Heat Pump & DX (Direct Expansion) Cycle?

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What is a Heat Pump?

What is a refrigerant DX Cycle?

DX system energy efficiency

Why refrigerants?


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 What is a Heat Pump? 

(Also known as Reverse (heating or cooling) DX Cycle,  
or a 
Vapor Compression Refrigeration System (VCRS).)

Heat Pump is a highly efficent4 sealed DX (Direct Expansion) Cycle2 that comprises of a Compressor, an Evaporator a Condenser and an Expansion Device. It uses refrigerants3 as a circulating media to transfer energy (heat) from outside to inside (heating) or from inside to outside (cooling).


A DX cycle utilising refrigerant can be:

  • Heating only DX cycle transfers Energy [heat] from Outside to Inside
  • Cooling only DX cycle transfer Energy [heat] from Inside to Outside
  • Heating or Cooling DX cycle where a Reversing Valve is incorporated that changes the evaporator to a condenser (heating cycle) or a condenser to an evaporator (cooling cycle), hence the Reverse Cycle terminology

Heat Pumps are a major contributor to Net Zero target4


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 What is a refrigerant DX (Direct Expansion) Cycle?

The refrigerant DX cycle (also known as Vapor Compression Refrigeration System - VCRS), Is a closed & sealed circuit and has been around for over 200 years. A highly efficent3 sealed DX (Direct Expansion) Cycle1 that comprises of a Compressor, an Evaporator a Condenser and an Expansion Device. It uses refrigerants3 as a circulating media to transfer energy (heat) from outside to inside (heating) or from inside to outside (cooling).


The DX cycle is used in all types of :
a)
 Refrigeration systems (mainly Cooling), (industrial, commercial or domestic including refrigerators & freezers,)
b) Air conditioning systems (Cooling or Heating / Heat Pump),  (Splits, VRF, Roof Tops, Chillers, AC in cars / trains / ships, etc,


Refrigerants3 used in a DX cycle are controlled substance (hi pressures, high and very low temperatures, levels of flammability & toxicity) and must be only handled by qualified & certified to correct class3 refrigeration engineers  C&G 2079 (A1 class) & C&G 6187 (A2/A2L/A3)


 

DX Cycle - Heating
                     Vapor Compression Refrigeration System - (VCRS)                      Air to Air Split- Heating 

 


 

DX Cycle - Cooling
                     Vapor Compression Refrigeration System - (VCRS)                      Air to Air Split- Cooling

  


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 DX cycle Energy Efficency Advantages due to Refrigerants 

Efficiency of DX cycle = Energy transferred / Energy used by the process

The efficiency of cooling EER- (Energy Efficiency Ratio) cycle or heating cycle COP - (Coefficient Of Performance) can range anything between 250% or (2.5/1) ratio to 500% or (5/1) ratio (that is for every energy unit, [say1 kW] electricity consumed, up to 5 kW can be absorbed from inside (unwanted heat) to be rejected outside - (cooling), or absorbed from outside (air, water or ground) and transferred to inside (heating) to heat a) occupied spaces or b) domestic hot water. These efficiencies (EER & COP) are based on a fixed ambient conditions (called Nominal Capacities).

There are seasonal efficiencies (SEER & SCOP) which are more accurately relates variable ambient conditions and hours of operation.
For example in the UK (considered as mild weather), the SEER can reach 700% or (7/1) ratio while SCOP can be 800% or (8/1) ratio, compared to 85% or (0.85/1) ratio for a gas or oil boilers. Hence a heat pump is a "major contributor to Net Zero target"

There are mainly 
Air to Air systems (most common, efficient and cost effective), Splits, Multi Splits, VRF, Roof Top, mobile etc 
Air to Water systems, Domestic Heat Pumps, Air cooled chillers, etc
Water to Water, 
Ground to Air systems,  
Splits, VRF (reverse cycle Heating & Cooling) etc 
Ground to Water systems. Ground Source Heat Pumps.
Systems can be
a) compact (all components in one box) Mono or
b) Single Splits (one indoor and one outdoor) or
c) Multi Splits/VRF (multiple indoor units connected to one outdoor unit).

The Indoor units can be a Wall, Floor, Ceiling or Ducted fan coils or a refrigerant to water heat exchanger to provide domestic hot water or medium temperature heating water for under floor heating, radiators of water fan coils. The interconnecting refrigeration pipe work would require specialist approved refrigeration engineers and materials. 

There are no extra safety control systems (provided the refrigerant is A1 class non-flammable), no chimneysno Natural Gas supplyno fuel storage (oil)less builder’s work, normal electrical supplies (domestic). Location of outdoor units can be as far as 100 meters away from indoor units in an Air to Air systems.

Refrigerants3 used in a DX cycle are controlled substance (hi pressures, high and very low temperatures, levels of flammability & toxicity) and must be only handled by qualified & certified to correct class3 refrigeration engineers 


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 Why refrigerant? 

- Refrigerant is a chemical gas (can be single or multiple component) that changes phase from gas to liquid and vice versa when it absorbs or rejects energy [heat].
- Refrigerants are extremely energy efficient and can carry more than 50 times energy per weight than air, and 10 times more than water.
- Refrigerants boiling temperature can be as low as -50°C (water +100°C) and condenses at as high as +140°C.
 

These characteristics makes refrigerants suitable for most habitable areas (ambient) of the world, with smaller inter-connecting refrigerant pipes, compact construction, makes it very cost effective and energy efficient against water and air systems..


Refrigerants are a controlled substance due to pressures, temperatures, flammability & toxicity and must only be handled by qualified & certified refrigeration engineers. C&G 2079 (A1 class) & C&G 6187 (A2/A2L/A3)


Refrigerant Standards & Classification

There 3 main Standards that governs the use of refrigerant, ISO817 & ASHRAE34 classify flammability & toxicity, while BS EN 378 sets safety requirements in the manufacture, design, construction, installation & maintenance of systems.

There are 2 safety categories identified by ISO817 & ASHRAE34 MUST be taken into account when handling or using refrigerants: 

1) Flammability 🔥: There are THREE Flammability classifications

  • A1 class - Non-flammable  (Suppresses fire when leaked)
  • A2 and A2L class - Flammables🔥 (Propagate fire when leaked and can be Explosive💥 under pressure). 
  • A3 class - Highly flammable (Propagate🔥 fire and extremely Explosive💥 when leaked

 

2) Toxicity (acute - short exposures due to a leak): There are 3 classification B1 (Lo), B2 (Med), B3 (Hi). & In  BSEN 378-1 (ISO 5149-1) which relates to the displacement of Oxygen in occupied spaces in the event of refrigerant. leak,

There are local standards such as Health & Safety obligations & Standards, Advertising Standards etc that needs to apply to any products including domestic appliances, AC & Heat Pump equipment.

 see also : EPA (US-Environmental Protection Agency) Refrigerant Safety definitions

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