adsorption charge

  • In addition to refrigerant, the bulb also contains a solid adsorbent such as charcoal or silica gel. The adsorbed refrigerant reacts more slowly to temperature changes than direct-charged bulbs, and gives a slower response.


  • The quantitative capacity of water to neutralize an acid, i.e. the measure of how much acid can be added to a liquid without causing a significant change in pH. Expressed as mg CaCO3/l.


  • Adenosine monophosphate.


  • American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

asymmetric operation

  • When the temperature change in a fluid (or the flow) on one side of the BPHE is much larger than on the other side.



  • SWEP's standard compact brazed heat exchanger, made of stainless steel 316 and copper.

back pressure

  • Higher pressure in the system after the heat exchanger than in the heat exchanger. This pressure prevents the condensate from being drained in steam applications.

boiling point

  • See: bubble point.


  • Brazed Plate Heat Exchanger.​

​bubble point

  • The temperature (pressure related) where boiling starts. 100% saturated liquid.


  • A pressure probe connected to the expansion valve in order to control the amount of superheating.

burnout point

  • At this point in the boiling process, vapor will form to such an extent that it hinders contact between the liquid refrigerant and the heat transfer area, leading to lower heat transfer efficiency.


Carnot heat engine

  • A heat engine is an engine that uses heat to produce mechanical work by carrying a working substance through a cyclic process. The Carnot heat engine uses a particular thermodynamic cycle, which is a four stage reversible sequence consisting of: 1. Isothermal expansion of the gas at high temperature 2. Adiabatic expansion 3. Isothermal compression of the gas at low temperature 4. Adiabatic compression

centrifugal compressor 

  • Also called a turbo compressor. Compresses the gas by accelerating it with an impeller. The velocity is then transformed to pressure.


  • ChloroFluoroCarbon


  • Combined Heat and Power plant, producing both electricity and heat.

circulation number

  • In flooded systems, the number of times a specific volume has to pass the evaporator to be fully evaporated.

cleaning in place (CIP)

  • The mineral scaling and other fouling that may form in a BPHE can be removed using this cleaning method.

close temperature programs

  • When the primary and the secondary fluid temperatures are close to each other.

compression ratio

  • Ratio of condenser and evaporator pressures.


  • Removes refrigerant vapor formed in the evaporator and brings it to a higher pressure. The compressor is usually powered by electricity.

compressor shut-off valves

  • Safety devices to protect the system from excessively high or low compressor pressures, i.e. low/high evaporation temperatures. Also: service valves.


  • A heat exchanger turning gas into liquid.


  • Heat transfer through a solid material.


  • Conductors are materials that contain movable electrical charges. When an electrical potential difference is applied across separate points on a conductor, an electric current flows between those points. While many conductors are metallic, there are also many non-metallic conductors.


  • Heat transfer by eddies in a fluid.


  • Coefficient Of Performance. COP is a measure of efficiency, and can be calculated for the chiller, the condenser and the heat pump. COPref = THA/W (=compressor work) and COPhp = THR/W. COPhp = COPref + 1

critical heat flux

  • At this point, vapor will form to such an extent in the evaporator that it hinders contact between the liquid refrigerant and the heat transfer area, leading to lower heat transfer efficiency.


  • Very low-temperature applications, e.g. liquid nitrogen at -120°C.


  • A cyclotron is a machine for accelerating beams of charged particles. It uses a high-frequency alternating voltage across a magnetic field to spiral the beam outwards and eventually to deflect it once the beam's radius equals it’s the container's. At this point the particles' speed is generally very high (approaching the speed of light). Cyclotrons are used today in the treatment of cancer, because the particles ionize tumors and help to stop or slow cancerous growth.



  • Detergents are surface-active agents, substances that lower the surface tension of water.

dew point

  • At the dew point, all liquid has evaporated and superheating starts. The gas is 100% saturated.

direct expansion (DX) system

  • The basic compression cycle is a DX system. The expansion valve is mounted directly before the evaporator.


electron gun

  • An electron gun is a component that produces an electron stream with a precise kinetic energy. It is used in all TVs and monitors using cathode ray tube (CRT) technology. The CRT has three electron guns, each producing a different stream of electrons. These make red, green or blue phosphors illuminate, each pixel of the TV screen being made by combining these three colors.

electronically controlled ON/OFF valves

  • The electronic ON/OFF valve is actually an electronically controlled solenoid valve that functions both as an expansion valve and as a solenoid valve. When functioning as an expansion valve, ON/OFF control is used.


  • Heat exchanger turning liquid into gas.


  • Entering Water Temperature

excited state

  • The energy state of each atom in the active medium (of a gas laser, for example) is characterized by a set of energy states in which it may exist. These states may be visualized as unevenly spaced steps on a ladder, with higher steps representing higher energy states. In stimulated emission, an atom in the excited state is stimulated by the presence of light to fall to a lower energy state, emitting additional light while doing so.

expansion valve

  • A valve that maintains the pressure difference between the high-pressure and low-pressure sides in a condenser/evaporator system. See also: valves.

external pressure equalization

  • An expansion valve with external pressure equalization compares the bulb pressure with the suction line pressure in order to control the superheat. Recommended in connection with BPHE evaporators due to the extra V-ring pressure drop.

externally equalized expansion valve

  • An expansion valve with external pressure equalization compares the bulb pressure with the suction line pressure in order to control the superheat. Recommended in connection with BPHE evaporators due to the extra V-ring pressure drop.


flooded (wet) evaporator

  • Generic term for thermosiphons and forced-flow systems. The forcedflow system is similar to a thermosiphon, but the forced-flow system has a pump installed before the evaporator.

flooded systems

  • Flooded evaporation is also called wet evaporation. In flooded systems, the refrigerant enters the evaporator as 100% liquid, and the outlet flow may contain liquid. The outlet flow must not be superheated.


  • Droplets of refrigerant entering the oil sump, where they are immediately evaporated.


  • In this context, the floor area underneath a heat exchanger.

forced circulation systems

  • The forced-flow system is similar to a thermosiphon, but the forced-flow system has a pump installed before the evaporator.

forwarding temperature

  • Temperature of the hot fluid when it enters the device in question (radiator, heat exchanger)

fouling factors

  • Contamination factors. Used as a measure of the contamination of (amount of dirt in) a BPHE.



  • With reference to refrigerants, glide is a phenomenon that occurs with some mixed refrigerants due to different refrigerants boiling off at different temperatures, i.e. at constant evaporation pressure, the temperature varies.


  • Global Warming Potential



  • Generic term for carbon compounds containing halogens (F, Cl, Br, I)


  • HydroChloroFluoroCarbon

heat flux

  • A measure to describe the heat transfer efficiency. The unit for this measure is transferred energy per unit area (kW/m2).

hermetic compressor

  • Houses both motor and compressor house inside a welded shell, which provides a truly hermetic seal to the surroundings. The welded shell of a hermetic compressor does not open.


  • HydroFluoroCarbon

high-pressure float valve

  • Located on the high-pressure side of the system and in open connection to the condenser. It controls the evaporator level indirectly by maintaining a constant level of refrigerant inside the float chamber. Float valves can be found in flooded systems.

high-theta pattern

  • Large-angled "herringbone" pattern. The pressure drop is higher in a BPHE with a high-theta pattern than in a BPHE with a low-theta pattern. Increased pressure drop leads to increased turbulence in the fluid = higher heat transfer efficiency.


  • A phenomenon that can happen in (e.g.) steam systems if the condensate drainage is incorrect. Condensate backs-up in the heat exchanger and induces on/off regulation. This leads to wear on the control valve, the risk of cavitation, and temperature fluctuations in the secondary fluid.

hydraulic block

  • In a boiler, block consisting of connections to the BPHE, instead of pipes, in order to save space. It contains a number of functions, such as threeway valve, bypass valve, fill-up valve and temperature sensors.


  • Absorbs water.



  • In an indirect system, the fluid is not directly heated by the heating source. The heating source heats an intermediate fluid that heats the fluid by means of a heat exchanger.


  • Compounds that make a chemical reaction go more slowly, by obstructing the reaction.

instantaneous tap water

  • Hot tap water produced only when the demand occurs (no accumulation).

internally equalized expansion valve

  • An expansion valve with internal pressure equalization compares the bulb pressure with the pressure in the gas/liquid line before the evaporator inlet in order to control the superheat. NOT recommended in connection with BPHE evaporators due to the extra V-ring pressure drop.

isentropic efficiency

  • Isentropic efficiency = actual work/isentropic work. Isentropic work = no heat losses to the surroundings, no heat gained from the surroundings, no change in entropy during the work process.


lag time

  • The time a process takes to react to various changes.

latent energy

  • The energy that is absorbed or rejected when a phase change occurs between two states of aggregation (gas/liquid/solid).

latent heat

  • The heat that is absorbed or rejected when a phase change occurs between two states of aggregation (gas/liquid/solid).


  • Bacteria that develop in stagnant hot water and cause the deadly Legionnaires' disease.

liquid cooler

  • Here, a BPHE being used to cool liquid.

liquid hammer

  • In compressors, if a considerable amount of liquid enters the compressor house, a very large pressure can be built up when the piston reaches its top position. This phenomenon is called liquid hammer, and may cause severe damage to the valves or crankshaft.

liquid heat capacity

  • Amount of energy needed to increase the temperature of a specific liquid 1 Kelvin.

liquid line

  • That part of the refrigeration cycle where the refrigerant is in the liquid phase (after the condenser, before the evaporator).

Liquid line filter driers

  • Positioned in the liquid line to protect the expansion valve from particle contamination and to absorb potential humidity in the refrigerant. N.B. Creates additional pressure drop.

liquid static head

  • A pressure head created by liquid.

liquid-charged bulb

  • A liquid-charged bulb has a large charge of refrigerant and will never "run dry". It will always contain both liquid and gaseous refrigerant (see: bulb).


  • Logarithmic Mean Temperature Difference.

low-pressure float valve

  • Controls the liquid level in the receiver, and is normally mounted in a chamber parallel to the liquid/vapor separator. Float valves can be found in flooded systems.

low-theta pattern

  • Small-angled "herringbone" pattern. The pressure drop is lower in a BPHE with a low-theta pattern than in a BPHE with a high-theta pattern. Reduced pressure drop leads to reduced turbulence in the fluid = lower heat transfer efficiency.


  • Leaving Water Temperature.


minimum stable signal (MSS)

  • Thermal expansion valves must always operate with a minimal working superheating to achieve stable regulation. The minimum stable signal (MSS) is sent to the TEV at theminimum superheating.

modulating electronic expansion valves

  • Modulating electronic expansion valves are controlled by temperature or pressure sensors.

MOP bulb

  • An MOP (Maximum Operating Pressure) bulb, also called a gas-charged bulb, has a much smaller quantity of refrigerant mixture inside the bulb than a liquid-charged bulb (see: bulb).


  • The fluid passes through more than one channel length before leaving the BPHE.


nominal operation point

  • The most usual operating case. The system design case.


  • Number of heat Transfer Units, an expression for the thermal length of a plate.



  • Ozone Depletion Potential

oil filters

  • Often installed in the oil return line between the oil separator and the compressor to protect the compressor from contaminants. If this filter is blocked, it can lead to excessive oil carry-over, which affects heat transfer in the evaporator.

oil separator

  • Separates oil from refrigerant and returns it to the compressor.

open compressor

  • The motor and compressor house are mounted without a shell, although the shaft has a seal.

opening superheating

  • The additional superheating (in addition to the static superheating) required to open the expansion valve for operation.

operating point

  • In a refrigerant system: the equilibrium point, where the performance of the evaporator matches the performance of the compressor.


  • The overdimensioning of a heat exchanger as a percentage. A heat exchanger with oversurface is larger than is necessary to accomplish the thermal duty.



  • The negative logarithm of the alkalinity, expressed as mg CaCO3 /L.


  • The negative logarithm of the calcium concentration, expressed as mg CaCO3 /L.


  • The pH of a solution is the negative common logarithm of the hydrogen ion activity, aH+: pH = -log10 (aH+)


  • The minimum temperature difference between the refrigerant and the secondary fluid in a counter-current condenser.

positive displacement compressors

  • Compressors where a certain volume of gas is trapped in a space that is continuously reduced by the compressing device (piston, scroll, screw or similar) inside the compressor.

pressure gauges

  • Permanently installed to monitor compressor suction and discharge pressures.

primary circuit

  • Intermediate fluid transporting the heat from the heating source to the fluid required to be heated (in heating systems).


  • The negative logarithm of the content of solid particles in a fluid.


  • Photovoltaic. A photovoltaic panel converts sunlight into electricity.



  • No medium need exist between the two bodies for heat transfer to take place (cf. conduction and convection). In radiation, the intermediaries are photons that travel at the speed of light.


  • Positioned after the condenser. Compensates for varying refrigerant flow in evaporator and condenser systems. Makes subcooling impossible.

reciprocating compressors

  • Another name for piston compressors.

recirculation type evaporator

  • Flooded evaporator.

redox potential

  • The redox potential is a measure (in volts) of the affinity of a substance for electrons compared with hydrogen (which is set at 0), it is the electronegativity of a substance. Substances more strongly electronegative than (i.e., capable of oxidizing) hydrogen have positive redox potentials. Substances less electronegative than (i.e., capable of reducing) hydrogen have negative redox potentials.


  • A fluid used in cooling systems, usually with a low boiling point at atmospheric pressure.


saturation point

  • A state where a phase change may occur without a change in temperature and pressure.


  • A fouling phenomenon: the inorganic salts dissolved in water precipitate and form a scale on a surface.

scroll compressors

  • These capture the gas in the volume formed between one fixed and one orbiting scroll.

secondary distribution loop

  • Safety circuit between corrosive/explosive, etc., fluids and the primary circuit. In the event of leakage, the hazardous fluid is confined in an area where it will not do any harm.

secondary heat exchanger

  • Transfers the heat from the primary to the secondary circuit.

semi-hermetic compressor

  • The motor and the compressor house are located in a two-piece shell. The covers are bolted together.

semi-instantaneous tap water

  • Hot water accumulated in a tank to ensure a short response time when a demand occurs.

sensible energy

  • The energy added to or rejected from a gas, liquid or solid without a phase change.

sensible heat

  • The heat absorbed by or rejected from a gas or liquid without any phase change.

sight glass

  • Used for inspection of the refrigerant flow before the expansion valve.


  • A BPHE where the fluids pass through the channels only once.

single-screw compressor

  • A configuration of a screw compressor consisting of a single screw rotor.

solenoid valve

  • The solenoid valve will maintain the pressure difference between the condenser and the evaporator sides during off periods and thereby prevent liquid from flowing into the evaporator.


  • The SWEP Software Package.

stalling point

  • Condition where the steam pressure inside the heat exchanger is equal to the back pressure in the condensate system.

static head

  • Liquid column in a vertical pipe.

static superheating

  • The minimum level of superheating that is required to allow the pressure from the bulb to start pushing back the spring and thus opening the expansion valve.

sub-cooled liquid

  • When saturated liquid is cooled at constant pressure, its temperature decreases and it becomes subcooled.


  • Installation transferring heat from the district heating/cooling network (primary system) to the building network (secondary system).

suction line

  • The part of the refrigerant cycle that is located between the evaporator and the compressor. The refrigerant is usually in the state of superheated refrigerant here.

suction line accumulators

  • Installed to avoid liquid carry-over to the compressor, which could affect lubrication and, in the worst-case scenario, lead to damage if liquid or oil foam enters the compression chamber.

suction line filters

  • Protects the compressor from particle contamination and absorbs potential humidity in the refrigerant. N.B. induces additional pressure drop. The evaporation pressure must be measured before the filter.

superheated gas

  • When a dry saturated gas is heated at constant pressure, its temperature rises and it becomes superheated.


  • Any substance that will reduce water's surface tension is called a surfaceactive agent, or surfactant. Using the analogy of the thin, elastic surface membrane, a surfactant cuts that membrane to ribbons.

system hunting

  • On-off regulation. See: hunting.


tandem compressors

  • Two or more compressor working in parallel.


  • Total Dissolved Solids


  • Thermal Expansion Valve.


  • Total Environmental Warming Impact


  • Total Heat of Absorption.


  • The junction between two metals generates a voltage that is a function of temperature. Thermocouples rely on this effect. Almost any metals can be used to make a thermocouple. Standard tables show the voltage produced by thermocouples at any given temperature.

thermosiphon systems

  • A flooded system where no pump is needed.


  • Total Heat of Rejection.

triplet compressors

  • Three compressors in parallel (also called trio).

twin-screw compressor

  • Consists of two rotors with matching profiles. The rotor profiles are designed to decrease the volume between them continuously from the inlet to the outlet of the compressor.

two-pass over one-pass

  • A BPHE where one of the fluids passes through the channels only once but the other passes through the channels twice.

two-phase static head

  • A pressure head created by a liquid/gas mixture.



  • A SWEP distribution device, securing a good distribution of gas and liquid in SWEP evaporators.

volumetric efficiency

  • The ratio between the actual vapor volume and the maximum volume that can be theoretically contained in the compressor cylinder.


Wankel engines

  • In the Wankel engine, the four strokes of a typical Otto cycle engine occur sequentially around an oval, unlike the reciprocating motion of a piston engine. In the basic single-rotor Wankel engine, a single oval housing contains a three-sided rotor that turns and moves within the housing. The sides of the rotor seal against the sides of the housing, and the corners of the rotor seal against the inner periphery of the housing, dividing it into three combustion chambers.

water hammer

  • Water propelled in the heat exchanger with a high velocity, which causes damage.

water hammer arrester

  • Absorbs unstable pressure due to sudden closing of gate, and moderates the impact of the fluid inside the pipe.

water heater

  • Provides only hot tap water (cf. boiler, which provides hot tap water and space heating).

working superheating

  • The real superheating that can be measured in the system. Working superheating = opening superheating + static superheating.