→ Installation of refrigeration units
Installation of main apparatus and auxiliary equipment
The main devices of a refrigeration unit include devices directly involved in mass and heat transfer processes: condensers, evaporators, subcoolers, air coolers, etc. Receivers, oil separators, dirt traps, air separators, pumps, fans and other equipment included in the refrigeration unit include to auxiliary equipment.
The installation technology is determined by the degree of factory readiness and design features of the devices, their weight and installation design. First, the main equipment is installed, which allows you to begin laying pipelines. To prevent moisture insulation on the supporting surface of devices operating at low temperatures, apply a layer of waterproofing, lay a thermal insulation layer, and then again a layer of waterproofing. To create conditions that prevent the formation of thermal bridges, all metal parts(fastening belts) are applied to the devices through wooden antiseptic bars or gaskets with a thickness of 100-250 mm.
Heat exchangers. Most heat exchangers are supplied by factories ready for installation. Thus, shell-and-tube condensers, evaporators, subcoolers are supplied assembled, elemental, spray, evaporative condensers and panel, submersible evaporators are supplied as assembly units. Finned tube evaporators, direct coils and brine evaporators can be manufactured installation organization in place from sections of finned pipes.
Shell-and-tube devices (as well as capacitive equipment) are mounted in a combined flow method. When laying welded apparatus on supports, make sure that all welds are accessible for inspection, tapping with a hammer during inspection, and also for repair.
The horizontality and verticality of the devices are checked by level and plumb line or using surveying instruments. The permissible deviations of the devices from the vertical are 0.2 mm, horizontally - 0.5 mm per 1 m. If the device has a collection or settling tank, a slope only in their direction is permissible. The verticality of shell-and-tube vertical condensers is especially carefully verified, since it is necessary to ensure film flow of water along the walls of the pipes.
Elemental capacitors (due to their high metal consumption they are used in rare cases in industrial installations) are installed on metal frame, above the receiver, element by element from bottom to top, checking the horizontality of the elements, the uniform plane of the fitting flanges and the verticality of each section.
Installation of irrigation and evaporative condensers consists of sequential installation of a pan, heat exchange pipes or coils, fans, oil separator, pump and fittings.
Devices with air cooled, used as condensers for refrigeration units, are mounted on a pedestal. To center the axial fan relative to the guide vane, there are slots in the plate, which allow the gear plate to be moved in two directions. The fan motor is centered to the gearbox.
Panel brine evaporators are placed on an insulating layer, on a concrete pad. The metal evaporator tank is installed on wooden beams, install the stirrer and brine valves, connect drain pipe and test the tank for density by filling it with water. The water level should not fall during the day. Then the water is drained, the bars are removed and the tank is lowered onto the base. Before installation, panel sections are tested with air at a pressure of 1.2 MPa. Then sections are mounted in the tank one by one, manifolds, fittings, and a liquid separator are installed, the tank is filled with water and the evaporator assembly is again tested with air at a pressure of 1.2 MPa.
Rice. 1. Installation of horizontal capacitors and receivers using the combined flow method:
a, b - in a building under construction; c - on supports; g - on overpasses; I - position of the capacitor before slinging; II, III - positions when moving the crane boom; IV - installation on support structures
Rice. 2. Installation of capacitors:
0 - elemental: 1 - supporting metal structures; 2 - receiver; 3 - capacitor element; 4 - plumb line for checking the verticality of the section; 5 - level for checking the horizontality of the element; 6 - ruler for checking the location of the flanges in the same plane; b - irrigation: 1 - draining water; 2 - pallet; 3 - receiver; 4 - sections of coils; 5 - supporting metal structures; 6 - water distribution trays; 7 - water supply; 8 - overflow funnel; c - evaporative: 1 - water collector; 2 - receiver; 3, 4 - level indicator; 5 - nozzles; 6 - drop eliminator; 7 - oil separator; 8 - safety valves; 9 - fans; 10 - precondenser; 11 - float water level regulator; 12 - overflow funnel; 13 - pump; g - air: 1 - supporting metal structures; 2 - drive frame; 3 - guide vane; 4 - section of finned heat exchange pipes; 5 - flanges for connecting sections to collectors
Submersible evaporators are mounted in a similar way and are tested at an inert gas pressure of 1.0 MPa for systems with R12 and 1.6 MPa for systems with R22.
Rice. 2. Installation of panel brine evaporator:
a - testing the tank with water; b - testing panel sections with air; c - installation of panel sections; d - test of the evaporator assembly with water and air; 1 - wooden beams; 2 - tank; 3 - stirrer; 4 - panel section; 5 - goats; 6 - air supply ramp for testing; 7 - water drain; 8 - oil sump; 9-liquid separator; 10 - thermal insulation
Capacitive equipment and auxiliary devices. Linear ammonia receivers are mounted on the side high pressure below the condenser (sometimes under it) on the same foundation, and the steam zones of the devices are connected by an equalizing line, which creates conditions for draining the liquid from the condenser by gravity. During installation, maintain a difference in heights from the liquid level in the condenser (the level of the outlet pipe from the vertical condenser) to the level of the liquid pipe from the oil separator overflow cup I of at least 1500 mm (Fig. 25). Depending on the brands of the oil separator and linear receiver, the differences in elevations of the condenser, receiver and oil separator are maintained: Yar, Yar, Nm and Ni, specified in the reference literature.
On the side low pressure install drainage receivers to drain ammonia from cooling devices when the snow coat is thawing with hot ammonia vapors and protective receivers in pumpless circuits to receive liquid in the event of its release from the batteries when the heat load increases, as well as circulation receivers. Horizontal circulation receivers are mounted together with liquid separators placed above them. In vertical circulation receivers, steam is separated from the liquid in the receiver.
Rice. 3. Installation diagram of a condenser, linear receiver, oil separator and air cooler in an ammonia refrigeration unit: KD - condenser; LR - linear receiver; HERE - air separator; SP - overflow glass; MO - oil separator
In aggregated freon installations, linear receivers are installed above the condenser (without an equalizing line), and the freon enters the receiver in a pulsating flow as the condenser is filled.
All receivers are equipped with safety valves, pressure gauges, level indicators and shut-off valves.
Intermediate vessels are installed on supporting structures on wooden beams, taking into account the thickness of the thermal insulation.
Cooling batteries. Direct cooling freon batteries are supplied by manufacturers ready for installation. Brine and ammonia batteries are manufactured at the installation site. Brine batteries are made from electric-welded steel pipes. For the manufacture of ammonia batteries, seamless hot-rolled steel pipes (usually with a diameter of 38X3 mm) are used from steel 20 for operation at temperatures down to -40 °C and from steel 10G2 for operation at temperatures up to -70 °C.
For cross-spiral finning of battery tubes, cold-rolled steel strip made of low-carbon steel is used. The pipes are finned using semi-automatic equipment in the conditions of procurement workshops with a random check with a probe for the tightness of the fins to the pipe and the specified fin pitch (usually 20 or 30 mm). Finished pipe sections are hot-dip galvanized. In the manufacture of batteries, semi-automatic welding in a carbon dioxide environment or manual electric arc is used. Finned tubes connect batteries with collectors or coils. Collector, rack and coil batteries are assembled from standardized sections.
After testing ammonia batteries with air for 5 minutes for strength (1.6 MPa) and for 15 minutes for density (1 MPa), the welded joints are galvanized with an electroplating gun.
Brine batteries are tested with water after installation to a pressure equal to 1.25 working.
The batteries are attached to embedded parts or metal structures on ceilings (ceiling batteries) or on walls (wall batteries). Ceiling batteries are mounted at a distance of 200-300 mm from the axis of the pipes to the ceiling, wall batteries - at a distance of 130-150 mm from the axis of the pipes to the wall and at least 250 mm from the floor to the bottom of the pipe. When installing ammonia batteries, the following tolerances are maintained: height ± 10 mm, deviation from verticality of wall-mounted batteries is no more than 1 mm per 1 m of height. When installing batteries, a slope of no more than 0.002 is allowed, and in the direction opposite to the movement of refrigerant vapor. Wall batteries are installed using cranes before installing floor slabs or using boom loaders. Ceiling batteries are mounted using winches through blocks attached to the ceilings.
Air coolers. They are installed on a pedestal (on-pedestal air coolers) or attached to embedded parts on the ceilings (mounted air coolers).
Pedestal air coolers are installed using a flow-combined method using a jib crane. Before installation, insulation is laid on the pedestal and a hole is made to connect the drainage pipeline, which is laid with a slope of at least 0.01 towards the drain into the sewer network. Mounted air coolers are installed in the same way as ceiling radiators.
Rice. 4. Battery installation:
a - batteries for an electric forklift; b - ceiling battery with winches; 1 - overlap; 2- embedded parts; 3 - block; 4 - slings; 5 - battery; 6 - winch; 7 - electric forklift
Cooling batteries and air coolers made of glass pipes. Glass pipes are used to make coil-type brine batteries. Pipes are attached to racks only in straight sections (rolls are not secured). The supporting metal structures of the batteries are attached to the walls or suspended from the ceilings. The distance between the posts should not exceed 2500 mm. Wall batteries to a height of 1.5 m are protected with mesh fences. Glass pipes of air coolers are also installed in a similar way.
For the manufacture of batteries and air coolers, pipes with smooth ends are taken, connecting them with flanges. After installation, the batteries are tested with water at a pressure equal to 1.25 working.
Pumps. Centrifugal pumps are used to pump ammonia and other liquid refrigerants, coolants and chilled water, condensate, as well as to empty drainage wells and circulate cooling water. To supply liquid refrigerants, only sealed, sealless pumps of the CG type with an electric motor built into the pump housing are used. The stator of the electric motor is sealed, and the rotor is mounted on the same shaft with the impellers. The shaft bearings are cooled and lubricated by liquid refrigerant taken from the discharge pipe and then transferred to the suction side. Sealed pumps are installed below the liquid intake point at a liquid temperature below -20 ° C (to avoid disruption of the pump, the suction head is 3.5 m).
Rice. 5. Installation and alignment of pumps and fans:
a - installation centrifugal pump along the joists using a winch; b - installation of the fan with a winch using guy ropes
Before installing stuffing box pumps, check their completeness and, if necessary, carry out an inspection.
Centrifugal pumps are installed on the foundation by a crane, a hoist, or along joists on rollers or a sheet of metal using a winch or levers. When installing the pump on a foundation with blind bolts embedded in its mass, wooden beams are placed near the bolts so as not to jam the threads (Fig. 5, a). Check the elevation, horizontalness, alignment, presence of oil in the system, smooth rotation of the rotor and packing of the stuffing box (oil seal). Stuffing box
The gland should be carefully stuffed and bent evenly without distortion. Excessive tightening of the gland leads to its overheating and increased energy consumption. When installing the pump above the receiving tank, a check valve is installed on the suction pipe.
Fans. Most fans are supplied as a ready-to-install unit. After installing the fan with a crane or winch with guy ropes (Fig. 5, b) on the foundation, pedestal or metal structures (through vibration-isolating elements), the elevation and horizontal position of the installation are verified (Fig. 5, c). Then remove the rotor-locking device, inspect the rotor and housing, make sure there are no dents or other damage, manually check the smooth rotation of the rotor and the reliability of fastening of all parts. Check the gap between the outer surface of the rotor and the housing (no more than 0.01 wheel diameter). The radial and axial runout of the rotor is measured. Depending on the size of the fan (its number), the maximum radial runout is 1.5-3 mm, axial 2-5 mm. If the measurement shows that the tolerance is exceeded, static balancing is carried out. The gaps between the rotating and stationary parts of the fan are also measured, which should be within 1 mm (Fig. 5, d).
At trial run within 10 minutes, check the level of noise and vibration, and after stopping the reliability of fastening of all connections, heating of the bearings and the condition of the oil system. The duration of the load tests is 4 hours, during which the stability of the fan operation is checked under operating conditions.
Installation of cooling towers. Small film-type cooling towers (I PV) are supplied for installation with a high degree of factory readiness. The horizontal installation of the cooling tower is verified, connected to the pipeline system, and after filling the water circulation system with softened water, the uniformity of irrigation of the nozzles made of miplast or polyvinyl chloride plates is adjusted by changing the position of the water spray nozzles.
When installing larger cooling towers after the construction of a swimming pool and building structures install the fan, check its alignment with the cooling tower diffuser, adjust the position of the water distribution gutters or collectors and nozzles for uniform distribution of water over the irrigation surface.
Rice. 6. Alignment of the impeller of the axial fan of the cooling tower with the guide vane:
a - by moving the frame relative to the supporting metal structures; b - cable tension: 1 - impeller hub; 2 - blades; 3 - guide vane; 4 - cooling tower casing; 5 - supporting metal structures; 6 - gearbox; 7 - electric motor; 8 - centering cables
Alignment is adjusted by moving the frame and electric motor in the grooves for the fastening bolts (Fig. 6, a), and in the largest fans, coaxiality is achieved by adjusting the tension of the cables attached to the guide vane and supporting metal structures (Fig. 6, b). Then check the direction of rotation of the electric motor, smoothness, runout and vibration level at operating shaft rotation speeds.
In the case when the consumption of the vapor phase of liquefied gas exceeds the rate of natural evaporation in the container, it is necessary to use evaporators, which, due to electrical heating, accelerate the process of evaporation of the liquid phase into the vapor phase and guarantee the supply of gas to the consumer in the calculated volume.
The purpose of the LPG evaporator is to convert the liquid phase of liquefied hydrocarbon gases (LPG) into a vapor phase, which occurs through the use of electrically heated evaporators. Evaporation units can be equipped with one, two, three or more electric evaporators.
Installation of evaporators allows the operation of both one evaporator and several in parallel. Thus, the productivity of the installation may vary depending on the number of evaporators operating simultaneously.
Operating principle of the evaporation unit:
When the evaporation unit is turned on, the automation heats up evaporation plant up to 55C. The solenoid valve at the liquid phase inlet to the evaporation unit will be closed until the temperature reaches these parameters. The level control sensor in the shut-off valve (if there is a level gauge in the shut-off valve) monitors the level and closes the inlet valve when overfilled.
The evaporator begins to heat up. When 55°C is reached, the inlet magnetic valve will open. The liquefied gas enters the heated pipe register and evaporates. At this time, the evaporator continues to heat up, and when the core temperature reaches 70-75°C, the heating coil will be turned off.
The evaporation process continues. The evaporator core gradually cools down, and when the temperature drops to 65°C, the heating coil will be turned on again. The cycle repeats.
Evaporation unit complete set:
The evaporation unit can be equipped with one or two regulatory groups to duplicate the reduction system, as well as the vapor phase bypass line, bypassing the evaporation unit for using the steam phase of natural evaporation in gas holders.
Pressure regulators are used to install set pressure at the exit from the evaporation plant to the consumer.
- 1st stage - medium pressure adjustment (from 16 to 1.5 bar).
- 2nd stage - low pressure adjustment from 1.5 bar to the pressure required when supplying to the consumer (for example, to a gas boiler or gas piston power plant).
Advantages of PP-TEC evaporation units “Innovative Fluessiggas Technik” (Germany)
1. Compact design, light weight;
2. Economical and safe operation;
3. Big thermal power;
4. Long service life;
5. Stable operation at low temperatures;
6. Duplicated control system for the exit of the liquid phase from the evaporator (mechanical and electronic);
7. Anti-icing of filter and solenoid valve (PP-TEC only)
Package Included:
Double thermostat for gas temperature control,
- liquid level control sensors,
- solenoid valves at the liquid phase inlet
- set of safety fittings,
- thermometers,
- ball valves for emptying and deaeration,
- built-in liquid phase gas separator,
- inlet/outlet fittings,
- terminal boxes for connecting power supply,
- electrical control panel.
Advantages of PP-TEC evaporators
When designing an evaporation plant, three elements must always be taken into account:
1. Ensure the specified performance,
2. Create the necessary protection against hypothermia and overheating of the evaporator core.
3. Correctly calculate the geometry of the location of the coolant to the gas conductor in the evaporator
The performance of the evaporator depends not only on the amount of power supply voltage consumed from the network. An important factor is the geometry of the location.
A correctly calculated arrangement ensures efficient use of the heat transfer mirror and, as a result, increases the efficiency of the evaporator.
In evaporators “PP-TEC “Innovative Fluessiggas Technik” (Germany), by correct calculations, the company’s engineers have achieved an increase in this coefficient to 98%.
Evaporative installations of the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) lose only two percent of heat. The remaining amount is used to evaporate the gas.
Almost all European and American manufacturers of evaporation equipment completely erroneously interpret the concept of “redundant protection” (a condition for the implementation of duplication of protection functions against overheating and overcooling).
The concept of “redundant protection” implies the implementation of “safety net” of individual working units and units or entire equipment, through the use of duplicated elements from different manufacturers and with different principles of operation. Only in this case can the possibility of equipment failure be minimized.
Many manufacturers try to implement this function (while protecting against hypothermia and the ingress of the liquid fraction of LPG to the consumer) by installing two magnetic valves connected in series from the same manufacturer on the input supply line. Or they use two temperature sensors for switching on/opening valves connected in series.
Imagine the situation. One solenoid valve is stuck open. How can you determine that the valve has failed? NO WAY! The installation will continue to operate, having lost the opportunity to ensure safe operation in time during overcooling in the event of failure of the second valve.
In PP-TEC evaporators this function was implemented in a completely different way.
In evaporation installations, the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) uses an algorithm for the combined operation of three elements of protection against hypothermia:
1. Electronic device
2. Magnetic valve
3. Mechanical shut-off valve in the shut-off valve.
All three elements have absolutely different principle actions, which allows us to speak with confidence about the impossibility of a situation in which non-evaporated gas in liquid form enters the consumer pipeline.
In the evaporation installations of the company “PP-TEC “Innovative Fluessiggas Technik” (Germany), the same thing was implemented when protecting the evaporator from overheating. The elements involve both electronics and mechanics.
The company “PP-TEC “Innovative Fluessiggas Technik” (Germany) was the first in the world to implement the function of integrating a liquid cut-off valve into the cavity of the evaporator itself with the possibility of constant heating of the cut-off valve.
No evaporation technology manufacturer uses this proprietary function. Using a heated cutter, evaporation units “PP-TEC “Innovative Fluessiggas Technik” (Germany) were able to evaporate heavy components of LPG.
Many manufacturers, copying from each other, install a cut-off valve at the outlet in front of the regulators. The mercaptans, sulfur and heavy gases contained in the gas, which have a very high density When entering a cold pipeline, they condense and are deposited on the walls of pipes, cut-off valves and regulators, which significantly reduces the service life of the equipment.
In PP-TEC “Innovative Fluessiggas Technik” (Germany) evaporators, heavy sediments in a molten state are kept in a separator until they are removed through a discharge ball valve in the evaporation unit.
By cutting off mercaptans, the company “PP-TEC “Innovative Fluessiggas Technik” (Germany) was able to achieve a significant increase in the service life of installations and regulatory groups. This means taking care of operating costs that do not require constant replacement of regulator membranes, or their complete expensive replacement, leading to downtime of the evaporation unit.
And the implemented function of heating the solenoid valve and filter at the inlet to the evaporation unit prevents water from accumulating in them even when freezing in solenoid valves disable when triggered. Or limit the entry of the liquid phase into the evaporation unit.
Evaporation plants German company“PP-TEC “Innovative Fluessiggas Technik” (Germany) is reliable and stable operation for for long years operation.
The MEL group of companies is a wholesale supplier of air conditioning systems to Mitsubishi Heavy Industries.
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Compressor-condensing units (CCU) for ventilation cooling are becoming increasingly common in system design central cooling buildings. Their advantages are obvious:
Firstly, this is the price of one kW of cold. Compared to chiller systems, supply air cooling using KKB does not contain an intermediate coolant, i.e. water or non-freezing solutions, therefore it is cheaper.
Secondly, ease of regulation. One compressor-condenser unit operates for one air-conditioning unit, so the control logic is uniform and is implemented using standard air-conditioning unit control controllers.
Thirdly, the ease of installation of the KKB for cooling the ventilation system. No additional air ducts, fans, etc. are needed. Only the evaporator heat exchanger is built in and that’s it. Even extra insulation supply air ducts often not required.
Rice. 1. KKB LENNOX and diagram of its connection to the air handling unit.
Against the backdrop of such remarkable advantages, in practice we come across many examples of air conditioning ventilation systems in which the air conditioning units either do not work at all or very quickly fail during operation. Analysis of these facts shows that often the reason is incorrect selection KKB and evaporator for cooling the supply air. Therefore, we will consider the standard methodology for selecting compressor-condenser units and try to show the mistakes that are made in this case.
INCORRECT, but the most common, method for selecting a KKB and evaporator for direct-flow air handling units
- As initial data, we need to know the air flow air handling unit. Let's set 4500 m3/hour as an example.
- The supply unit is direct-flow, i.e. no recirculation, operates on 100% outside air.
- Let's determine the construction area - for example, Moscow. Calculated parameters of outdoor air for Moscow are +28C and 45% humidity. We take these parameters as the initial parameters of the air at the entrance to the evaporator of the supply system. Sometimes the air parameters are taken “with a reserve” and set at +30C or even +32C.
- Let's set required parameters air at the outlet of the supply system, i.e. at the entrance to the room. Often these parameters are set 5-10C lower than the required supply air temperature in the room. For example, +15C or even +10C. We will focus on the average value of +13C.
- Further using i-d charts(Fig. 2) we build the air cooling process in the ventilation cooling system. We determine the required cooling flow under given conditions. In our version, the required cooling flow is 33.4 kW.
- We select the KKB according to the required cooling flow of 33.4 kW. There is a nearby large and a nearby smaller model in the KKB line. For example, for the manufacturer LENNOX these are models: TSA090/380-3 for 28 kW of cold and TSA120/380-3 for 35.3 kW of cold.
We accept a model with a reserve of 35.3 kW, i.e. TSA120/380-3.
And now we will tell you what will happen at the site when the air handling unit and the air handling unit we selected work together according to the method described above.
The first problem is the overestimated productivity of KKB.
The ventilation air conditioner is selected for outdoor air parameters of +28C and 45% humidity. But the customer plans to operate it not only when it’s +28C outside; the rooms are often already hot due to internal heat excess starting from +15C outside. Therefore, the controller sets the supply air temperature to best case scenario+20C, and at worst even lower. KKB produces either 100% performance or 0% (with rare exceptions of smooth control when using VRF outdoor units in the form of KKB). When the outside (intake) air temperature decreases, the KKB does not reduce its performance (and in fact even slightly increases due to greater subcooling in the condenser). Therefore, when the air temperature at the inlet to the evaporator decreases, the KKB will tend to produce a lower air temperature at the outlet of the evaporator. Using our calculation data, the output air temperature is +3C. But this cannot be, because... The boiling point of freon in the evaporator is +5C.
Consequently, lowering the air temperature at the evaporator inlet to +22C and below, in our case, leads to an overestimated performance of the KKB. Next, the freon does not boil enough in the evaporator, the liquid refrigerant returns to the compressor suction and, as a result, the compressor fails due to mechanical damage.
But our problems, oddly enough, do not end there.
The second problem is a LOWERED EVAPORATOR.
Let's take a closer look at the selection of the evaporator. When selecting an air handling unit, specific parameters for the operation of the evaporator are set. In our case, this is the air temperature at the inlet +28C and humidity 45% and at the outlet +13C. Means? the evaporator is selected EXACTLY for these parameters. But what will happen when the air temperature at the evaporator inlet is, for example, not +28C, but +25C? The answer is quite simple if you look at the formula for heat transfer of any surfaces: Q=k*F*(Tv-Tph). k*F – heat transfer coefficient and heat exchange area will not change, these values are constant. Tf - the boiling point of freon will not change, because it is also maintained at a constant +5C (in normal operation). But TV - the average air temperature has dropped by three degrees. Consequently, the amount of heat transferred will become less in proportion to the temperature difference. But KKB “does not know about this” and continues to provide the required 100% productivity. Liquid freon returns to the compressor suction again and leads to the problems described above. Those. The calculated evaporator temperature is the MINIMUM operating temperature of the KKB.
Here you can object: “But what about the work of on-off split systems?” The design temperature in the splits is +27C in the room, but in fact they can operate up to +18C. The fact is that in split systems the surface area of the evaporator is selected with a very large margin, at least 30%, just to compensate for the decrease in heat transfer when the temperature in the room drops or the fan speed of the indoor unit decreases. And finally,
Problem three – selection of KKB “With RESERVE”...
The productivity reserve when selecting a KKB is extremely harmful, because The reserve is liquid freon at the compressor suction. And in the end we have a jammed compressor. In general, the maximum evaporator capacity should always be greater than the compressor capacity.
Let's try to answer the question - how to CORRECTLY select KKB for supply systems?
Firstly, it is necessary to understand that the source of cold in the form of a compressor-condensing unit cannot be the only one in the building. Conditioning the ventilation system can only remove part of the peak load entering the room with ventilation air. And maintaining a certain temperature indoors in any case falls on local closers ( indoor units VRF or fan coils). Therefore, the KKB should not maintain a certain temperature when cooling the ventilation (this is impossible due to on-off regulation), but should reduce heat input into the premises when a certain outside temperature is exceeded.
Example of a ventilation and air conditioning system:
Initial data: Moscow city with design parameters for air conditioning +28C and 45% humidity. Supply air flow 4500 m3/hour. Excess heat in the room from computers, people, solar radiation, etc. are 50 kW. Estimated room temperature +22C.
The air conditioning capacity must be selected in such a way that it is sufficient for worst conditions(maximum temperatures). But ventilation air conditioners should also work without problems even with some intermediate options. Moreover, most of the time, ventilation air conditioning systems operate just at 60-80% load.
- We set the calculated temperature of the external air and the calculated temperature of the internal air. Those. The main task of the KKB is to cool the supply air to room temperature. When the outside air temperature is less than the required indoor air temperature, the KKB DOES NOT TURN ON. For Moscow, from +28C to the required room temperature of +22C, we get a temperature difference of 6C. In principle, the temperature difference across the evaporator should not be more than 10C, because the supply air temperature cannot be less than the boiling point of freon.
- We determine the required performance of the KKB based on the conditions for cooling the supply air from the design temperature of +28C to +22C. The result was 13.3 kW of cold (i-d diagram).
- We select 13.3 KKB from the line of the popular manufacturer LENNOX according to the required performance. We select the nearest SMALLER KKB TSA036/380-3с with a productivity of 12.2 kW.
- We select the supply evaporator from the worst parameters for it. This is the outside air temperature equal to the required indoor temperature - in our case +22C. The cold productivity of the evaporator is equal to the productivity of the KKB, i.e. 12.2 kW. Plus a performance reserve of 10-20% in case of contamination of the evaporator, etc.
- We determine the temperature of the supply air at an outside temperature of +22C. we get 15C. Above the boiling point of freon +5C and above the dew point temperature +10C, this means that insulation of the supply air ducts does not need to be done (theoretically).
- We determine the remaining excess heat in the premises. It turns out 50 kW of internal heat excess plus a small part from the supply air 13.3-12.2 = 1.1 kW. Total 51.1 kW – calculated performance for local control systems.
Conclusions: the main idea that I would like to draw attention to is the need to calculate the compressor capacitor unit not to the maximum outside air temperature, but to the minimum within the operating range of the ventilation air conditioner. Calculation of the KKB and evaporator carried out for the maximum supply air temperature leads to the fact that normal operation will only occur in the range of external temperatures from the design temperature and above. And if the outside temperature is lower than the calculated one, there will be incomplete boiling of freon in the evaporator and the return of liquid refrigerant to the compressor suction.
In the evaporator, the process of transition of the refrigerant from the liquid phase state to the gaseous state occurs with the same pressure; the pressure inside the evaporator is the same everywhere. During the process of transition of a substance from liquid to gaseous (its boiling away) in the evaporator, the evaporator absorbs heat, unlike the condenser, which releases heat to the environment. That. through two heat exchangers, the process of heat exchange occurs between two substances: the cooled substance, which is located around the evaporator and the outside air, which is located around the condenser.
Liquid freon flow diagram
Solenoid valve - shuts off or opens the flow of refrigerant to the evaporator, is always either completely open or completely closed (may not be present in the system)
Thermostatic expansion valve (TEV) is a precise device that regulates the flow of refrigerant into the evaporator depending on the intensity of the refrigerant boiling in the evaporator. It prevents liquid refrigerant from entering the compressor.
Liquid freon enters the expansion valve, the refrigerant is throttled through the membrane in the expansion valve (freon is sprayed) and begins to boil due to the pressure drop, the droplets gradually turn into gas throughout the entire section of the evaporator pipeline. Starting from the throttling device of the expansion valve, the pressure remains constant. Freon continues to boil and in a certain section of the evaporator it completely turns into gas and then, passing through the evaporator, the gas begins to be heated by the air that is in the chamber.
If, for example, the boiling point of freon is -10 °C, the temperature in the chamber is +2 °C, freon, having turned into gas in the evaporator, begins to heat up and at the exit from the evaporator its temperature should be equal to -3, -4 °C, thus Δt ( the difference between the boiling point of the refrigerant and the gas temperature at the evaporator outlet) should be = 7-8, this is the mode normal operation systems. For a given Δt, we will know that at the exit from the evaporator there will be no particles of unboiled freon (there should not be any); if boiling occurs in the pipe, then not all the power is used to cool the substance. The pipe is thermally insulated so that the freon does not heat up to ambient temperature, because The refrigerant gas cools the compressor stator. If liquid freon still gets into the pipe, it means that the dose supplied to the system is too large, or the evaporator is weak (short).
If Δt is less than 7, then the evaporator is filled with freon, it does not have time to boil away and the system does not work correctly, the compressor is also filled with liquid freon and fails. Overheating on a larger side is not as dangerous as overheating on a smaller side; at Δt ˃ 7, overheating of the compressor stator may occur, but a slight excess of overheating may not be felt by the compressor and is preferable during operation.
With the help of fans located in the air cooler, cold is removed from the evaporator. If this did not happen, then the tubes would become covered with ice and at the same time the refrigerant would reach its saturation temperature, at which it stops boiling, and then, even regardless of the pressure drop, liquid freon would enter the evaporator without evaporating, flooding the compressor.
