with 90 percent of the piping on the roof. Some elastomeric insulation products are being promoted as ultraviolet (UV) resistant and acceptable for use outdoors without the additional protection of coatings, jackets, or cladding materials. But UV protection is not the only issue when it comes to outdoor applications. Mechanical abuse (by birds, cats, people, etc.) and environmental abuse (by hail, sand, dirt, wind, rain, etc.) play a role in the reliability and longevity of the insulation system. For optimum performance, coatings, jacketing, or cladding should be used for outdoor applications.

The insulation system installed on a supermarket refrigeration system must be highly reliable, as it will operate 24 hours a day for up to 10 years. System failure may result in a large loss of perishable foods. Closed-cell elastomeric materials have been used in this application for many years because they are extremely reliable and cost-effective. It is a primary product used for supermarket refrigeration applications because of its low WVT, allowable use temperature range, and ease of installation.

Another rather unique, behind-the-scenes refrigeration application is in beverage-dispensing units for large stadiums or coliseums, where satellite food court areas are all supplied from a central unit. In this case, multiple lines are bundled together and insulated with a single large-inner-diameter insulation coil. This keeps all the lines together and saves materials, space, and installation time. The coils are produced in long lengths to reduce seams, which also saves time and improves system reliability. Closed-cell flexible insulation materials are preferred for this application because of their flexibility during installation.

Chillers come in various sizes and models. All require insulation of the piping and chiller barrels. As with other refrigeration applications, use of new refrigerants and space constraints have caused concerns. In addition, many manufacturers have changed manufacturing locations, which means units are being shipped farther or stored longer prior to shipment. This increases the units’ exposure to weather and the resulting challenges. Plus, end users demand not only performance, but also the acceptable appearance of the delivered unit. As a result, manufacturers are moving toward insulation with preapplied UV weather and abuse protection, such as coatings or claddings that offer both performance and appearance benefits. Indoor safety and health issues at the manufacturing location may not allow the use of solvent-based adhesives, so manufacturers are using products with preapplied adhesive on either the sheet or tubular insulation. Facing increased labor costs, inventory constraints, and space limitations, some find purchasing ready-to-use kits supplied by fabricators a cost-effective option.

Using a coating or flexible jacketing can improve the appearance, durability, weather resistance, and longevity of the insulation on a unit. Flexible, closed-cell elastomeric insulation is the predominate product used in this application. A 3/4-inch thickness is commonly used.

Food processing, freezing, storage, and distribution applications often use ammonia refrigeration because of its lower operating costs. System practices for ammonia and carbon dioxide refrigerants are outlined in Chapter 3 of the ASHRAE handbook. Processed, precooked, and prepared foods all come under the regulations of the Food and Drug Administration (FDA) 21 Code of Federal Regulations (CFR), and facilities using meat products come under the regulations of the United States Department of Agriculture (USDA).

As ammonia systems are designed for smaller applications at an economical up-front cost, they are getting more consideration than more expensive operating systems. While most areas do not exceed high temperatures above 160°F, some sections may cycle from -40°F to 250°F. Lower temperatures (down to -60°F) mean greater insulation thickness (2 to 3 inches) is usually required to prevent condensation. Typically, 3 inches of insulation is used to prevent condensation, as many of these applications are in high-humidity areas. Ammonia refrigeration applications are demanding, and performance and longevity expectations are high. The majority of the insulation is installed outdoors, so jacketing selection is critical. As a result of the cost and thickness required, polystyrene and polyisocyurante with a stainless steel jacket are the most common materials used. Of key concern is corrosion of iron pipe. Moisture resistance and control of the insulation material is critical. Proper insulation installation (with no open or through seams) is a major concern, and use of secondary vapor-retarder systems is the norm. New materials that can save on material or installation costs are always being evaluated, but long-term reliability is of prime importance because downtime and iron-pipe replacement is more costly than the dollars saved up front on installation.

The refrigeration market covers a broad spectrum of applications, each with unique requirements but all with a common goal: prevention of moisture intrusion and condensation to maintain long-term system reliability. Installation techniques are just as critical as material selection. The consequences of system failure can include degraded thermal performance of the insulation, higher system operating cost, inadequate cooling capacity, mold and mildew, ice formation, ruined ceilings, slippery floors, equipment downtime, and corroded pipes.

In below-ambient systems like refrigeration applications (including chilled water and cryogenic systems), closed-cell insulation products are preferred because of their low WVT and inherent moisture resistance. It’s important to select the right insulation product for the application. Customer expectations must be matched to product performance and cost. Refrigeration applications are demanding and require careful consideration in material selection and installation to obtain optimum performance for the end user.

Insulation Installing

Avoid gaps and missing insulation along the refrigeration lines

Proper placement and securing of insulation on air conditioner or heat pump refrigeration lines is important to avoid condensation leaks into the building. One, or on some systems both refrigeration lines can be cool or cold under some operating conditions.

The cold copper tubing in contact with warm humid air causes moisture in the air to condense onto and then drip off of the refrigration lines.

The result can be leaks into the building, as our photo at left illustrates.

Missing or damaged refrigerant line insulation insulation on the refrigerant lines, particularly on the larger suction line, will cause condensation and drips from the lines in humid areas.

In our photo at above left where refrigerant line insulation is incomplete, the drip stains on the attic floor may well indicate a point at which leak stains or even mold appear on the ceiling below.

In our photo at left none of the refrigerant lines are insulated where they emerge from the building wall. If the lines were also uninsulated within the wall, depending on their location and the wall's dew point properties, a condensation, leak, mold, rot, or insect problme can ensue.

We have seen very costly building damage where lines were not properly insulated indoors: condensate drips wet gypsum board walls, leading to a costly mold remediation project.

Missing refrigerant line insulation also may increase system operating cost or in addition to a condensation worry, uninsulated high pressure refrigerant lines may result in unwanted heat transmission into some building areas.

At left we illustrate a neat insulation job visible on the outdoor portion of refrigerant piping for a split system ductless air conditioner installlation.

According to McQuay International, a large producer of refrigeration equipment,

Suction lines are cold – 40°F (4.4°C) SST – and cause condensation, even in conditioned spaces. In addition, any heat that enters the refrigerant adds to the superheat and reduces system efficiency.

For these reasons, suction lines should be insulated with a vapor proof insulation. This is a requirement of many building codes. Rubratex is the most common form of refrigerant line insulation.

Liquid lines generally are insulated. They are warm to hot (110°F (43.3°C) for air-cooled). If liquid lines pass through a space that is warmer than the refrigerant (i.e. the roof of a building at roof level), or if they could be considered hot enough to pose a safety risk, then insulation should be added.

Discharge lines are generally uninsulated. They may be very hot, in excess of 150°F (66°C), so insulation may be warranted as a safety consideration, or if the heat loss from the discharge gas line would be considered objectionable to the space.

Hot gas bypass lines should be insulated, especially if the runs are long or if the piping is exposed to cold temperatures.

Do Not Compress Insulation on A/C or Heat Pump Refrigeration Lines

The same split system air conditioner installer we described above at A/C Condensate Disposal for Split System Air Conditioners violated the manufacturer's recommendations against compressing the insulation on the refrigerant lines - one more picky issue that we decided to let go since the wall was to be insulated with blown-in foam.

But he made the same mistake on the insulation on the refrigeration lines and condensate drain where they extend outdoors between the building wall and the compressor/condenser unit.

Our photographs illustrate that the importance of not compressing refrigeration line insulation is no joke. In our photo at below left, notice those drip stains below the condensate lines at each location where the insulation was compressed by a too-tight plastic tie?

And in the two photos at below right, notice the incomplete insulation on the refrigeration line? It leaves me worried about condensation and water accumulation inside the wall cavity as well. Since I know this installer is not stupid we're left thinking he has a bit of contempt for his customers, or a limited concept of workmanship.

Imagine that same dripping and accumulation of water where the installer made the same mistake in a fiberglass-batt insulated wall or a wall or ceiling inside which the dew point may be reached on the refrigeration lines? The accumulation of water in a building cavity is asking for a costly mold, insect, or rot damage problem later on.

Missing insulation on the refrigeration lines outdoors is not a catastrophe - at least for a short un such as at this split system compressor/condenser unit. Perhaps a little loss in efficiency of the system operation in some weather conditions. On a long refrigeration line run, say between an attic air handler and a ground level compressor/condenser, the effects may be more significant.

We removed the leaky, incomplete, and ugly squashed insulation on the refrigeration lines for this system (above left), replacing the squashed foam insulating tubing with new insulation (above right).

Incidentally, just clipping off the old plastic wire ties to "release" the squashed refrigerant line foam insulation won't work: after a few months the insulating foam remains permanently squashed, as you can see in our photo at left.

We paid particular attention to sealing and insulating the refrigeration line at the exit point from the building wall, reducing the chances of leaks into the wall at that point. To keep the refrigeration line insulation in place you can still use a plastic tie if you like - just don't tighten it so far as to squash the insulation.

The manufaturer (Sanyo) recommends covering the foam insulation on the refrigeration line with weatherproof tape which we did at the end of this job.

I admit that we "over-designed" the final insulation job shown in our last photo with that extra layer of foam that surrounds both lines, as we used more thickness of insulation than necessary.

We did so to end with a neat, weather-protected job that, combined with the application of black weatherproof tape, should last for a long time.

Notice that the aluminum or plastic ties used to hold components in place were left loose - we did not squash the new refrigerant line insulation, and we made sure it was continuous, neat, and protected from the weather.

A neat installation takes what, maybe five minutes longer than a sloppy one, but it took about an hour to buy the replacement refrigerant line insulation, remove the original sloppy installation, and do the job right the second time.

Protect outdoor refrigerant line insulation from the weather

Manufacturers also recommend wrapping the insulated refrigerant lines exposed to outdoor weather, using an appropriate weatherproof tape.

Design of Refrigeration Cycles

Goal

We want to design a vapor-compression refrigeration cycle to absorb heat from a cool environment and reject it to a warm environment. The design is to be based upon the ideal vapor-compression refrigeration cycle, with four components: a cooler (where we reject the heat), a throttle, a heater (where we absorb the heat), and a compressor.

Basics of Vapor-Compression Refrigeration Cycles

The general idea

The challenge in refrigeration (and air conditioning, etc.) is to remove heat from a low temperature source and dump it at a higher temperature sink. Compression refrigeration cycles in general take advantage of the idea that highly compressed fluids at one temperature will tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this case, we can use it to cool at a low temperature and reject the heat to a high temperature.

Vapor-compression refrigeration cycles specifically have two additional advantages. First, they exploit the large thermal energy required to change a liquid to a vapor so we can remove lots of heat out of our air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid to the temperature of whatever is being cooled. This is a benefit because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. The isothermal process allows the fastest rate of heat transfer.

More details

An ideal refrigeration cycle looks much like a reversed Carnot heat engine or a reversed Rankine cycle heat engine. The primary distinction being that refrigeration cycles lack a turbine, using a throttle instead to expand the working fluid. (Of course, a turbine could be incorporated into a refrigeration cycle if one could be designed to deal with liquids, but the useful work output is usually too small to justify the cost of the device.)

The cycle operates at two pressures, Phigh and Plow, and the statepoints are determined by the cooling requirements and the properties of the working fluid. Most coolants are designed so that they have relatively high vapor pressures at typical application temperatures to avoid the need to maintain a significant vacuum in the refrigeration cycle.

The T-s diagram for a vapor-compression refrigeration cycle is shown below.

Figure 1: Vapor-Compression Refrigeration Cycle 

T-s diagram

Below is a possible CyclePad design of a refrigeration cycle. The layout shown below is a clickable image. To jump to the part of this page that details the assumptions of a particular device or statepoint, just click on it.

Figure 2: Basic refrigeration cycle layout

Cooler Inlet Cooler (Condenser) Cooler Outlet Throttle Heater (Evaporator) Compressor Inlet Compressor

Example Design Constraints

Cooling requirements

For purposes of illustration, we will assume that a refrigeration system used to cool air for an office environment. It must be able cool the air to 15.5°C (about 60°F) and reject heat to outside air at 32°C (90°F).

The working fluid

We have several working fluids available for use in refrigeration cycles. Four of the most common working fluids are available in CyclePad: R-12, R-22, R-134, and ammonia. (Nitrogen is also available for very low temperature refrigeration cycles.) We will choose R-22 for this example.

Description of Cycle Stages

We will examine each statepoint and component in the refrigeration cycle where design assumptions must be made, detailing each assumption. As we can see from the example design constraints, very few numbers need be specified to describe a vapor-compression refrigeration cycle. The rest of the assumptions are determined by applying reasoning and background knowledge about the cycle. The two principle numerical design decisions are determining Phigh and Tlow, at the cooler outletand the compressor inlet.

Cooler (Condenser) inlet (S1)This state need not involve any design decisions, but it may be important to come back here after the cycle has been solved and check that T2, which is the high temperature of the cycle, does not violate any design or safety constraints. In addition, this is as good a place as any to specify the working fluid.

Cooler (Condenser): Heat Rejection (CLR1)

The cooler (also known as the condenser) rejects heat to the surroundings. Initially, the compressed gas (at S1) enters the condenser where it loses heat to the surroundings. During this constant-pressure process, the coolant goes from a gas to a saturated liquid-vapor mix, then continues condensing until it is a saturated liquid at state 2. Potentially, we could cool it even further as a subcooled liquid, but there is little gain in doing so because we have already removed so much energy during the phase transition from vapor to liquid.

Cooler (Condenser) outlet (S2)

We cool the working fluid until it is a saturated liquid, for reasons stated above. An important design question arises at this state: how high should the high pressure of the cycle be?

We choose Phigh so that we can reject heat to the environment. Phigh is the same as P2, and P2 determines the temperature at state S2, T2. (T2 is just the saturation temperature at Phigh). This temperature must at least be higher than that of the cooling source, otherwise no cooling can occur.

However, if T2 is too high (that is, higher than the critical temperature TC for the working fluid), then we will be beyond the top of the saturation dome and we will loose the benefits of the large energy the fluid can reject while it is being cooled. Furthermore, it is often impractical and unsafe to have very high pressure fluids in our system and the higher P2 we choose, the higher T1 must be, leading to additional safety concerns. To find an applicable pressure, use the saturation tables to find a pressure which is somewhere between the saturation pressure of the warm air yet still in the saturation region.

For reference, TC for our four working fluids are given below.

Critical Temperatures of some refrigerants
substance	TC (°C)
R-12 (CCL2F2)	111.85
R-22 (CHCLF2)	96.15
R-134a (CF3CH2F)	101.05
ammonia (NH3)	132.35

For our example using R-22, we must be able to reject heat to air that is 32°C. We can choose if T2 to be anywhere between that number and the 96°C TC. We'll choose it to be 40°C for now.

The figure above gives a general idea of the improvements we can expect with lower temperatures in the cooler. Keep in mind that the practical limitation here is heat transfer to the surrounding air. While lower temperatures will make the cycle more efficient theoretically, setting Thigh too low means the working fluid won't surrender any heat to the environment and won't be able to do its job.

Throttling (THR1)

The high-pressure, saturated liquid is throttled down to a lower pressure from state S2 to state S3. This process is irreversible and there is some inefficiency in the cycle due to this process, which is why we note an increase in entropy from state S2 to S3, even though there is no heat transfer in the throttling process. In theory, we can use a turbine to lower the pressure of the working fluid and thereby extract any potential work from the high pressure fluid (and use it to offset the work needed to drive the compressor). This is the model for the Carnot refrigeration cycle. In practice, turbines cannot deal with the mostly liquid fluids at the cooler outlet and, even if they could, the added efficiency of extracting this work seldom justifies the cost of the turbine.

Heater (Evaporator): Heat Absorption (HTR1)

The working fluid absorbs heat from the surroundings which we intend to cool. Since this process involves a change of phase from liquid to vapor, this device is often called the evaporator. This is where the useful "function" of the refrigeration cycle takes place, because it is during this part of the cycle that we absorb heat from the area we are trying to cool. For an efficient air conditioner, we want this quantity to be large compared to the power needed to run the cycle.

The usual design assumption for an ideal heater in a refrigeration cycle is that it is isobaric (no pressure loss is incurred from forcing the coolant through the coils where heat transfer takes place). Since the heating process typically takes place entirely within the saturation region, the isobaric assumption also ensures that the process is isothermal.

Compressor Inlet (S4)

Where do we want S4?

Typically, we want state S4 to be right at the saturated vapor side of the saturation dome. This allows us to absorb as much energy from the surroundings as possible before leaving the saturation dome, where the temperature of the working fluid starts to rise and the (now non-isothermal) heat transfer becomes less efficient.

Of course, we would get the same isothermal behavior if we were to start the compression before the fluid was completely saturated. Further, there would seem to be a benefit in that statepoint S1 (see Figure 1) would be closer to the saturation dome on the Phigh isobar, allowing the heat rejection to be closer to isothermal and, therefor, more like the Carnot cycle. 

It turns out that, for increased efficiency, we can choose S4 such that S1 is on the saturation dome, instead of outside of it in the superheat region. Figure 4 shows the T-s diagrams for two refrigeration cycles, one where S4 is a saturated vapor and the other (in light green) where S4 has been moved further into the saturation dome to allow S1 to be a saturated vapor.

Figure 4: T-s diagram for different compressor conditions

The advantage in the second case is that we have reduced the compressor work. We have also reduced the heat transfer somewhat, but the reduced compressor work has a greater effect on the cycle's coefficient of performance. Figure 6 shows the cycle's COP versus the quality of S4. We note that the change in COP is noticable, but not terribly impressive.

Figure 5: COP versus compressor inlet quality

However, in setting S4 below the saturated vapor line, we assume our compressor can work with fluid that is substantially liquid at statepoint S4. Since the liquid part of the fluid is incompressible, this is likely to damage the compressor. It is for this reason that we choose the inlet to the compressor to be completely saturated vapor, ensuring that the compressor can do its work entirely in the superheat region. When we are told we have compressors capable of dealing with fluids whose quality is slightly less than 100% (these are sometimes available), we can adjust the position of S4 to improve cycle efficiency.

How to choose Tlow

This brings us to another design issue: Now that we know that S4 is on the saturated vapor line, where on the line is it? In other words, how low can Tlow go?

Tlow occurs within the saturation dome, so it determines Plow as well. We know that Tlow must at least be cooler than the desired temperature of the stuff we wish to cool, otherwise no cooling will occur. An examination of the saturation tables for our refrigerants shows that setting Tlow at, for instance 15� C, still allows for fairly high pressures (4 to 7 atmospheres, typically). So, while this tells us how low Plow must be, it does not tell us how low it can be.

There are several major practical considerations limiting Plow. Fundamentally, we must concern ourselves with the properties of our working fluids. Examination of the saturation table for R-22 shows that at atmospheric pressure, the saturation temperature is already very cold (about -40°C). For small-scale air-conditioning applications, we have no desire to create a stream of extremely cold air, both due to safety concerns and because cold air holds very little moisture and can be uncomfortably dry. For larger-scale applications, this is less of a concern because we can always mix the cold, dry air with warmer, wetter air to make it comfortable.

Another hardware consideration is that it is fairly difficult to maintain a very low-pressure vacuum using the same compressor that will achieve high pressure at its outlet. Choosing a Tlow that results in a Plow of 0.1 atmospheres is probably not practical if we intend to have Phigh up near 10 atmospheres.

This brings us to the other reason we cannot make Tlow too small. Examining Figure 1 again, we see that the lower Plow is, the further out to the right (higher entropy) the saturated vapor will be at statepoint S4. Statepoint S4 has the same entropy as S1, and the further to the right S1 is along the Phigh pressure isobar, the hotter S1 must be. This high temperature is undesirable from both efficiency and safety standpoints.

The figure below shows the relationship between Tlow and the cycle's coefficient of performance (COP). We note that the higher Tlow, the better the COP. The practical limit on Tlow is heat transfer rate in the evaporator; having Tlow too close to the temperature of the stuff we wish to cool results in low heat transfer rates.

Figure 6: Vapor-Compression Refrigeration Cycle 

COP versus Tlow

So, ultimately, we want a low pressure such that its saturation temperature is below the desired cool air temperature but high enough that the temperature at state one is not too hot. For our example, where we need to cool air down to 15.5°C, we will choose Tlow to be 10°C.

Compressor (COMP1)

Ideal compressors are like ideal pumps, adiabatic and isentropic. We also note that the compressor is the only device in the system that does work to the fluid. For an efficient air conditioner, we want this quantity to be small.